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.

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.

Fruit Thinning Decisions for Apple Orchards in Winchester/Frederick County- May 3, 2022.

Tomorrow and after (May 4-5) could be a good (but not ideal) time for fruit thinning applications for apple orchards located in Winchester and the surrounding area. In our Winchester research orchard, the average fruit diameter for Gala, Honeycrisp, Pink Lady, Red Delicious, and Fuji is about 6.5 to 8.5mm, which is inside the size range (6-15mm) we usually target for thinning treatments. Also, the carbohydrate thinning model shows low carbohydrate levels in the past two days, today, and the following four days, which should also ensure a good response to thinning treatments applied tomorrow and Thursday (May 5). However, it should be noted that we are not within the “perfect” thinning window yet. We typically target accumulated degree days between 200-250 DD. We got only 143 DD, which is not bad…but also not ideal. Also, based on the weather forecast, it will be rainy tomorrow until 8:00 am and windy (>10 mph) by noon, which gives you only a few hours to apply thinning treatments. The daytime temperature tomorrow and Thursday is fine for 6-BA and NAA treatments, but I would generally wait for extended periods of 75 to 85 oF for a better thinning response.

The second potential thinning window will likely be on May 10-12. As we approach there, fruit size will be around 11-12 mm, degree days will be within the perfect range (200-250), the temperature will be just perfect (based on the current forecast), but we can’t predict if the tree carbohydrate balance would remain on the deficit side by that time. We can, however, increase the rate of thinning materials to compensate for high carbohydrate levels.

So, after considering all these factors, I would focus tomorrow on thinning the hard-to-thin cultivars (e.g. Gala, Fuji, Golden Delicious, and Gold Rush) that generally require more than one thinning treatment. But, for other cultivars, I would wait for the May 10-12 window. It should be worth noting that blocks you decide to spray tomorrow can’t/shouldn’t be resprayed with thinning treatments on May 10-12. For these blocks, I would suggest that you give it 2-3 weeks at least to see the effect of the first thinning treatment and decide if you will need additional applications. For tomorrow’s thinning applications, the model recommends that you apply the standard chemical thinning rate.

The Cornell Apple Carbohydrate Thinning Model (May 3, 2022).

Finally, if you are nearby, you may need to consider attending our in-depth meeting tomorrow, May 4 at 7:00 pm at the Alson H. Smith, Jr., Agricultural Research and Extension Center (AHS AREC): 595 Laurel Grove Road, Winchester, VA 22602. I will cover critical topics regarding crop load management and return bloom, and other specialists will provide seasonal updates on disease and insect management.

The Carbohydrate Model & Thinning Recommendations-Central Virginia-May 1, 2022

Our collaborating apple growers in Central Virginia informed me that fruit sizes for most apple cultivars are now between 6-12 mm; the prime window for fruit thinning applications. I, therefore, ran the apple carbohydrate thinning model this morning to determine the optimal application timing based on the carbohydrate status, growing degree days, and thinning efficacy. As you will see below, we have already approached accumulated degree days of 235 (base temp= 4C), which is within the optimal range (200-250) for thinning applications. This range often coincides with fruit size 6-15 mm, which has always been considered the sweetest spot for fruit thinning. As for the carbohydrate status, the daily carbohydrate level is predicted to be in the deficit status for the next three days (May 2-4), and the 7-days weighted average will be at -2.49 g/day for tomorrow.

As you know, trees respond better to thinning materials when the 7-day weighted average values are in the range of -10 to -40 g/day, which is unlikely to be achieved in the following three days and therefore, the model predicts a mild response to thinning materials, and suggests increasing thinning rates by 30%. So, if you decide to apply thinning sprays tomorrow and if you usually use per acre rates of (64 fl oz of Maxcel and 1 qt of Carbaryl, and 1 pt of Regulaid/100 gal) for thinning your Gala trees, you should use (83 fl oz of Maxcel and 1.3 qt of Carbaryl and 1 pt of Regulaid/100 gal/acre) to compensate for the mild thinning action. As you may have noticed, the rate of the non-ionic surfactant remains the same per 100 gal.

My recommendations: after considering the fruit size, the carbohydrate level, and the predicted thinning action, I would seriously consider applying thinning sprays in the following three days (May 2-4); and with the forested thunderstorms on Tuesday and Wednesday, I would surely consider finishing a major portion of thinning applications tomorrow (Monday, 5/2). The temperatures in the following three days are around 80 F, perfect for NAA and 6-BA uptake and effectiveness. Also, the cloudy days on May 5-7 might reduce the carbohydrate level, at least partially, allowing for better thinning. If the current forecast is accurate, the second potential thinning window will be on Monday and Tuesday next week (May 9-10). I would use this window for cultivars currently at 6-8mm fruit diameter.

The Cornell Apple Carbohydrate Thinning Model (May 1, 2022).

