Sprinklers for Frost Protection: What You Need to Know Before Turning Them On

This week and weekend, the temperature is expected to drop below freezing, and some areas throughout the state may even experience snow showers. While the apples, cherries, and peaches in our location in Winchester, VA aren’t at great risk yet because the flowering buds are not yet at a developmental stage that raises red flags, other parts of the state, particularly around central and southwestern Virginia, could be at a much more advanced stage, particularly for stone fruits. These freezing temperatures might be a cause for concern.

That’s why some growers might be thinking about using irrigation sprinklers for frost protection. It’s a convenient method if you already have them in your orchard. But, I’ve noticed over the years that some might be using sprinklers the wrong way or when they’re not necessary, which can cause more harm than good.

I hope we don’t experience any damaging frost this season, but if we do, and you’re considering turning on your sprinklers as your last resort in the face of spring frost, I’ve recycled some information from old blog posts to remind you of the science and application of using sprinklers for frost mitigation. So, here’s a quick read to help you understand how to use sprinklers effectively and safely.

If you’re looking for a way to protect your trees from frost damage, using sprinklers (e.g. overhead or under-tree irrigation sprinklers) may be a viable solution. When you use sprinklers, you are essentially harnessing the latent energy of water molecules as a source of heat to warm up your trees. But how does this work exactly?

To understand this phenomenon, it’s important to note that water exists in three states: liquid, solid (ice), and gas (vapor). The transition among these states can either produce energy (exothermic) or consume energy (endothermic).

When the air temperature drops below freezing point (< 32°F), and you use sprinklers, you encourage the transition of water from its liquid phase into ice. This process releases latent energy into sensible energy (heat) that plant tissues can use to warm up.

While this may seem like an ingenious solution, there are some potential drawbacks to using sprinklers. To ensure that using sprinklers to reduce frost damage doesn’t harm your plants, there are three crucial things to consider. Let’s take a closer look at each one.

First and foremost, it’s essential to keep an eye on the wind. This is because wind can encourage the transition of water from its liquid phase into its gaseous phase (vapor). During this process, heat in the air and around your plants will be consumed in an endothermic reaction. That’s why it’s recommended that you avoid using sprinklers if the wind speed is above 10 mph.

To help you determine the amount of water needed under different wind conditions and temperatures, refer to the table below (Table 1).

Table 1 provides information on the amount of water (in inches per hour) that should be provided for frost protection under different wind speeds (in miles per hour). This table is based on information from the University of Florida Extension Circular 287.

The second thing you need to consider when using sprinklers to reduce frost damage is the dew point. In simple terms, low dew points mean low humidity. If the dew point is too low, the water you add through sprinklers will quickly evaporate to compensate for the low humidity. This will cool down the air around your plants, which will cause more damage.

Under moderate dew points, a portion of the water you add through sprinklers will turn into vapor and consume heat, while another part will turn into ice and produce heat. After some time, the net energy will be positive, and you’ll reap the benefits of using sprinklers. However, if the dew point is too low, it means that the air is too dry and will take much longer to become saturated with vapor. During this time, the air temperature may drop to a critical temperature that can damage your buds or flowers. In this scenario, using sprinklers may cause more harm than good to your plants.

To help you determine whether sprinklers will be useful in your situation or not, refer to the table below (Table 2) to determine the temperature at which you should turn your sprinklers on/off. If the dew point and the critical damage temperatures are not within the range shown in the table, it’s best to avoid using sprinklers altogether.

Table 2 provides information on the minimum temperature at which sprinklers should be turned on/off. This data comes from UC-Davis and is part of their FP005 Quick Answers guide.

To determine when to turn your sprinklers on and off for frost protection, use a critical temperature for frost damage (columns) and dew-point temperature (rows) chart. The point where the row and column intersect is the temperature at which you should turn on or off your sprinklers. It is generally recommended to use sprinklers to protect against temperatures in the range of 24 to 32F.

To find out the critical temperature at which 90% of the flower buds can be killed, please refer to the following link: https://blogs.ext.vt.edu/tree-fruit-horticulture/2022/03/27/hard-freeze-is-expected-this-tuesday-march-29/

You can get the dew point information for your location by using your zip code on Intellicast at http://www.intellicast.com/Local/Weather.aspx?location=USVA0837.

The third factor to consider is the amount of water your sprinklers can provide. If you’re uncertain whether your sprinklers can provide an adequate amount of water, it’s best not to use them at all. Refer to Table 1 to determine the amount of water required under different conditions.

