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.

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

Apple Orchard Thinning Recommendations for Central Virginia and Winchester

Despite the frost damage we witnessed on March 20 statewide and in some locations on April 8-9, you might be astonished by the quantity of fruits that still need chemical or hand thinning to attain the targeted size and quality. That being said, it is crucial to assess the extent of frost damage in your various blocks and varieties before deciding whether thinning treatments are necessary this season. To learn about the materials used for apple fruit thinning and the stages at which these chemicals are applied, please refer to my previous blog article. The present blog post aims to share the results of the apple carbohydrate thinning model and provide recommendations on timing, rates, and other considerations for orchards in Central Virginia and the Winchester/Frederick County area.

For Central Virginia, I ran the model today (April 16 at 12:00 pm) using the green tip and full bloom dates for Gala as March 6 and March 29, respectively. You should take into account your own dates and varieties when running the model. However, I believe these two dates are suitable for most apple varieties and orchards in Central Virginia. According to this data, we have already reached an accumulated growing degree day (base temperature = 4C) of 209, which falls within the optimal range (200-250) for thinning applications. This range often coincides with a fruit size of 6-15 mm, traditionally considered the most favorable period for fruit thinning. In terms of carbohydrate status, the daily carbohydrate level is predicted to experience a surplus for the next two days (April 17-18), but the 6-day weighted average will reach -15 g/day tomorrow. Trees generally respond more effectively to thinning materials when the 6-day weighted average values range from -10 to -40 g/day. Furthermore, the current forecast indicates temperatures between 65 to 85F this week (Monday to Friday), which is ideal for NAA and 6-BA uptake and effectiveness.

Considering all these factors, I strongly recommend applying your main fruit thinning application within the next four days. If you can complete all thinning applications within two days, opt for Wednesday and Thursday, as temperatures are expected to be around 80F. For tomorrow’s treatment, a standard rate of thinning materials is advised. For example, if you typically apply 64 fl oz of 6-BA (e.g., Maxcel or Exilis plus), 1 qt of Sevin or Carbaryl, and 1 pt of Regulaid per 100 gal/acre as the standard rate, use the same rate tomorrow. For applications on Wednesday and Thursday, more carbohydrate deficiency is expected which may lead to a recommended 15% reduction in your standard rate. However, this cannot be confirmed yet, as it is based on the 6-day average (2 days before thinning and the following four days). Nevertheless, with a projected carbohydrate surplus on Monday and Tuesday and a deficit in the subsequent three days, I believe a standard rate should be used for Wed and Thu applications.

The apple carbohydrate thinning model outputs for Gala apples, incorporating data from the Crozet (Chiles Peach Orchard) weather station, a green tip date of March 6, a bloom date of March 29, and a flowering spur percentage range of 51-75%

For orchards in the Winchester/Frederick County area, I ran the model for Gala with green tip and bloom dates of March 7 and April 12, respectively. As the model outputs below indicate, the optimal thinning window has not yet arrived. The accumulated DD is below the recommended 200 value, and our average fruit size is still under 5 mm. Therefore, I anticipate that our main thinning window will be around Monday-Tuesday next week (April 24-25), but I will keep you updated if conditions change.

The apple carbohydrate thinning model outputs for Gala apples, incorporating data from Winchester (VT AHS AREC) weather station, a green tip date of March 7, a bloom date of April 12, and a flowering spur percentage range of 51-75%

Chemical Thinning Decisions: A Refresher on Key Factors and Tools

Chemical fruit thinning is a critical management practice in commercial apple orchards, aiming to optimize crop production by regulating the number of fruit per tree. This technique involves the application of chemical thinners during the spring to trigger intentional fruit abscission or shedding, known as “thinning.” This blog post delves into the purpose of chemical fruit thinning, its effectiveness, factors influencing its efficacy, and tips for growers to better predict and achieve desired thinning outcomes.

Chemical fruit thinning serves to:

  • Increase fruit size: By directing carbohydrates to fewer fruits, thinning promotes the growth of larger fruits, preventing the production of small, undersized fruit.
  • Improve fruit quality: Thinning enhances fruit color by increasing light and nutrient availability.
  • Prevent biennial bearing: Thinning helps maintain an annual bearing cycle by encouraging the development of sufficient fruiting spurs for the next year.

