Thinning Update: Week of May 6 – Winchester and Central Virginia

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If you haven’t applied any thinning treatments yet and are planning to start this week, I’ve run the carbohydrate thinning model for both Winchester and Central Virginia to guide your decisions.

For Winchester/Frederick County:
Last week offered ideal thinning conditions—fruit size ranged from 6–15 mm, the carbohydrate model indicated a deficit (favorable for thinning), and the temperatures were warm enough for effective 6-BA and NAA activity. If you applied thinning sprays last week, you’re in great shape—no further action is needed this week or until at least next Monday. Keep in mind that it usually takes 10–14 days under current weather conditions to evaluate thinning response. If you didn’t thin last week and want to start now, here are your options:

Before applying anything, I recommend closely inspecting your trees and gently shaking some branches to check for natural fruit drop. If you still believe thinning is needed, don’t delay—waiting until next week may be too late.

Cornell Apple Carbohydrate Thinning Model for Winchester, VA. Green tip date is March 16, and full-bloom date is April 12.

For Central Virginia:
The same guidance applies. If you applied thinning sprays last week, no action is needed this week. If not, refer to the thinning recommendation table below for this week’s guidance.

Cornell Apple Carbohydrate Thinning Model for Roseland, VA. Green tip date is March 14, and full-bloom date is April 10.

Ideal Conditions for Apple Fruit Thinning This Week: Model Results for Winchester and Central Virginia

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As you know, despite the freeze damage we experienced on April 9, several apple cultivars across multiple locations are still carrying at least a few times more fruit than desired. Chemical fruit thinning remains our best option before it’s too late and we are left with the laborious task of hand thinning. The best timing for applying 6-BA and NAA for fruit thinning is during the 6–18 mm fruit size window, and we are currently within that window for ‘Gala’ and most other apple cultivars in the Winchester/Frederick County area. To help guide thinning applications and ensure trees are responsive to treatments, I ran the Cornell Apple Carbohydrate Thinning Model (https://newa.cornell.edu/apple-carbohydrate-thinning). Based on key parameters, the model helps determine both the timing and rate of thinning applications. Using data from the weather station located at our research center in Winchester, and with a green tip date of March 16 and a full bloom date of April 12 for ‘Gala’, the model predicts that starting tomorrow—Tuesday, April 29—and continuing through the end of the week, conditions will be ideal for thinning treatments. The trees’ accumulated carbohydrate balance is generally negative, which is exactly what we want for effective thinning, and temperatures—especially on Wednesday and Friday—will be perfect for the performance of 6-BA and NAA. The model also recommends using the standard rate for thinning materials.

Cornell Apple Carbohydrate Thinning Model for Winchester, VA. Green tip date is March 16, and full-bloom date is April 12.

For growers in Central Virginia, I also ran the model using the Roseland, VA weather station, with ‘Gala’ green tip and full bloom dates set as March 14 and April 10, respectively. As shown below, if you haven’t already applied your thinning sprays, we are still within the optimal thinning window. The model suggests that if you apply tomorrow (April 29), you should increase the thinning rate by 30%. However, based on the daily carbohydrate balance, I believe that applications made this Wednesday and Thursday will likely require only the standard rate.

Cornell Apple Carbohydrate Thinning Model for Roseland, VA. Green tip date is March 14, and full-bloom date is April 10.

Apple Carbohydrate Thinning Model Outputs – Central Virginia – Roseland, VA

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The Apple Carbohydrate Thinning Model uses key environmental variables—primarily temperature and solar radiation—to estimate the carbohydrate status of apple trees. This status helps predict how responsive trees will be to chemical thinning agents. When trees are carbohydrate-deficient, they are more likely to shed fruit, making them more responsive to thinning. Conversely, when conditions are favorable for photosynthesis—sunny and warm—trees tend to retain their fruit and respond less to thinning applications.

Importantly, the model does not rely on a single day’s data. Instead, it calculates a six-day weighted average of carbohydrate balance to determine whether the tree is in a state of surplus or deficit. This running average guides decisions on whether thinning materials should be applied, what kind of response to expect, and how much product to use to avoid over- or under-thinning. You can refer to Column 6 in the table below to view the six-day average.

In addition, the model tracks accumulated degree days (DD) from full bloom, which is why recording the exact date of full bloom is essential. The optimal thinning window usually occurs between 200 and 250 DD—coinciding closely with the 6–18 mm fruit size window. See Column 7 in the table below for the current degree-day accumulation.