By considering these three factors, you can ensure that using sprinklers to reduce frost damage is effective and safe for your trees.

Freeze Injury Updates (Winchester-March 29): Sweet cherry blooms lost to freezing temperatures; apples and peaches made it through.

Temperature profile of orchard blocks in the AHS Jr. AREC’s research farm (Winchester, VA) on March 28-29, 2022.

Our digital temperature data loggers recorded temperatures as low as 17 oF that continued for > 2h this morning (Tuesday, March 29). Some of our apple cultivars, including Gala and Pink Lady, were at the tight cluster and first pink; whereas, others like Fuji and Honeycrisp were between half-inch green and tight cluster. Temperatures below 21F were enough to wipe out the flowers of these cultivars completely, but we were pleasantly surprised to find out that all apple cultivars we assessed this afternoon survived this frost event with minimal damage. We recorded between 15 – 35% bud mortality in the king blooms and less than 15% in the side-blooms of Honeycrisp, Gala, and Pink Lady, with the latest showing the highest percentage of damage. Similarly, the four peach cultivars (Rich May, Sunhigh, Red Haven, and John Boy) we evaluated showed less than 15% flower mortality.

I generally believe that 10-30% flower mortality after frost is not a bad thing and could even be considered desirable as a natural thinning process and to ensure a good return bloom next year. It is generally evident that thinning during bloom has a very positive impact on return bloom; and whether this thinning is done chemically (e.g. liquid lime sulfur), mechanically (e.g. hand thinning), or by mother nature, the impact on return bloom will probably be the same.

On the other hand, our sweet cherry cultivars failed to sustain these low temperatures, and despite being still in the tight-cluster stage, they were severely damaged, with an average bud mortality rate of around 80% in Ebony Pearl and Selah. Regina, a relatively late-blooming cultivar, was still at the swollen bud stage, showing 0% damage.

It is also worth noting that flower development can vary among cultivars, species, and locations in the state. Also, temperatures recorded this morning in Winchester were seriously low, but they might not be the lowest in other areas. So, you can depend on this report only if your trees are in the same developmental stage and your temperature didn’t go below 17 oF this morning. If otherwise, I suggest that you evaluate the damage in your block as per the method and images I shared in previous posts.

Flower buds were collected from three apple cultivars (Pink Lady, Gala and Honeycrisp) and assessed for damage after the frost event that occurred on the morning of March 29, 2022 in Winchester/Frederick county area.
Flowers were collected from four peach cultivars (Sun High, Rich May, John Boy and Red Haven) and assessed for damage after the frost event that occurred on the morning of March 29, 2022 in Winchester/Frederick county area.
Flowers were collected from three sweet cherry cultivars (Ebony Pearl, Regina and Selah) and assessed for damage after the frost event that occurred on the morning of March 29, 2022 in Winchester/Frederick county area.

More About Apple Chemical Thinning-May 2024

From the thinning treatments conducted between April 28-30 in the Winchester/Frederick County area, we can now observe a noticeable difference in fruit size within the same cluster. The smaller fruits, which are likely to fall off, can now be easily detached with a gentle touch or slight pull. As for the chemical thinning applied last week (May 2-3), the results are starting to become visible for some, but not all, varieties. It’s important to note that due to several days of low carbohydrate levels, as expected, some fruits have started to drop naturally—without any chemical intervention. These fruitlets measure between 8-11 mm in diameter. The fruits that are responding to our thinning from 10-12 days ago are about 13-15 mm in diameter. Therefore, hold off on deciding whether to apply another thinning treatment based on the smaller fruits under 11 mm. Wait a bit longer to see how the last treatment worked, checking for clear size differences in the same cluster or if the fruits can now be easily pulled off.

Apple fruitlets from a Pink Lady apple tree, displayed in two groups. The upper section shows fruitlets with diameters of 8-11 mm that have either fallen or are about to fall due to natural fruit abscission. The lower section presents fruitlets measuring 13-16 mm, which detach easily upon touch or gentle pull, indicating a response to the chemical thinning applied on April 29th. Some king fruits now measure 24 mm.

If you find that your fruit size is still under 18 mm and you need another thinning pass, you can still use 6-BA and NAA products along with carbaryl and a surfactant—or better yet, oil. For fruits averaging 15-20 mm, Accede can be used. It contains 10% of ACC, a precursor to the ethylene hormone, crucial for this process. You should apply 23-46 fluid ounces per acre, ideally when the king fruit is between 15-20 mm. If the temperature is above 90°F on the day you plan to apply, consider reducing the rate. For best results, apply Accede in slow drying conditions, like early morning or at night.