Various factors influence the effectiveness of thinning sprays, including:

  • Chemical selection and rates: Choose chemical thinners based on the fruit growth stage and desired level of thinning, and adhere to label guidelines for proper rates.
  • Fruit growth stage and application timing: Apply chemical thinners according to the fruit growth stage, as this can impact efficacy. Most thinning sprays should be completed by 15 mm fruit diameter.
  • Use of oils and surfactants: Adding spray oils and surfactants to chemical thinners can improve efficacy but should be used cautiously unless heavy thinning is required.
  • Weather: Light and temperature conditions before, during, and after application affect thinning spray efficacy. Low light and high temperatures can increase thinning response.
  • Fruit variety: Different apple varieties respond differently to chemical thinners. Consult the following table for a categorization of easy, moderate, and hard-to-thin apple varieties.
  • Block health and age: Healthy, vigorous, and mature orchard blocks tend to be more challenging to thin than stressed or younger blocks.
  • Pruning: Previous dormant pruning can impact thinning spray efficacy, with heavily pruned blocks potentially requiring little to no thinning.

Rates of Chemical Thinners

The rate of chemical thinners is another important aspect that growers need to carefully consider. Most chemical thinners except for carbaryl, oxamyl, and ethephon have rates listed in parts per million (ppm) and fluid ounces (fl. oz.) according to the spray volume. The labels for each chemical thinner commonly supply a conversion chart from ppm to fluid ounces based on the spray volume (gallons per acre). The labels for most chemical thinners typically list a broad range of rates. The level of thinning action is dependent on the rates. When lower rates are used, a lower thinning action can be expected, and when higher rates are used a higher thinning action occurs. When deciding what rates to apply, carefully consider the number of set fruit (crop load), factors which can affect the efficacy (i.e. variety), and always follow the label.

Thinning Spray Applications

The factors of application timing, chemical thinners, and rates should be carefully considered, evaluated, and selected to develop a thinning spray application which will achieve the desired thinning response. The following tables list several common thinning sprays according to the fruit growth stage, materials, rate ranges, and perceived thinning actions.

Effective thinning sprays (petal fall – 5 mm fruit diameter)

Effective thinning sprays (6 mm – 15 mm fruit diameter)

Effective thinning sprays (16 mm – 25 mm fruit diameter)

Carbohydrate Thinning Model

The Cornell Apple Carbohydrate Thinning Model is a valuable tool that assists growers in making informed chemical thinning decisions. Developed by scientists at Cornell University, this model enhances the effectiveness of thinning sprays by calculating the carbohydrate balance in trees, accounting for weather factors such as solar radiation and temperature. Growers can easily access and operate the model through the Network for Environment and Weather Applications (NEWA) webpage, hosted by Cornell University, at the following web address: https://newa.cornell.edu/apple-carbohydrate-thinning.

To utilize the model, begin by selecting the weather station nearest to your orchard blocks. If you prefer to use your own weather station, detailed instructions for uploading your weather station data are available on the NEWA webpage. Once the closest weather station is selected, input your current date, green tip date, and bloom date for the orchard block you want to analyze. The model will then generate a table displaying daily maximum and minimum temperatures, solar radiation, total carbohydrate status, and accumulated degree days since bloom. In the far-right column titled “thinning recommendations,” you will find chemical thinner rate suggestions based on the weather forecast.

Screenshot: Navigating the Cornell Apple Carbohydrate Thinning Model

Disclaimer: The application rates mentioned in this article are based on a concentrate spray volume of 100 gallons per acre and product labels at the time of publication. When applying chemical thinners, use the rates indicated on the labels of the products you are using. The degrees of thinning action are listed according to the author’s personal field experiences. Thinning action may vary from orchard to orchard and block to block.

For additional information on apple fruit thinning, please refer to our extension publication, “Crop Load Management in Commercial Apple Orchards: Chemical Fruit Thinning,” available at https://www.pubs.ext.vt.edu/SPES/SPES-134P/SPES-134P.html.

Assessing the Extent of Frost Damage: Unexpectedly Devastating Effects on Apples and Severe Damage in Peaches and Cherries

Temperature data loggers, situated at various locations within our research farm in Winchester, VA, have documented temperatures as low as 18F this morning (March 20, 2023). In several fruit-growing regions across the state, temperatures in the range of 14-18 F have also been recorded, resulting in a severely detrimental hard freeze, causing considerable damage to most fruit trees in Virginia, including apples, peaches, cherries, apricots, and plums.