Based on weather data from the Roseland, VA station and the growth stage of ‘Gala’ apples, the model projects that trees will reach approximately 200 DD by Saturday, April 26. This signals the start of the ideal thinning window, which will extend through the 250 DD threshold. With forecasted temperatures staying above 75°F from Saturday through Wednesday, conditions are favorable for effective uptake of thinning agents such as NAA and 6-BA.

However, the model also indicates a carbohydrate surplus from April 26 through at least the next three days. As a result, it recommends increasing the thinning material rate by 30% above standard. You can find these suggested rates in the last column of the table below. Under surplus conditions, the risk of over-thinning or excessive natural fruit drop is reduced. Still, conditions may shift by Tuesday or Wednesday, so rate adjustments might be necessary.

I’ll re-run the model early next week and share updated guidance. In the meantime, you can run the model yourself at: https://newa.cornell.edu/apple-carbohydrate-thinning
This will allow you to select the closest weather station and enter your specific green tip and full bloom dates for a more localized prediction.

Bottom line: The thinning window begins this Saturday and will continue through the coming week. Plan your applications accordingly.

Apple Carbohydrate Thinning Model-Weather Station (Roseland, VA), Gala- Green tip (March 14), Full Bloom (April 10)

Rescuing Apple Crops After a Frost or Hailstorm: Can 6-BA + GA Treatments Help?

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One way growers have managed to salvage part of their crop in years when a hard freeze or hailstorm hits during apple bloom is by applying a combination of plant growth regulators—specifically 6-benzyladenine (6-BA) and gibberellins GA₄+₇—found in products like Promalin® and Perlan®. These products essentially mimic the natural hormones produced by developing seeds after successful fertilization. By applying them during bloom or shortly after a frost or hail event, it’s possible to induce parthenocarpic fruit set—a process in which fruit develops without fertilization, resulting in seedless apples.

How It Works: When fertilization doesn’t occur—due to frost injury, hail damage, or disrupted pollination—flowers typically abscise. But applying GA₄+₇ and 6-BA can supplement those missing signals. Gibberellins stimulate fruit cell enlargement, and cytokinins like 6-BA promote cell division. Together, they initiate fruit development in flowers that would otherwise abort. This treatment has shown success across many trials and real-world orchard scenarios, with growers reporting the ability to recover 20–30% of a normal crop. While that might not seem like much in a bumper crop year, it can make a huge difference in disaster years—sometimes being the difference between breaking even and facing a total loss.

For best results, the sprays should be applied as soon as possible after the damage, ideally within 24 to 48 hours. There’s still a chance of success if applied up to 4–6 days later, but the earlier the application, the better the outcome. It’s important to remember that completely dead flowers cannot be revived, but if the blooms are partially damaged and still viable, there’s a good chance they will respond. The optimal timing is from bloom through petal fall. Later applications, once fruitlets are already forming, are far less effective. Conditions at and following application also matter—moderate temperatures and no immediate refreeze improve absorption and success.

Managing Expectations: While the benefits are real, it’s important to manage expectations:

  • Results vary by cultivar, timing, extent of damage, and orchard conditions.
  • The treatment won’t help if all flowers are completely destroyed.
  • Parthenocarpic fruits are usually of good eating quality, but they may be slightly smaller, more elongated, and are sometimes more prone to dropping if they’re growing alongside seeded fruit.

Nonetheless, when applied under the right conditions, GA₄+₇ + 6-BA sprays have repeatedly increased fruit set and overall yield, even in the absence of pollination. The resulting apples typically retain their flavor, sweetness, and firmness. In most cases, there’s no downside in terms of market quality, especially when crop load is light.

In addition to PGRs, applying low-biuret urea at a rate of 3 lb/acre during bloom can also help improve fruit set, particularly under stressful conditions. Urea serves as a readily available nitrogen source, supporting early cell division and metabolic activity in developing fruitlets. When used in conjunction with GA₄+₇ and 6-BA, it can enhance the fruit set response and overall effectiveness of frost recovery treatments.