On the other hand, Ethephon is best for ‘rescue thinning’ when your fruits are between 18-25 mm. Ethephon works best when temperatures are between 70-80°F on the day of application and the following two days. Avoid using Ethephon when it’s cooler than 70°F or hotter than 80°F, as it may be ineffective or cause overthinning. Also, don’t use it if the forecast predicts temperatures over 85°F for the application day or the next two days. When applying Ethephon, use at least 100 gallons per acre to ensure effective distribution. Please refer to the attached table for the rates of ethephon and carbaryl for different cultivars, based on the research by Autio and Cowgill (https://ag.umass.edu/fruit/fact-sheets/f-129r-late-season-rescue-thinning-with-ethephon).

Apple Thinning Advisory: May 6-10 Insights for Winchester/Frederick County and Central Virginia Growers

In the Winchester/Frederick County area, it appears that apple trees may be overly responsive to thinning treatments applied this week (May 6-10). Therefore, I generally advise against implementing any thinning treatments during this period, especially if you have already done so at petal fall (April 25-26) or the previous week (April 29-May 3). Examining the carbohydrate balance chart below, it’s evident that trees have been consistently deficient since April 24. While this deficiency is typically conducive to chemical thinning treatments, it also poses a risk of natural fruit abscission. Given the forecasted temperatures exceeding 80°F with mostly cloudy days this week, I anticipate further carbohydrate deficiency. If you administered the first thinning treatment on April 25-27, you should observe its effects this week (May 7-8). However, if your initial treatment was conducted last week (April 29-May 2), simply wait until May 12-13. By then, you should notice some fruits dropping off easily upon touch, with a noticeable difference in size between retained and dropping fruits.

Apple Carbohydrate Thinning Model-Weather Station (Winchester, VA), Gala- Green tip (March 7), Full Bloom (April 14)

The situation in Central Virginia presents less risk, with more days of carbohydrate surplus following full bloom. Nevertheless, there has been a significant decline in carbohydrate balance this past week, likely to persist due to high temperatures and reduced solar radiation on certain days this week. Hence, I also advise against thinning treatments this week, recommending waiting until May 10th to ascertain if carbohydrate levels stabilize. Growers who have already applied two thinning treatments (at petal fall and last week) likely have completed thinning for the season, achieving an optimal crop load. For those who applied their first treatment on April 28-30, trees are likely to respond favorably to thinning materials, with a noticeable size separation expected this week (May 8-10).

Apple Carbohydrate Thinning Model-Weather Station (Roseland, VA), Gala- Green tip (March 12), Full Bloom (April 12)

Ideal Thinning Conditions for Apples in Winchester-Frederick County Area (April 29)

With the high daily temperatures we’re experiencing this week, the degree days for Gala and most other apple cultivars in the Winchester/Frederick County area will likely reach the 200DD mark today. This signals the optimal timing for chemical fruit thinning using 6-BA-carbaryl and NAA-carbaryl mixes. I utilized the carbohydrate thinning model on NEWA (https://newa.cornell.edu/apple-carbohydrate-thinning), based on a green tip date of March 7 and full bloom date of April 14 for Gala in my location. The model indicates a severe carbohydrate deficiency, which is very conducive to thinning conditions. It also recommends reducing the chemical thinning materials by 15% compared to standard rates. Please refer to my previous posts ((https://blogs.ext.vt.edu/tree-fruit-horticulture/2024/04/18/apple-fruit-thinning-general-notes-and-specific-recommendations-for-central-virginia/) for the standard rates I’ve recommended and adjust your calculations accordingly. You have a four-day window (April 30 to May 3) to complete your thinning applications. The temperatures over the next few days are expected to be above 80 degrees Fahrenheit, which is ideal for both the uptake and activity of 6-BA and NAA products.