Important remarks regarding the Cornell apple carbohydrate thinning model:

  • The model interface on NEWA (https://newa.cornell.edu/apple-carbohydrate-thinning) is slightly different this year, but the model inputs and outputs are the same.
  • The model collects data for temp and solar radiation from the weather station to calculate the tree’s carbohydrate daily balance and 7-day average (2 days before, the day of thinning and the 4 days after ).
  • For orchards in Central Virginia, I usually use the weather station in Tyro (Silver Creek), but I could not see this station listed on NEWA this morning, probably for a technical reason, and therefore I used the Crozet (Chiles peach orchard) instead. It would help if you used the nearest station to your location.
  • The model requires inputs for the green tip and full bloom dates. For the model I ran today, I used a green tip date of March 15 and a full bloom date of April 10 for Gala apples. If your dates are different, the outputs and recommendations will be different.

Apple Fruit Thinning: An Overview & Recommendations

After all the frost events and subfreezing temperatures in the past four weeks, we (the state of Virginia) still have a medium-heavy apple crop, and we should start planning for the fruit thinning treatments. However, given the geographical distribution of apple orchards throughout the state, which results in different developmental stages, growing degree days, elevation, etc., thinning recommendations would vary from one location to another. So, this post aims to give a general overview of thinning materials, rates, and application timing as well as a few suggestions to consider when thinning certain cultivars or using specific materials. But I will follow this with other blog posts focusing on particular locations as required.

Table 1: Common chemical thinners, tradenames, and manufacturers.

  • Notes
  • Exilis 9.5 SC contains 9.51% of 6-BA, compared to 1.9% in Maxcel and 2% in Exilis Plus.
  • There are two forms of Refine; Refine 3.5 and Refine 6.25. The later contains a higher concentration of NAA.

Table 2: Effective thinning sprays at petal fall to 5 mm fruit diameter

The application rates are based on PoMaxa for NAA, Amid-Thin for NAD and Carbaryl 4 L for Carbaryl.

Table 3. Thinning materials and rates for 6-18 mm fruit diameter

The application rates are based on PoMaxa for NAA, Maxcel for 6-BA, Amid-Thin for NAD and Carbaryl 4 L for Carbaryl.
  • Do not use NAD and NAA with Red Delicious or Fuji, as this will result in pygmy fruits. 
  • To achieve better results with NAD, use it in 100 gallons or more per acre.
  • Carbaryl can be used alone for fruit thinning between petal fall and 15 mm fruit diameter. However, it is better to combine it with either NAA or 6-BA for thinning fruits at 7-15 mm. 
  • Carbaryl is not rate-responsive when used alone, so increasing the rates of carbaryl will not necessarily improve thinning efficiency.
  • 6-BA enhances cell division and fruit size compared to other fruit thinners. Therefore, it is recommended to use it when thinning small-fruited cultivars such as Gala and Ginger Gold. 
  • Temperatures between 75 – 85 oF are ideal for 6-BA uptake and effectiveness. 6-BA is not effective when temperatures are below 68°F.
  • NAA can reduce the overall fruit size of the harvested crop compared to other chemicals used in fruit thinning.
  • NAA applications for fruit thinning can also enhance return bloom in biennial bearing cultivars, such as Honeycrisp and Golden Delicious. Research from my lab at Virginia Tech showed that NAA applications within the first 30 days of bloom are critical for flower bud formation and return bloom. NAA applications at 40 and 50 days of bloom have minimal effect on return bloom in Honeycrisp. 
  • Adding a non-ionic surfactant (e.g. Regulaid @ 1pt/100 gal) to 6-BA and NAA spray mixtures improves thinning efficiency. 
  • We use the Cornell Apple carbohydrate thinning model on the NEWA website (https://newa.cornell.edu/apple-carbohydrate-thinning) to determine the optimal timing for thinning treatments. The model collects temperature and solar radiation data from the nearest weather station to your location to predict the carbohydrate status of the tree. Under the carbohydrate deficit status, trees become more responsive to thinning treatments; and the opposite is true under the carbohydrate surplus conditions. 
  • Sunny, cool days and cold nights promote carbohydrate accumulation, resulting in low thinning efficiency; whereas cloudy, hot days, and warm nights lead to carbohydrate deficiency, which is good for fruit thinning.
  • The upper parts of the canopy are harder to thin than the lower parts.
  • If you have a heavy crop, thinning will be easier than having a light crop.
  • Vigorous trees are harder to thin compared to compact and dwarf trees. 

Table 4: Effective thinning treatments at 16 mm-25 mm fruit diameter.