Severe Frost Damage Hits Early Bloom: A Look at the Impact on Apple and Stone Fruit Blossoms

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I’m sure many of you have noticed how inaccurate the weather forecast was over the past 24 hours. While the minimum temperature was predicted to be 27°F, we recorded temperatures below 24°F for over an hour early this morning (4/9) at the ASH Jr. AREC research farm in Winchester. Unfortunately, this unexpected dip resulted in severe damage to both our apple and stone fruit crops. The extent of injury far exceeds what anyone might have anticipated based on the forecasted temperature profiles from Monday and Tuesday. Following the freeze event we witnessed this morning, my team collected flower samples from multiple cultivars and species across the farm. Below is a summary of our findings:

In stone fruits, Red Haven peaches showed 66% flower mortality, while SunHigh peaches experienced complete (100%) loss. Cherries were also heavily impacted, with 64% of flowers dead. Among apples, Pink Lady suffered total flower loss—100% mortality of both king and lateral flowers. ‘Honeycrisp’ on MM.111 rootstock showed the most severe damage, with 92.5% king and 87.2% lateral flower mortality. On B.9 rootstock, ‘Honeycrisp’ fared slightly better but still showed high injury levels (90% king and 47% lateral mortality). ‘Gala’ trees showed 75% mortality in king flowers and 47% in lateral flowers, while ‘Fuji’ was the least affected—likely due to being slightly behind in development—with only 18% and 15% mortality, respectively. ‘Red Delicious’ experienced consistent and heavy damage, with 100% king flower mortality and lateral flower losses ranging from 75% to 85%, averaging around 81%.

Representative flower buds of cherries, peaches, and apples (top to bottom), showing live (left) and dead (right) flowers. These cross-sections were taken from trees at the AHS Jr. AREC research farm following a brief but intense freeze event on April 9, 2025.

I strongly encourage all growers to carefully assess their own orchards—don’t rely solely on visual inspection from the outside. I was surprised to find that several peach flowers appeared normal until I dissected them, only to find the ovaries completely dead. Keep in mind, your apple thinning decisions will hinge heavily on the extent of damage caused by this freeze. I’ve attached several images (see above) to help illustrate what we’re considering alive versus dead when evaluating flower buds.

Spring Freeze on Wednesday Morning: Possible, But Likely Not Too Severe

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It’s that time of year when we all stay on our toes, watching closely for any spring freeze that might harm our delicate pome and stone fruit blossoms. So far, flower development has been progressing slowly but steadily, and everything looks fine. This year is not shaping up to be a heavy bloom year for apple orchards—especially in Frederick County and, to some extent, in southwest Virginia, though for two entirely different reasons.

In Frederick County, I predicted last year that flower development for 2025 would likely be suboptimal. This is due to a severe carbohydrate deficit we experienced around the time of flower bud initiation (mid-April to the first 10 days of May 2024). That deficit not only contributed to fruit abscission last season but also appears to have impacted flower bud development for this year. So if you’re seeing fewer blooms, it’s not your fault. In Southwest Virginia, on the other hand, any reduction in bloom is more likely tied to the secondary bloom that occurred last fall following Hurricane Helene. I don’t expect this to significantly affect all crops or locations, but if you noticed extensive secondary bloom in your blocks last fall, those trees will likely show reduced bloom this spring.

Now, regarding the upcoming frost: I remain optimistic that things will turn out just fine, even with the predicted freeze on Wednesday morning. This will be a typical radiative freeze—clear skies, low wind—which causes cold air to settle near the ground and warmer air to rise. The lowest temperatures are expected right before sunrise, and once the sun is up, things should return to normal quickly. Here in the Winchester area, the forecast shows a low of 26°F lasting for about one hour. Other areas, such as Rappahannock, Madison, and Nelson counties, are projected to reach 28–29°F.

Is that critical? Not really. Even if apples, peaches, or cherries are at full bloom, such a brief dip will likely cause only about 10% blossom damage. That level of damage is often considered a natural and even beneficial thinning. In fact, such frost events can act as a signal to the tree to initiate more flower buds for the following season, helping ensure a strong bloom next year. So—no major concerns, unless you observe temperatures that fall within the critical ranges capable of killing 90% or more of your flowers (refer to the critical temperature table https://irp.cdn-website.com/45c0e18f/files/uploaded/Critical_Temperatures_Frost_Damage_Fruit_Trees_Utah.pdf

Should you turn on wind machines or irrigation (under-tree or over-tree sprinklers)?
That’s your call. Personally, I wouldn’t, as long as temperatures stay within the forecasted range. But if you want to play it safe, it wouldn’t hurt—especially in low-elevation or frost pocket areas where these tools can truly make a difference. Since it’s a radiative freeze, both wind machines and sprinklers can be effective. Let’s hope for the best and I look forward to seeing you all at our upcoming orchard meetings!