Apple Carbohydrate Thinning Model-Weather Station (Winchester, VA), Gala- Green tip (March 07), Full Bloom (April 14

Apple Fruit Thinning for Central Virginia (April-23)

I believe this is the ideal time to schedule your thinning applications for Gala and other cultivars with a green tip date around March 12 and a full bloom date of April 12. In some areas, Honeycrisp and Fuji reached full bloom a week after Gala. Therefore, make sure to check the Carbohydrate Thinning Model on the NEWA website (https://newa.cornell.edu/apple-carbohydrate-thinning) and input the exact dates for your cultivars. Using the weather station data from Roseland and Gala’s phenology data, I ran the model and here are the outputs: three positive indicators for good thinning conditions. First, the accumulated degree days are expected to reach the optimal range of 200-250 DD by this Sunday, April 28. Second, the 6-D weighted average of tree carbohydrate balance is showing a promising decline as we approach April 28. Third, the current weather forecast predicts next week’s temperatures will exceed 80°F, which is ideal for the effectiveness of 6-BA and NAA treatments, with partially cloudy days contributing to the carbohydrate deficiency needed for effective thinning. If this is your first thinning treatment for Gala this year, aim for Sunday, April 28 through Tuesday, April 30. However, if you have already applied a thinning treatment at petal fall, delay your next treatment until the end of next week, around May 2 and May 3, to observe the effects of the initial application.

Apple Carbohydrate Thinning Model-Weather Station (Roseland, VA), Gala- Green tip (March 12), Full Bloom (April 12)

Also, for detailed guidelines on thinning practices during petal fall and the 6-18 mm window for Gala, Honeycrisp, Fuji and other cultivars, please refer to my previous post (https://blogs.ext.vt.edu/tree-fruit-horticulture/2024/04/18/apple-fruit-thinning-general-notes-and-specific-recommendations-for-central-virginia/).

Apple Cultivar Stages and Thinning Practices in Winchester/Frederick County (April-23)

In the scenic Winchester/Frederick County area, apple growers are witnessing varied stages of growth across different apple cultivars. Currently, cultivars such as Fuji and Honeycrisp are at the partial petal fall stage, while Gala and Red Delicious have reached complete petal fall. Most notably, the Pink Lady cultivar has reached the critical >6 mm diameter stage, making it the ideal time for fruit thinning.

It’s important to time the application of chemical thinning agents, such as 6-BA and NAA, accurately. According to the Apple Carbohydrate Thinning Model on Cornell’s NEWA website (https://newa.cornell.edu/apple-carbohydrate-thinning), the current predictions for accumulated degree days (DD) for Pink Lady apples in Winchester suggest reaching the 200-250 DD window by Thursday, April 25, and continuing through most of the next week. While the model indicates a daily carbohydrate surplus, which isn’t ideal for thinning, a forecasted drop in carbohydrate levels by April 27 should be closely monitored. This dip, though minor, will likely influence the 6-D weighted average, which is a crucial metric for thinning effectiveness.

If you’re targeting a thinning application for cultivars like Pink Lady, which had a green tip date of March 2nd and a full bloom date of April 8, post-April 25th is advisable for starting treatments. The weather forecast suggests that April 28, April 29, and May 1 will offer optimal conditions—both temperature-wise and in terms of carbohydrate levels—for the effectiveness of 6-BA and NAA treatments.

Apple Carbohydrate Thinning Model-Weather Station (Winchester, VA), Pink Lady- Green tip (March 2), Full Bloom (April 8)

Turning our attention to the Gala cultivar, it hasn’t yet reached the 200-250DD window. However, for those keen on applying a thinning treatment at petal fall, the upcoming Sunday or Monday is recommended. Following this initial treatment, it’s advisable to wait at least 10 days before the subsequent application, ideally timed for when the cultivar enters the 200-250DD window.

Apple Carbohydrate Thinning Model-Weather Station (Winchester, VA), Gala- Green tip (March 7), Full Bloom (April 14)

For detailed guidelines on thinning practices during petal fall and the 6-18 mm window for Gala, Pink Lady, and other cultivars, please refer to my previous post (https://blogs.ext.vt.edu/tree-fruit-horticulture/2024/04/18/apple-fruit-thinning-general-notes-and-specific-recommendations-for-central-virginia/).

Apple Fruit Thinning: General Notes and Specific Recommendations for Central Virginia

In various parts of Virginia, apple orchards are currently transitioning through stages ranging from full bloom and petal fall to having fruits about 6-18 mm in size. Given this period of active growth, it’s an ideal time to discuss the crucial practice of apple fruit thinning—especially vital this year due to the heavy bloom and minimal spring frost damage. Proper thinning is essential; without it, the excess blossoms and small fruitlets from this year could negatively impact both next year’s yield and the quality of this year’s harvest. The development of next year’s flowering buds in apples is largely determined during the first four to five weeks post-full bloom. In years with heavy bloom and fruit set, like this one, failing to adequately thin can lead to too many fruits drawing on the tree’s carbohydrates. This overburden can drastically reduce the number of viable buds for the following year, potentially resulting in no crop for varieties prone to alternate bearing, such as Honeycrisp, Fuji, and Winesap.

