As we move into early May, the forecast looks favorable—no frost events predicted for now. But as we know all too well, spring weather can change quickly. If frost holds off, that means we’ll need to be proactive in managing a full crop set through pruning, blossom thinning, and carefully timed chemical thinning.

I’ve previously written about setting fruit targets per tree—if you need a refresher,  here’s the link to that article Once you’ve set your crop target, the precision thinning process begins. That starts at bloom and continues through the petal fall and 12–18 mm fruit stages. Getting started early, especially with cultivars like ‘Honeycrisp’ that are prone to biennial bearing, is key.

Why Bloom Thinning Matters—Especially for Biennial Cultivars

Early thinning is particularly critical for cultivars like ‘Honeycrisp’ that have strong alternate bearing tendencies. Removing excess flowers early reduces the number of seeds that will produce gibberellins (GAs)—hormones that inhibit flower formation for next year. This is why combining good pruning and blossom thinning is such an effective strategy for these cultivars.

Bloom Thinning Options

There are two main types of chemical blossom thinners: caustic and hormonal.

1. Caustic Products

• Ammonium Thiosulfate (ATS):
  • Rate: 2.5–3%
  • Mode: Burns flower stigmas
  • Timing: Use at 60% pollen tube growth (not 100%)
• Lime Sulfur (LS) + Oil (2.5% LS and 2% oil):
  • More aggressive: Burns both stigma and style
  • Timing: Use at 80% pollen tube growth
  • Caution: May cause russeting if applied too close to petal fall, since fruit cuticles aren’t fully developed
  • IMPORTANT: Do not apply Captan 7 days before or after LS + oil; risk of phytotoxicity due to enhanced uptake from oil
  • NOTE: We don’t have a label in WI to use LS as a thinner
• Hormonal Products
• NAA (10 ppm) or NAD (50 ppm):
  • Mode: Disrupt seed development
  • Timing: Closer to full bloom, once seeds begin forming
  • Advantages: Gentle and safe, even at high rates
  • Limitation: Less effective at removing lateral blooms

Choosing Between Caustic and Hormonal Thinners

When deciding which type of blossom thinner to use, consider current and forecasted weather conditions, as well as bloom progression and bee activity. Caustic products, such as ATS and lime sulfur, are best suited for seasons with favorable conditions—that means no recent frost events or frost in the forecast, moderate temperatures between 70–80°F, good bee activity, and a steady bloom progression. These conditions allow caustic materials to work effectively without risking damage to developing fruit or compromising pollination. On the other hand, hormonal products like NAA and NAD are a better choice when conditions are less ideal—such as frost events, rapid bloom, hot weather, or poor bee flight. They’re also a good alternative if you’re concerned about potential phytotoxicity from caustic materials. Since hormonal thinners act more gently and target developing seeds rather than floral structures, they offer more flexibility under stressful conditions.

So far this year, conditions have been moderate and favorable—no frost and good bee activity. That makes caustic products a strong choice for bloom thinning.

Timing Is Everything: Use Models!

• Pollen Tube Growth Model: Critical for bloom thinning timing. Here’s a link to a previous article on how to use this model.Carbohydrate Model: Helps guide rates and thinning success. If carbohydrate balance is positive, include a surfactant to increase thinner activity. If it’s negative, skip the surfactant to avoid over-thinning.

Flower Assessment and Sprayer Setup

• Assess Flower Quality in Each Block
  • If king blooms are dominant: Use full rates and follow the carb model closely
• Sprayer Nozzle Positioning by Stage
  • Bloom thinning: Uniform coverage, small droplet size, top to bottom
  • Petal fall (110–130 GDD): 1/3 bottom, 2/3 top
  • 12 mm fruit (200–250 GDD): 2/3 top, 1/3 bottom
  • 18–20 mm fruit (300–350 GDD): Spray only the tops

Final Thoughts

We’re off to a good start this season, but don’t wait to begin your thinning program. Start with a crop target, assess flower quality, and use your models to time sprays correctly. With good planning and execution, you can set up your trees for both a successful harvest this year and strong return bloom for next.

This article was posted in Apples and tagged Amaya Atucha, Apples, bloom thinning, crop load management.