Pre-Bloom & Bloom Nutrition and Growth Management for Apple Orchards

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It’s a beautiful day out there, so let’s enjoy it as it is and stay positive—no need to keep checking the weather station just yet! Since most of our apple cultivars are between tight cluster and full bloom, I wanted to send a quick reminder about some important considerations for early-season nutrient and growth regulator applications during this critical stage.

Pre-Bloom Foliar Nutrient Sprays: The Nutrient Cocktail

A common and effective approach during the tight cluster to pink bud stage is to apply a foliar “nutrient cocktail” of nitrogen, boron, and zinc (if possible).
Nitrogen (Urea)
A foliar urea spray can boost nitrogen levels in developing buds and spur leaves. This helps strengthen bloom and improve early fruit set.
• Apply a low-biuret urea at 3 lb per 100 gallons of water (dilute spray). Make sure the biuret content is below 0.25% to avoid leaf damage.
• Nitrogen is vital for rapidly expanding tissues. Pre-bloom applications support both flower and shoot development.
Boron
If your boron levels were low last season or you’re looking to support flower fertility, now is the time.
• Apply a spray-grade boron product (e.g., Solubor®) at 1 lb per 100 gal (~0.2 lb actual B).
• Boron is essential for pollen viability and pollen tube growth. Even mild deficiencies can reduce fruit set.
Zinc
Zinc deficiency is fairly common, especially in certain soils and older blocks.
• Include zinc EDTA chelate at label rate in your foliar spray. Avoid zinc sulfate at this timing if frost or oil applications are nearby, as it can cause tissue damage.
• Zinc plays a key role in enzyme activation and pollen formation. Early sprays ensure flowers and young leaves develop properly.
Pro Tip: Tank-mix this nutrient spray with early-season fungicides but always check for compatibility.

Bloom-Time Nutrient Applications

Boron at Bloom
A repeat boron application at pink or full bloom supports fertilization and early fruit development.
• Apply 2.5–5 lb/acre of Solubor (equivalent to 0.5–1.0 lb of actual B).
• This timing helps secure pollen germination and supports calcium movement into developing fruit. Just be careful not to overdose.
Early Calcium Sprays
Calcium uptake is critical in the early weeks after bloom, especially for sensitive varieties like Honeycrisp.
• What to do: Begin foliar calcium applications at pink or early bloom, and continue through the early cover sprays.
• Rates: Use 2–4 lb of calcium chloride (CaCl₂) per 100 gallons of spray. Include a non-ionic surfactant and apply during fast-drying conditions to avoid leaf burn.
• Why it matters: Calcium builds strong fruit cells and reduces the risk of bitter pit and other disorders. Early sprays are particularly effective.
• Spray safety: Avoid mixing calcium sprays with Apogee or Kudos.
Foliar Urea at Bloom
Foliar nitrogen can give your fruit set a helpful boost during bloom, especially if your trees are weak or suffered frost damage.
• What to do: Apply 3 lb of urea per 100 gal at bloom, and 5–6 lb per 100 gal at petal fall.
• Why it matters: Urea helps prolong flower viability and encourages strong fruit development. It supports cell division in young fruit, improving size at harvest.
• Caution: Avoid using foliar urea beyond the second cover spray — late-season nitrogen can hurt color development.
Prohexadione-Calcium (Apogee® or Kudos®): Managing Tree Growth
When applied at the right time, prohexadione-calcium helps you control excessive shoot growth, open up your canopy, and even reduce fire blight risk — all while improving fruit calcium allocation.
Application Timing
• When to apply: At the pink bud stage, or early bloom at the latest, when shoots are 1–3 inches long.
• Split plan: Apply 6 oz/acre at pink, and repeat in 2–3 weeks (e.g., at petal fall).
Mixing Tips
• If your water is hard, add 1–2 lb of ammonium sulfate (AMS) per 100 gal to condition it.
• Add a nonionic surfactant to improve uptake (follow label rates).
• Do not mix Apogee with calcium or boron — these reduce its effectiveness.
Honeycrisp Consideration
Apogee is especially valuable on Honeycrisp. It helps reduce the competition for calcium, lowering the risk of bitter pit. Just be sure there’s enough leaf tissue present before spraying to ensure good absorption.

More About Apple Chemical Thinning-May 2024

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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

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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)

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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://treefruithorticulturevaes.wp.prod.es.cloud.vt.edu/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