Moreover, maintaining too many fruits on the tree can also degrade fruit size and overall quality at harvest—particularly for the Gala variety. It’s important to note that fruit size at harvest is primarily influenced by the rate of cell division and the total number of cells per fruit, rather than merely by cell expansion. This cell division occurs mainly in the initial weeks following full bloom, making it critical to reduce competition among the fruitlets so that more resources are directed to fewer fruits, thus enhancing their growth and quality.

With these points in mind, I’d like to offer some specific recommendations for thinning applications at petal fall and the 6-18 mm fruit size window (the main thinning window). At the end of this post, I will also share outputs from the Carbohydrate Thinning Model and my personal advice for apple growers in Central Virginia and other regions currently within the 6-18 mm fruit size stage. For those in the Winchester and Frederick County areas, where orchards are still transitioning between full bloom and petal fall, other posts will follow as needed.

Effective thinning sprays at petal fall to 5 mm fruit diameter.

• To prevent the formation of pygmy fruit, avoid applying NAD and NAA to Red Delicious and Fuji apples after petal fall.
• When using NAD as a thinning spray, ensure that the spray volume is at least 100 gallons per acre, as lower spray volumes may decrease efficacy.
• Carbaryl can be used alone as a thinning spray between petal fall and when the fruit diameter reaches 15 mm. However, it is more commonly used in combination with either NAA or 6-BA when the fruit diameter is between 7 and 15 mm.
• When using carbaryl alone, increasing the rate of application will not enhance thinning activity, as carbaryl’s effectiveness is not highly responsive to changes in application rate.

Recommended chemical materials and rates for thinning fruits (6-15 mm)

• Exilis 9.5 SC contains a high concentration of 6-BA (9.51%) compared to Maxcel (1.9%) and Exilis Plus (2%). Refine is available in two concentrations: Refine 3.5 and Refine 6.25, with the latter having a higher concentration of NAA. All dosages in the table are based on the concentrations found in Maxcel (6-BA) and PoMaxa (NAA).
• The recommended application rates in the table are for mature trees (6 years or older). For younger trees (4th and 5th leaf), reduce the rate by 25% to 50%.
• Do not use less than 100 gallons per acre for thinning treatments.
• Thinning is more difficult in the upper parts of the canopy compared to the lower parts.
• Thinning is easier when the crop is heavy, as opposed to when the crop is light.
• Use the carbohydrate thinning model to time your thinning applications. Alternatively, as a general rule of thumb: Thinning is more challenging when it is sunny; overcast conditions for 2 to 3 days facilitate easier thinning; thinning materials are less effective in cool weather (below 65°F) and more effective in warm weather (above 70°F).

Apple Carbohydrate Thinning Model Outputs-Pink Lady- Roseland-VA

The Carbohydrate Thinning Model uses critical environmental factors such as temperature and solar radiation to assess the carbohydrate status of trees. This status helps determine how trees will respond to chemical thinning agents. When trees are in a carbohydrate-deficient state, they are more likely to shed fruit, making them more receptive to thinning applications. Conversely, when conditions are favorable for photosynthesis—sunny and warm—trees are less likely to respond to thinning as they prefer to retain their fruits. Remember, fruits carry seeds, and seeds represent the future generation of trees—an easy concept to grasp. The model doesn’t rely on a single day’s data; instead, it averages the carbohydrate balance over six days to decide if a tree has a deficiency or surplus. This helps determine whether to apply treatments, how much response to expect from the trees, and whether to adjust the quantity of thinning materials to avoid over- or under-thinning. The model also tracks accumulated degree hours from the time of bloom, which is why knowing the exact day of full bloom is crucial. It predicts the optimal thinning period, which usually occurs between 200 to 250 degree days (DD)—coinciding nearly perfectly with the 6-18 mm fruit size window.

Based on data from the weather station in Roseland, VA, and the growth stages of Pink Lady and Gala apples, the model predicts that by Saturday, April 20, Pink Lady trees will reach about 200DD. This is the ideal time to start applying thinning materials, continuing through the 250 DD mark. However, the model also indicates a carbohydrate surplus on April 19 and the following three days. Therefore, it recommends increasing the thinning material rate by 30% above standard. Since the forecast predicts temperatures above 70°F this Saturday, which enhances the uptake and efficiency of agents like NAA and 6-BA, thinning applications are best done on that day.

