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.
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)
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.
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).
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)
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
In the scenic Winchester/Frederick County area, apple growers are witnessing varied stages of growth across different apple cultivars. Currently, cultivars such as Fuji and Honeycrisp are at the partial petal fall stage, while Gala and Red Delicious have reached complete petal fall. Most notably, the Pink Lady cultivar has reached the critical >6 mm diameter stage, making it the ideal time for fruit thinning.
It’s important to time the application of chemical thinning agents, such as 6-BA and NAA, accurately. According to the Apple Carbohydrate Thinning Model on Cornell’s NEWA website (https://newa.cornell.edu/apple-carbohydrate-thinning), the current predictions for accumulated degree days (DD) for Pink Lady apples in Winchester suggest reaching the 200-250 DD window by Thursday, April 25, and continuing through most of the next week. While the model indicates a daily carbohydrate surplus, which isn’t ideal for thinning, a forecasted drop in carbohydrate levels by April 27 should be closely monitored. This dip, though minor, will likely influence the 6-D weighted average, which is a crucial metric for thinning effectiveness.
If you’re targeting a thinning application for cultivars like Pink Lady, which had a green tip date of March 2nd and a full bloom date of April 8, post-April 25th is advisable for starting treatments. The weather forecast suggests that April 28, April 29, and May 1 will offer optimal conditions—both temperature-wise and in terms of carbohydrate levels—for the effectiveness of 6-BA and NAA treatments.
Apple Carbohydrate Thinning Model-Weather Station (Winchester, VA), Pink Lady- Green tip (March 2), Full Bloom (April 8)
Turning our attention to the Gala cultivar, it hasn’t yet reached the 200-250DD window. However, for those keen on applying a thinning treatment at petal fall, the upcoming Sunday or Monday is recommended. Following this initial treatment, it’s advisable to wait at least 10 days before the subsequent application, ideally timed for when the cultivar enters the 200-250DD window.
Apple Carbohydrate Thinning Model-Weather Station (Winchester, VA), Gala- Green tip (March 7), Full Bloom (April 14)
In various parts of Virginia, apple orchards are currently transitioning through stages ranging from full bloom and petal fall to having fruits about 6-18 mm in size. Given this period of active growth, it’s an ideal time to discuss the crucial practice of apple fruit thinning—especially vital this year due to the heavy bloom and minimal spring frost damage. Proper thinning is essential; without it, the excess blossoms and small fruitlets from this year could negatively impact both next year’s yield and the quality of this year’s harvest. The development of next year’s flowering buds in apples is largely determined during the first four to five weeks post-full bloom. In years with heavy bloom and fruit set, like this one, failing to adequately thin can lead to too many fruits drawing on the tree’s carbohydrates. This overburden can drastically reduce the number of viable buds for the following year, potentially resulting in no crop for varieties prone to alternate bearing, such as Honeycrisp, Fuji, and Winesap.
Moreover, maintaining too many fruits on the tree can also degrade fruit size and overall quality at harvest—particularly for the Gala variety. It’s important to note that fruit size at harvest is primarily influenced by the rate of cell division and the total number of cells per fruit, rather than merely by cell expansion. This cell division occurs mainly in the initial weeks following full bloom, making it critical to reduce competition among the fruitlets so that more resources are directed to fewer fruits, thus enhancing their growth and quality.
With these points in mind, I’d like to offer some specific recommendations for thinning applications at petal fall and the 6-18 mm fruit size window (the main thinning window). At the end of this post, I will also share outputs from the Carbohydrate Thinning Model and my personal advice for apple growers in Central Virginia and other regions currently within the 6-18 mm fruit size stage. For those in the Winchester and Frederick County areas, where orchards are still transitioning between full bloom and petal fall, other posts will follow as needed.
Effective thinning sprays at petal fall to 5 mm fruit diameter.