Upon assessing the damage to our blocks, we were surprised to find that our apple cultivars, which were transitioning from half-inch green to tight clusters, exhibited severe damage to both king and side blooms. In both Pink Lady and Gala, we recorded a staggering 87% flower damage in king blooms and 60% damage in side blooms. Peach flowers (mostly at the pink stage) sustained damage ranging between 20 to 70%, while sweet cherry flowers (mostly at the tight cluster stage) sustained damage around 80%. Given that our location is approximately 10-14 days behind most of the other orchards in central and southwest Virginia with regards to flower development, the presented figures offer only a glimpse into the overall extent of the damage caused across the state.

While we maintain hope that our other apple cultivars, currently in the green-tip and ¼ inch green stages, have survived the freeze, a comprehensive assessment of the buds’ survival can only be performed later, once they have grown larger. I have included some images below that you can use as a guide to determine the extent of damage to your orchards.

Assessment of blossom damage was conducted on sweet cherry flowers at the ‘tight cluster’ stage, using longitudinal sections. This evaluation followed a freeze event that took place on Monday, March 20, 2023, in Winchester, VA. Blue and red arrows refer to viable and damaged flower styles and ovaries.
Assessment of blossom damage was conducted on ‘Pink Lady’ appleflowers at the ‘tight cluster’ stage.This evaluation followed a freeze event that took place on Monday, March 20, 2023, in Winchester, VA. Blue and red arrows refer to viable and damaged flower styles and ovaries.
After a freeze event that occurred on March 20, 2023, longitudinal sections of ‘Sunhigh’ peach flowers at the ‘Pink’ stage reveal viable (blue arrow) and damaged (red arrow) flower styles and ovaries.

Anticipating Potential Frost Damage Tomorrow, March 20, 2023

As of 5:00pm on Sunday, March 19, the weather forecast predicts low temperatures ranging from 18-23°F tomorrow morning. These low temperatures could potentially damage the flowers and flower buds of various fruit trees, particularly stone fruits. However, in the Winchester/Frederick County area, the cold weather over the past two weeks has helped delay the progress of flower buds, minimizing the risk of critical damage. Most apple cultivars in the area are currently between green-tip and half-inch green stages, while peaches are between calyx red and first bloom (for early-blooming cultivars). Sweet cherries are mostly between the tight cluster and open cluster stages. For all these species, the critical temperature that would kill 90% of the buds is higher than what is forecasted for tomorrow. As a result, tomorrow’s frost should not be a significant concern for most orchards in the Winchester area, unless your orchard is located at a low elevation or has early-blooming peach cultivars. Additionally, apricot or plum trees in bloom might experience some damage.

Orchards in central and southwest Virginia face a greater risk from tomorrow’s frost. Current forecasts predict temperatures of 22-23°F in Nelson County, 24°F in Roanoke, and as low as 18°F in Carroll County. Orchards in these locations are generally 10 days to two weeks ahead of those in Winchester, with many peach cultivars possibly in full bloom and post-bloom stages, and apple cultivars between half-inch green and tight cluster stages. If this is the case, tomorrow’s temperatures could cause a 90% kill rate for peach flowers in most of these locations if the frost lasts for 30-60 minutes. For apples, the critical temperatures are 15°F for half-inch green and 21°F for tight cluster stages, so some damage may occur if temperatures drop below 20°F. Please refer to this link for critical temperatures at each developmental stage. http://royaloakfarmorchard.com/pdf/Critical_Temperatures_Frost_Damage_Fruit_Trees_Utah.pdf

What can you do now? Given the weather parameters for tomorrow’s frost, which is characteristic of radiation frost typically accompanied by a clear sky, low wind, and gradual temperature decline starting at midnight and reaching its lowest point just before sunrise, some tools may be effective in raising the temperature around tree canopies by 2-3°F. Wind machines, under-tree and overhead sprinklers, propane heaters, and fire dragons may be useful if the difference between the lowest temperature and the critical temperature is only 3°F. Unfortunately, no measures will be effective if temperatures drop to 18°F tomorrow. We can only hope that the weather forecast is inaccurate and conditions turn out to be more favorable.