Apple Carbohydrate Thinning Model-Weather Station (Roseland, VA), Pink Lady- Green tip (March 4), Full Bloom (April 6)

For Gala apples, which reached green tip on March 12 and full bloom on April 12, the 200-250DD window has not yet been reached. Nonetheless, if you wish to apply chemical thinning now using 6-BA and carbaryl to promote cell division, you can use 48 fl oz of 6-BA and 1 pt/100 gal of carbaryl. However, it’s important to wait at least 10 days to observe the effects of this treatment before proceeding with another. Based on the current weather forecast and degree days, the 6-18 mm fruit size window should still be open in 10 days if you decide on a second thinning application.

Apple Carbohydrate Thinning Model-Weather Station (Roseland, VA), Gala- Green tip (March 12), Full Bloom (April 12)

Optimizing Fruit Size and Preventing Bitter Pit in Apple Crops with Early Season Interventions

Calcium applications:

  • Apply 4-14 pounds of actual calcium per acre each season to help prevent bitter pit and cork spots. This amount is equivalent to applying 15-50 pounds of calcium chloride (CaCl2) or distributing 2-8 pounds of CaCl2 per cover spray.
  • For apple varieties that are particularly susceptible to bitter pit, such as Honeycrisp, Golden Delicious, and York Imperial, it is advisable to use the maximum recommended concentration of calcium.
  • Initiating calcium applications early in the season, starting at the Pink stage of bud development, is more beneficial than applications later in the season.
  • Foliar applications of calcium nitrate are not advised for Delicious and York apple varieties, as they can induce symptoms similar to cork spot.
  • In orchards with significantly low calcium levels, applying calcium sulfate (gypsum) in a band beneath the trees at a rate of 3 tons per acre can improve calcium content in the leaves and fruit. However, it may take two years or more to observe positive outcomes.
  • Avoid applying calcium chloride under conditions that slow drying, such as in the early morning, as it can harm the foliage. This is especially important for apple varieties sensitive to calcium chloride, including Idared and Golden Delicious.
  • A deficiency in boron can hinder the movement of calcium within the tree, potentially affecting fruit quality.

Boron applications:

  • Boron plays a crucial role in the development of flowers and the setting of fruit. A lack of boron can adversely affect both yield and the size of the fruit.
  • Furthermore, boron is necessary for the transportation of calcium within the plant. A deficiency in boron can lead to disorders related to calcium deficiency, such as bitter pit and cork spot in apples.
  • To avoid these problems, it is recommended to apply 0.5-1 pound of boron per acre, which is equivalent to applying 2.5-5 pounds of Solubor per acre.
  • Boron should be applied either at the pink or bloom stage of flower development, mixed in the tank with calcium chloride, or 7-10 days following the fall of the petals.

Prohexadion calcium:

  • Applying prohexadione calcium (PC), found in products like Kudos and Apogee, early in the season at the Pink stage can significantly reduce the incidence of bitter pit and blossom blight, as well as decrease the risk of shoot blight.
  • For optimal results from PC treatments, consider the following guidelines:
    • Apply PC at a rate of 6 ounces per acre.
    • PC’s effectiveness decreases in environments with high pH levels or in water with a high concentration of calcium carbonate (hard water). To counteract this, add ammonium sulfate (AMS) to the spray mixture.
    • Avoid combining PC with calcium or boron in the spray mix, as interactions may diminish efficacy.
    • Enhance the performance of PC applications by including a surfactant in the tank mix. This helps in better distribution and adherence of the product on plant surfaces.
    • Should the surfactant cause foaming, incorporate an anti-foaming agent to mitigate this issue.
    • Both Kudos and Apogee are approved for application at the Pink stage for apples, indicating a targeted timing for use.
    • Research by Sherif in 2019 (unpublished) suggests that applying PC concurrently with or before thinning agents like 6-BA or NAA does not compromise the effectiveness of thinning treatments.