  • Ethephon should be applied as a “rescue thinning” treatment if first thinning sprays were insufficient. It is most commonly used when fruit size is between 18 and 26 mm.
  • When aggressive thinning is needed, mix carbaryl or NAA with ethephon.
  • Ethephon can result in severe over-thinning, particularly at high temperatures (> 90 oF).
  • Accede, a new thinning product from Valent USA, can also be used at this stage. For more information about Accede, read our blog post: https://blogs.ext.vt.edu/tree-fruit-horticulture/?s=Accede

Table 5: Easy, moderate, and hard to thin apple cultivars.


Application rates mentioned in this article are based upon 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 that you are using. The degree of thinning action are listed according to the author’s personal field experiences. The degree of thinning action may vary from orchard to orchard and block to block.

For more information regarding apple fruit thinning, you can read our extension pub “Crop Load Management in Commercial Apple Orchards: Chemical Fruit Thinning” at https://www.pubs.ext.vt.edu/SPES/SPES-134P/SPES-134P.html.

Another, probably less devastating, frost event is expected tomorrow, April 18

Another frost/freeze event is expected from midnight today to 9:00 am tomorrow (Monday, April 18), with temp as low as 30 oF in some portions of central, northern, and northwest Virginia. On their webpage, the National Weather Service stated that “this frost and freeze conditions will kill crops, other sensitive vegetation and possibly damage unprotected outdoor plumbing”. I can’t comment about plumbing, but I can probably provide insights into how this frost event would impact apples, peaches, and apples in the state. In our location in Winchester, VA, most apple cultivars, including Honeycrisp, Gala, Fuji, Reds, and Goldens, are between 20%- 90% bloom. Some early cultivars, e.g., Pink Lady and Zestar, are already post-bloom. For cultivars at early bloom and full bloom, 10% kill happens at temperatures below 28 oF for > 30 min; and 90% kill happens if the temp drops below 25 oF for >30 min. Based on the current forecast, the lowest temp in most of the locations I looked at in Frederick, Rappahannock/Maddison, Roanoke, Nelson, and Carroll counties, is 30 oF, which should not, theoretically, cause any significant damage to apple blossoms. For cultivars at post-bloom and petal fall, subfreezing temp (below 32 oF) can cause damage to the skin of developing fruitlets, leading to “frost rings”. Fruits with a frost ring will likely grow normally until harvest, but blossoms damaged by the frost will likely abscise within the next 2-4 weeks. Temperatures above 28 oF should not cause significant damage to flowers and developing fruits of peaches and sweet cherries.

Early season applications to improve fruit size and control bitter pit in apple. 

Prohexadione calcium to reduce bitter pit incidence: 

– Shoot growth reduction by prohexadione calcium (PC) early in the season (at Pink) can significantly reduce bitter pit incidence in ‘Honeycrisp’ apples.

– PC applications at pink also reduce the incidence of shoot/fire blight.

  • Use at the rate of 6 oz/acre.
    • PC should not be mixed with calcium or boron.
    • Adding a non-ionic surfactant (e.g., Regulaid or LI700) to the tank increases the efficiency of PC applications. 
    • If the water source contains high levels of calcium carbonate (hard water), add spray-grad ammonium sulfate (AMS) to the spray tank at a rate of (1 lb AMS to 1 lb PC) or according to the product label.
    • Don’t spray if rain is expected within 8h of application.
    • Both Kudos and Apogee are PC products labeled for use at the pink stage.

Calcium applications:

– Between 15-50 lb of calcium chloride (CaCl2) should be added per acre per year to reduce bitter pit incidence. In other words, 2-8 pound/cover spray.

  • Early season applications of calcium (starting @Pink) are more critical than late-season applications.
    • Foliar applications of Ca nitrate are not recommended for Delicious and York. They might cause cork-spot-like symptoms. 
    • Avoid CaCl2 applications at temperatures above 75 oF or under slow drying conditions (e.g., early morning) as this might cause damage to the foliage, especially in sensitive apple cultivars (e.g., Idared and Golden Delicious).  
    • Soil applications of Ca are not efficient.
    • Boron deficiency may reduce Ca movement in the tree.

Boron applications:

– Boron is an essential element for flower development and fruit set, and boron deficiency can negatively affect yield and fruit size.

– Boron is also crucial for Ca movement, and Boron deficiency may lead to Ca-deficiency disorders, including bitter pit.  

  • Maintenance rate: 0.5-1 lb of Boron/acre (2.5 – 5 lb of Solubor/acre)
    • Apply at Pink stage, tank mixed with calcium chloride. 
      • OR, 7-10 after petal fall; or with the first cover spray.
    • Research at WA state showed a significant positive relationship between splitting in Gala and fruit boron content. So, it is recommended to use the lowest rate of boron with Gala.

Urea applications:

Foliar applications of urea at bloom (3 lb/100 gal) and at petal fall and early cover sprays (5-6 lb/100 gal) can promote cell division, especially for Gala. These applications are particularly important when king blooms, which usually give the largest fruit, are lost to frost.