What can you do after the freeze? Generally, six hours after a frost event is sufficient to begin observing damage to blossoms. You can collect branches from various locations within the orchard and use a razor blade to cut through peach flowers and apple clusters, looking for signs of damage to the ovary and style (female parts) in the center of each flower. Consult the following link for examples from our previous posts:

Effects of Ethylene Inhibitors on Pre-harvest Drop, Fruit Quality, and Stem-end Cracking of ‘Gala’ Apples 

The pre-harvest drop refers to the abscission of fruits from the tree before horticultural maturity. Depending on the cultivar and growing season, yield losses due to pre-harvest drop can reach 30%. Factors such as heat and drought stress, heavy insect infestation, and late summer pruning can increase the severity of fruit drop. Early-maturing cultivars (e.g., Gala and Honeycrisp) are usually more prone to fruit drop than late-maturing cultivars (e.g., Fuji and Pink Lady). Ethylene, the ripening hormone, is considered the primary driver of pre-harvest drop, and therefore ethylene inhibitors are used to control the pre-harvest drop in apple orchards. ReTain (from Valent Bioscience) and Harvista (from AgroFresh) are the two ethylene inhibitors labeled for pre-harvest drop control in Virginia. ReTain inhibits the biosynthesis/production of ethylene, whereas Harvista prevents ethylene reception and action. The purpose of this blog post is to share with you the results of two experimental trials we conducted in the 2018 and 2019 seasons to explore the effects of different rates and application timings on the efficacy of these materials.

Over the past four years, we conducted four field trials to explore the optimal timing and rate of ReTain and Harvista and investigate the effects of these materials on fruit drop (%), fruit quality, and stem-end cracking in ‘Gala’ apples. The findings of the 2018 and 2019 seasons were reported in a previous blog post (https://blogs.ext.vt.edu/tree-fruit-horticulture/wp-admin/post.php?post=1919&action=edit), but they can be summarized as follow:

  • A full rate of ReTain (333g/acre) applied 3 weeks before the anticipated harvest date was more effective in controlling fruit drop than a half rate (166g/acre), but not significantly different than a double rate (two pouches, 666g/acre).
  • There was no significant effect on fruit size and weight when ReTain or Harvista were used.
  • Fruits treated with ReTain were firmer than control, but had poor red skin coloration.
  • Fruit coloration was not significantly affected by Harvista.
  • ReTain applications showed a significant reduction in ethylene content, but it required two applications of Harvista to obtain the same effect.

In 2021, we conducted two other field trials to examine the effects of ReTain (full rate), Harvista (full rate), ReTain (half rate) + Harvista and ReTain (full rate) + Provide (GA4+7) on fruit drop, fruit quality and stem-end cracking. The design and findings of these trials were reported in the following link: https://drive.google.com/file/d/1NL8xa_wK3iTZQa61KEJGCn-qOfG-YYLw/view?usp=sharing

The following is the conclusion of these trials:

  • A single application of ReTain (full rate) applied 3 WBH reduced fruit drop (%) compared to control, but was statistically similar to a single or double application of Harvista.
  •  A double application of Harvista showed better control of pre-harvest drop than the single application, but differences were not statistically significant.
  • ReTain applied alone or with ProVide or Harvista reduced fruit coloration compared to control.
  • The single or double applications of Harvista didn’t negatively affect fruit skin coloration.
  • ReTain alone treatment was more effective than Harvista and ProVide in reducing fruit cracking in Gala.

Note: you may need to copy and past the webpage links in a separate window to open them

Fruit Thinning for Apple Orchards in Winchester/Frederick County- May 9, 2022

For those who responded humbly to the recommendations of my previous blog post and sprayed their Galas, Fujis and other hand-to-thin cultivars with chemical thinners on May 4 and 5…Lucky You…I believe that was a wise decision. We had several cold and/or rainy days in the past few days, along with carbohydrate surplus conditions that were unsuitable for any thinning treatments. This week, there is another potential window for thinning for those who chose to wait for a larger fruit size or more perfect thinning conditions. According to the carbohydrate model outputs (see below), the accumulated degree days will be within the perfect range (200-250 DD) on May 13 & 14. There is also a potential decline in tree’s carbohydrate level which should increase the response of apple trees to thinning materials applied in these two days. As of today, May 9, the average fruit size for our Fuji, Gala, Goldens and Honeycrisp in Winchester’s research farm is evolving around 10 mm which is also the perfect size for thinning. Having said that, I think it won’t be possible to spray on Friday and Saturday due to the rain; so, you better apply your thinning treatment on Wed and Thu this week (May 11 and 12); or Sunday (May 15) if weather allows. For blocks treated for thinning on May 4&5, no additional thinning treatments are needed at this time. You need to wait at least 2 weeks after the 1st application to decide if additional thinning is required.

The Cornell apple carbohydrate thinning model outputs for Winchester.

It’s worth noting that the outputs and recommendations of the model above are based on our Gala’s green tip date of Mar 15 and full bloom date of April 17. If you have different dates for your Gala or other apple cultivars, you will see different model outputs. To access the model, use this link: https://newa.cornell.edu/apple-carbohydrate-thinning.