Urea applications

  • Applying urea to the foliage during the bloom period (at a rate of 3 pounds per 100 gallons of water) and subsequently at petal fall and the first cover spray (5-6 pounds per 100 gallons of water) can significantly support cell division. This practice is especially beneficial for Gala apples. Such applications become critical when the primary, or king blooms, which typically produce the largest fruits, are lost to frost damage.
  • The foliar application of urea at the time of bloom can also improve fruit set by prolonging the period of effective pollination, enhancing the chances of fertilization and subsequent fruit development.
  • It’s important to note that foliar-applied nitrogen, while beneficial for fruit set and sizing, does not substitute for soil-applied nitrogen fertilizers. Unlike ground applications, foliar-applied nitrogen does not move into the tree’s woody structure but is instead directly utilized for the development of the fruit.

Fruit Russeting in Golden Delicious and Scarf Skin in Gala:

  • Fruit russeting and scarf skin in apples is often associated with high humidity during the first 30-40 days of fruit development.
  • To address this issue, apply 2-4 applications of GA4+7 (such as ProVide 10SG or Novagib 10L), starting from petal fall and continuing at 7-10 day intervals.

Optimal Timing for Thinning Apple Trees and Increasing Return Bloom

As we enter the optimal window for thinning treatments in the Winchester/Frederick County area, it’s important to consider the best options for your apple trees. In a follow-up to our previous post, we have some important updates for those who chose option A or B.

If you went with option A, and applied your thinning treatments last week (April 25-27) because you had larger fruit, you won’t need to do any further treatment for at least two weeks after this treatment. Wait until you can see signs of fruit abscission and/or fruits showing distinct segregation in size before deciding if another thinning treatment is necessary. For those who went with option B due to the low temperatures, this Friday and over the weekend may provide better thinning conditions. Temperatures are expected to rise above 65F on Friday and reach 72F on Sunday, with even higher temperatures on Monday. If your average fruit size is 15mm or above, start thinning on Friday and Saturday. If your fruits are less than 15mm, wait until Monday when temperatures are expected to reach around 78F and remain warm and cloudy for the rest of the week. Trees are currently in a carbohydrate surplus state, which could sustain until Friday, so you may need to pump up the rate to 30% more than the standard rate. Check the model the day before the application to see the recommended rate in the last column.

Green tip (Mar 7), full bloom (April 12), weather stations (Winchester, VT AHS AREC), percent flowering spurs (51-75%). The carbohydrate thinning model can be accessed at: https://newa.cornell.edu/apple-carbohydrate-thinning

For apple growers in Central Virginia, the same advice applies, except that you can start thinning treatments on Thursday if the forecasted temperature stays at 65F or higher. If your current fruit size is less than 15mm, consider Monday (5/8) as the main thinning day. Use the model to determine whether to apply the standard rate or increase it. If the model suggests adding oil to the tank to increase efficiency, it’s worth considering, especially if your fruits have exceeded 15mm and this is your final chance to get the top part of the canopy in shape. However, keep in mind that Captan should not be used 4-5 days before and after applications containing oil, as this can cause significant damage to fruit quality. In summary, timing and temperature are critical when it comes to thinning treatments for apple trees. Follow these guidelines to ensure optimal results for your fruit harvest.

Green tip (Mar 6), full bloom (Mar 29), weather stations (Crozet, Chiles peach orchard), percent flowering spurs (51-75%). The carbohydrate thinning model can be accessed at: https://newa.cornell.edu/apple-carbohydrate-thinning

When it comes to thinning apple trees, it’s important to consider the optimal thinning conditions that increase return bloom, especially for cultivars with a tendency for biennial bearing, such as Honeycrisp, Golden Delicious, and Fuji. If you’re looking to improve return bloom for Honeycrisp and Golden Delicious, we have a recipe that has been endorsed by many and is based on our own research over the past few years.

First, use NAA applications for fruit thinning at a fruit size of 6-15 mm, or within 30 days of full bloom. The recommended mixture is NAA @ 3 oz + Sevin @ 1 qt + Regulaid @ 1pt/100 gal/acre. In addition to NAA applications for thinning, NAA applications (@ 2-4 oz/100 gal without carbaryl or oil) should start at a fruit size of 30-35 mm and repeated 2 times at 7-10 day intervals. The rates of NAA are based on Refine 3.5 and PoMaxa. Make rate adjustments if using Refine 6.25.

For improving return bloom in Fuji and Red Delicious, we recommend using ethephon. Don’t use NAA with Fuji and Red Delicious. Apply 1-4 applications of ethephon at 0.5 pt/100 gal (without carbaryl or oil) starting at a fruit size of 30-35 mm in diameter and repeated at 7-10 day intervals. A surfactant is not necessary if ethephon is applied with cover sprays. Ethephon at this low concentration and stage of fruit development will not cause any thinning, but it should improve return bloom.