Notes: • To prevent the formation of pygmy fruit, avoid applying NAD and NAA to Red Delicious and Fuji apples after petal fall. • When using NAD as a thinning spray, ensure that the spray volume is at least 100 gallons per acre, as lower spray volumes may decrease efficacy. • Carbaryl can be used alone as a thinning spray between petal fall and when the fruit diameter reaches 15 mm. However, it is more commonly used in combination with either NAA or 6-BA when the fruit diameter is between 7 and 15 mm. • When using carbaryl alone, increasing the rate of application will not enhance thinning activity, as carbaryl’s effectiveness is not highly responsive to changes in application rate.
Recommended chemical materials and rates for thinning fruits (6-15 mm)
Notes: • Exilis 9.5 SC contains a high concentration of 6-BA (9.51%) compared to Maxcel (1.9%) and Exilis Plus (2%). Refine is available in two concentrations: Refine 3.5 and Refine 6.25, with the latter having a higher concentration of NAA. All dosages in the table are based on the concentrations found in Maxcel (6-BA) and PoMaxa (NAA). • The recommended application rates in the table are for mature trees (6 years or older). For younger trees (4th and 5th leaf), reduce the rate by 25% to 50%. • Do not use less than 100 gallons per acre for thinning treatments. • Thinning is more difficult in the upper parts of the canopy compared to the lower parts. • Thinning is easier when the crop is heavy, as opposed to when the crop is light. • Use the carbohydrate thinning model to time your thinning applications. Alternatively, as a general rule of thumb: Thinning is more challenging when it is sunny; overcast conditions for 2 to 3 days facilitate easier thinning; thinning materials are less effective in cool weather (below 65°F) and more effective in warm weather (above 70°F).
Apple Carbohydrate Thinning Model Outputs-Pink Lady- Roseland-VA
The Carbohydrate Thinning Model uses critical environmental factors such as temperature and solar radiation to assess the carbohydrate status of trees. This status helps determine how trees will respond to chemical thinning agents. When trees are in a carbohydrate-deficient state, they are more likely to shed fruit, making them more receptive to thinning applications. Conversely, when conditions are favorable for photosynthesis—sunny and warm—trees are less likely to respond to thinning as they prefer to retain their fruits. Remember, fruits carry seeds, and seeds represent the future generation of trees—an easy concept to grasp. The model doesn’t rely on a single day’s data; instead, it averages the carbohydrate balance over six days to decide if a tree has a deficiency or surplus. This helps determine whether to apply treatments, how much response to expect from the trees, and whether to adjust the quantity of thinning materials to avoid over- or under-thinning. The model also tracks accumulated degree hours from the time of bloom, which is why knowing the exact day of full bloom is crucial. It predicts the optimal thinning period, which usually occurs between 200 to 250 degree days (DD)—coinciding nearly perfectly with the 6-18 mm fruit size window.
Based on data from the weather station in Roseland, VA, and the growth stages of Pink Lady and Gala apples, the model predicts that by Saturday, April 20, Pink Lady trees will reach about 200DD. This is the ideal time to start applying thinning materials, continuing through the 250 DD mark. However, the model also indicates a carbohydrate surplus on April 19 and the following three days. Therefore, it recommends increasing the thinning material rate by 30% above standard. Since the forecast predicts temperatures above 70°F this Saturday, which enhances the uptake and efficiency of agents like NAA and 6-BA, thinning applications are best done on that day.
Apple Carbohydrate Thinning Model-Weather Station (Roseland, VA), Pink Lady- Green tip (March 4), Full Bloom (April 6)
For Gala apples, which reached green tip on March 12 and full bloom on April 12, the 200-250DD window has not yet been reached. Nonetheless, if you wish to apply chemical thinning now using 6-BA and carbaryl to promote cell division, you can use 48 fl oz of 6-BA and 1 pt/100 gal of carbaryl. However, it’s important to wait at least 10 days to observe the effects of this treatment before proceeding with another. Based on the current weather forecast and degree days, the 6-18 mm fruit size window should still be open in 10 days if you decide on a second thinning application.
