Bloom thinning apples: How the pollen tube growth model works

 Article written by Madeline Wimmer, UMN Extension Fruit Production Educator.

Images: King blossoms are the first apple blossoms to open and generally produce the highest quality fruit. Their early timing is key to bloom thinning strategies that rely on pollen tube growth models. Photos taken at ApplesRus in Olmsted County, Minnesota (Zone 5a). 

Apples benefit from fruit thinning.

The first step to managing an apple’s crop load each year starts with dormant pruning. While pruning helps to reduce the total number of fruiting spurs, apples still produce more blossom/fruits per fruit spur than is ideal. This is in part because densely clustered apples increase the chance for development of fruit rot later as moisture can become trapped between large apples. Apples with shorter stems can additionally put pressure on one another, leading to increased drops before harvest.

Biennial bearing is another condition some apple trees are also prone to that can be relieved with thinning practices. Biennial bearing happens when an apple variety is sensitive to producing an overcrop, and sets dramatically less fruiting buds for the following year (i.e., low bloom return). Thus, thinning can improve an apple crop for the current and following growing season. Goals for apple thinning include deciding how many apples are best for a given variety/block, or other unit, with the additional goal to thin each blossom cluster to one apple and spacing about 4-6 inches between the remaining fruits.

Chemical vs mechanical, or hand thinning

Prior to the development of chemical thinning methods, hand thinning and natural-occurring  thinning due to spring frosts and other external factors were the primary influence on apple crop load at bloom, or shortly after. Keep in mind that apples do naturally drop apples during an event often referred to as “June-drop.” Because these natural drops are based on a competition for nutrients between fruits, it can be less pronounced if the crop load has already been reduced by this time. 

While hand thinning is one option to manage fruit set and reduce the current season’s crop load, it is often time-consuming, cannot be completed uniformly, and can have less impact on biennial bearing varieties when done after fruit buds have initiated. However, it can work well and is the only option for some orchards. It's also a good practice to remove less visually desirable fruits.

How does chemical thinning work and when can it be done?

Chemical thinning works by either damaging parts of the flower, causing general stress to the tree, or mimicking physiological signaling to induce fruit abortion. Chemical thinning can be done during bloom—usually after a set number of king flowers have opened for a given amount of time and estimated to be pollinated—or just shortly after fruit set. The process for chemical thinning at bloom is different from after petal fall based on which products can be used, and the method for estimating when to apply a given product. This article will focus on bloom thinning, but information is out there for growers who are interested in thinning post petal fall at a given fruitlet size.

Chemical thinning products.

Products used to thin apples at bloom are classified into caustic thinners, like lime sulfur and ammonium thiosulfate (ATS), or hormone thinners (i.e., plant growth regulators). 

Caustic thinners:

Thinners in this category damage female parts of the apple flower, and will generally have more dramatic effects on thinning than hormone thinners. Some caustic thinners are allowable under USDA Organic regulations, which opens the door for organically managed orchards.

Hormone thinners:

Hormone thinners include products like 6-BA, NAA, and NAD. Most hormonal thinners are known for having a weaker effect on thinning, and increase in effectiveness with warmer temperatures. This is helpful to keep in mind as warm temperatures above 65 F are in the forecast for the next week as many orchards enter bloom in southern parts of Minnesota.

For more information about chemical thinning products, go to page 44 of the Midwest Fruit Pest Management Guide. 

Pollen tube growth models for bloom thinning.

Bloom thinning is a method of reducing apple crop load during flowering, using specific thinning products that disrupt pollination in flowers that are not needed for this year’s crop. Growers who plan to use bloom thinning as their primary thinning method will benefit from following a pollen tube growth model like the model presented by Cornell’s Network for Environment and Weather Applications (NEWA). This type of model is also suitable for orchards that produce Honeycrisp, for which biennial bearing can be an issue and early thinning can be beneficial. 

Below is more information about what pollen tube models are, and how they work. Bear in mind that these models are helpful and can still have limitations, including the limited number of varieties that can be entered into the model, and the assumptions the model makes based on information it receives from an orchard.   

“What is a pollen tube?” you might be asking.

Image: Understanding the anatomy of an apple flower is helpful in learning about how the pollen tube model works. Apple flowers have both male and female parts, but cannot be fertilized by their own pollen and require cross pollination from other apple varieties. The amount of time it takes for pollen to reach the ovules depends on the length of the style and temperature. Image from the University of Missouri Extension website. 

Pollination occurs in apples when the pollen of one apple variety lands on top of the female flower part (i.e., stigma) of a second apple variety. After the pollen granule lands, it begins to develop the pollen tube, which grows down the female flower part (i.e., style) to an ovule, which is then fertilized to become an apple seed. How fast it takes for pollination to complete depends on how long the road is to the ovules (i.e., how long the style is), as well as how warm external temperatures are, where pollination happens faster with warmer temperatures (typically above 60-65° F). 

When apple blossoms transition from pink to bloom, the king flower is generally the first to unfold and is known for producing the highest quality fruit. It then stays open by itself for a while before the other blossoms open. This delay in other flowers opening can be taken advantage of, as its pollination will also finish before other flowers. The process of bloom thinning waits for king bloom pollination (or a set number of king blooms) to finish pollination, and then inhibits pollination for other flowers in a pre- or mid-pollination state. 

Images: The length of the female flower part, known as the style, can be measured at bloom from a number of representative blossoms, with a final average style length calculated.

Steps needed for entering information into a pollen tube growth model include: 

1. Estimate your desired crop load as the number of king blossoms (i.e., the first opening blossom) per tree. 
2. When king blossoms begin to open, measure the length of the style from the stigma (top) down to the bottom of the base of the style, just above the ovary, using either a caliper or a ruler for 30-50 king flowers. 
3. Continue to track the opening of king blossoms per tree (this can be more exact for high-density orchards and roughly estimated for free-standing trees) for five representative trees in a given orchard block. 
4. When you have the desired number of open blossoms, this is when information can be entered into the model to begin estimating when pollination will complete for the open king blossoms (note: this is not when thinners are applied). 

The first thinning product can finally be applied once the pollen tube model forecasts the main pollen tube length has grown 100-110% of the measured average style length. When it hits this threshold, what it is essentially informing a grower is when the pollen has fertilized the ovules and completed pollination for the king blossoms.

Second and occasionally third sprays can be incorporated into thinning programs. This depends on a grower’s goals, how uniform other blossoms open, and how quickly the product is washed off, for instance.

Final note.

Bloom thinning is a practice that takes consideration and planning to execute, and may be a viable option for some growers, or helpful to try out on varieties prone to biennial bearing, like Honeycrisp. 

Always be sure to follow the label as law, and take caution to follow best practices to prevent issues like phototoxicity and russeting with caustic products. During years where spring frosts occur near bloom, over-thinning is possible and should be done with consideration. The 2025 growing season is generally not posing a frost risk for MN apple orchards, but future years could present frost complications. 

References and further learning:

• Bloom Thinning (YouTube Webinar; University of Wisconsin and University of Minnesota Extension)
• Apple Chemical Thinning (Washington State University)
• Pollen Tube Growth Model Developed by Virginia Tech (Cornell University)
• The Pollen Tube Growth Model: Past, Present, and Future (YouTube Webinar; Cornell University)

This information is provided for educational purposes only. Always follow product labels and consult with your crop advisor when needed.