Thinning Recommendations and Timing for Central Virginia and Winchester/Frederick County Apple Orchards

Clusters of apples on the same branch exhibit noticeable variations in fruit sizes, posing a challenge for thinning decisions

We base our thinning recommendations on the carbohydrate thinning model results from the NEWA website, which I previously discussed in earlier posts. Last week, I ran the model for Central Virginia using Gala’s green tip and bloom dates of March 6 and 29, respectively, along with data from the Crozet weather station (Chiles peach orchard). Based on the model, I recommended that growers apply their primary thinning treatment between April 17 and 20, ideally on Wednesday or Thursday, when both temperature and tree carbohydrate balance were optimal for thinning with 6-BA and NAA products. Those who have already followed these recommendations and applied treatments last week should now see that it was the right decision, given the cool weather and carbohydrate surplus this week.

For growers who did not apply a thinning treatment last week but plan to start this week, I suggest one of two options: A) If the majority of your crop has an average fruit size more than 12mm, apply the treatment tomorrow (April 26), or Thursday or Friday (weather permitting). B) If the majority of your crop has a fruit size under 12mm, wait another week for a warmer temperature window and/or lower carbohydrate levels. This year, it is not uncommon to see two distinct crops with different sizes on the same tree, particularly for Gala, Pink Lady, Fuji, and Reds. If the majority of your crop has an average size of >12mm, follow option A; otherwise, go with option B.

Green tip (Mar 6), full bloom (Mar 29), weather stations (Crozet, Chiles peach orchard), percent flowering spurs (51-75%). Note: If thinning treatments are scheduled for tomorrow, the model suggests increasing the thinning materials by 30% to account for carbohydrate surplus.

Regarding thinning recommendations for apple blocks in the Winchester/Frederick County area, the model results, using Gala’s green tip and bloom dates of March 7 and April 12, respectively, along with data from the Winchester weather station (VT AHS AREC), show that we have not yet reached the 200-250 accumulated degree days (DD). These values are expected to be reached this weekend (April 29 and 30), coinciding with low carbohydrate levels and relatively acceptable temperatures. However, rain is anticipated over the weekend. With this in mind, I suggest the same two options as I did for Central Virginia. Based on apple blocks at our research center, I would choose option B for the majority of our varieties and wait another week, hoping for better thinning conditions in terms of temperature and tree carbohydrate status. I typically recommend applying 6-BA and NAA only when the temperature is above 65°F on the application day, with the optimal range being 80-85°F. I hope to see warmer temperatures by next Thursday or Friday, allowing for our primary thinning treatment.

Green tip (Mar 7), full bloom (April 12), weather stations (Winchester, VT AHS AREC), percent flowering spurs (51-75%). Note: If thinning treatments are scheduled for tomorrow, the model suggests increasing the thinning materials by 30% to account for carbohydrate surplus.

Carbohydrate Thinning Model Updates for Central Virginia- April 18

In my latest blog entry, I highlighted that this week, particularly on Wednesday and Thursday, presents an optimal window for fruit thinning treatments, especially for Gala, Pink Lady, and possibly Fuji. Taking into account the green tip and full bloom dates of March 6th and March 29th respectively for Gala apples in Central Virginia and data from the Crozet weather station, the model forecasts a moderate to severe shortfall in daily carbohydrate levels over the next three days, impacting the 6-day weighted average. As such, it is crucial to apply thinning treatments judiciously, with a focus on 6-BA and NAA. However, over-thinning may occur if high rates are used. Therefore, the model suggests reducing thinning material rates by 15% for applications scheduled on Wednesday, April 19th. This recommendation may extend to Thursday and Friday, based on current forecasts. In practical terms, if your standard application rate for Gala consists of 64 fl oz of Maxcel, 1 qt of Sevin, and 1 pt of Regulaid per 100 gal/acre, a 15% reduction corresponds to 54 fl oz of Maxcel and 27 fl oz of Sevin. No adjustments to the surfactant are needed.

I encourage you to utilize the model with the weather station nearest to your location, inputting accurate green tip and bloom dates for your orchard. If the degree days (DD) column displays values under 200, but your king fruits measure over 7mm in size (averaging around 15-20 fruitlets), I still advise proceeding with thinning treatments this week, rather than postponing to the following week and potentially encountering difficulty in finding an appropriate temperature window for application. To access the model, use the following link: https://newa.cornell.edu/apple-carbohydrate-thinning