Apple Carbohydrate Thinning Model-Weather Station (Roseland, VA), Gala- Green tip (March 12), Full Bloom (April 12)
Apply 4-14 pounds of actual calcium per acre each season to help prevent bitter pit and cork spots. This amount is equivalent to applying 15-50 pounds of calcium chloride (CaCl2) or distributing 2-8 pounds of CaCl2 per cover spray.
For apple varieties that are particularly susceptible to bitter pit, such as Honeycrisp, Golden Delicious, and York Imperial, it is advisable to use the maximum recommended concentration of calcium.
Initiating calcium applications early in the season, starting at the Pink stage of bud development, is more beneficial than applications later in the season.
Foliar applications of calcium nitrate are not advised for Delicious and York apple varieties, as they can induce symptoms similar to cork spot.
In orchards with significantly low calcium levels, applying calcium sulfate (gypsum) in a band beneath the trees at a rate of 3 tons per acre can improve calcium content in the leaves and fruit. However, it may take two years or more to observe positive outcomes.
Avoid applying calcium chloride under conditions that slow drying, such as in the early morning, as it can harm the foliage. This is especially important for apple varieties sensitive to calcium chloride, including Idared and Golden Delicious.
A deficiency in boron can hinder the movement of calcium within the tree, potentially affecting fruit quality.
Boron applications:
Boron plays a crucial role in the development of flowers and the setting of fruit. A lack of boron can adversely affect both yield and the size of the fruit.
Furthermore, boron is necessary for the transportation of calcium within the plant. A deficiency in boron can lead to disorders related to calcium deficiency, such as bitter pit and cork spot in apples.
To avoid these problems, it is recommended to apply 0.5-1 pound of boron per acre, which is equivalent to applying 2.5-5 pounds of Solubor per acre.
Boron should be applied either at the pink or bloom stage of flower development, mixed in the tank with calcium chloride, or 7-10 days following the fall of the petals.
Prohexadion calcium:
Applying prohexadione calcium (PC), found in products like Kudos and Apogee, early in the season at the Pink stage can significantly reduce the incidence of bitter pit and blossom blight, as well as decrease the risk of shoot blight.
For optimal results from PC treatments, consider the following guidelines:
Apply PC at a rate of 6 ounces per acre.
PC’s effectiveness decreases in environments with high pH levels or in water with a high concentration of calcium carbonate (hard water). To counteract this, add ammonium sulfate (AMS) to the spray mixture.
Avoid combining PC with calcium or boron in the spray mix, as interactions may diminish efficacy.
Enhance the performance of PC applications by including a surfactant in the tank mix. This helps in better distribution and adherence of the product on plant surfaces.
Should the surfactant cause foaming, incorporate an anti-foaming agent to mitigate this issue.
Both Kudos and Apogee are approved for application at the Pink stage for apples, indicating a targeted timing for use.
Research by Sherif in 2019 (unpublished) suggests that applying PC concurrently with or before thinning agents like 6-BA or NAA does not compromise the effectiveness of thinning treatments.
Urea applications
Applying urea to the foliage during the bloom period (at a rate of 3 pounds per 100 gallons of water) and subsequently at petal fall and the first cover spray (5-6 pounds per 100 gallons of water) can significantly support cell division. This practice is especially beneficial for Gala apples. Such applications become critical when the primary, or king blooms, which typically produce the largest fruits, are lost to frost damage.
The foliar application of urea at the time of bloom can also improve fruit set by prolonging the period of effective pollination, enhancing the chances of fertilization and subsequent fruit development.
It’s important to note that foliar-applied nitrogen, while beneficial for fruit set and sizing, does not substitute for soil-applied nitrogen fertilizers. Unlike ground applications, foliar-applied nitrogen does not move into the tree’s woody structure but is instead directly utilized for the development of the fruit.
Fruit Russeting in Golden Delicious and Scarf Skin in Gala:
Fruit russeting and scarf skin in apples is often associated with high humidity during the first 30-40 days of fruit development.
To address this issue, apply 2-4 applications of GA4+7 (such as ProVide 10SG or Novagib 10L), starting from petal fall and continuing at 7-10 day intervals.
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
