chemical thinning POMEWEST
TURNING CHEMICAL THINNING
from an art into a science
Practical lessons from the APAL Future Orchard Walk, Donnybrook, July 2026
For many apple growers, chemical thinning can feel like a bit of a gamble. One year the results are excellent, the next year the same programme seems to deliver something completely different. At the recent APAL Future Orchard Walk in Donnybrook, AgFirst consultant Jonathan Brookes challenged growers to move away from “magic” and focus on a more structured, data-driven approach to crop load management.
Words Susie Murphy-White, Project Manager, Pomewest
THE message was simple: successful thinning starts long before the spray tank is filled.
First, know what success looks like
Every block should have a clear target. Before making thinning decisions, growers need to think about:
• What fruit size does the market want?
• What yield is realistic?
• How much fruit can the canopy support?
• What are the block’s limitations, such as soil, water, heat, pests or disease?
• How much hand-thinning cost is acceptable?
A productive crop is not necessarily the biggest crop. The best financial outcome often comes from achieving the right balance between yield, fruit size and quality.
Your best guide is last year’s data
When planning for the season ahead, growers should reflect on key questions from the previous year: Did yield meet expectations, and was fruit size where it needed to be? Were bud numbers above or below target, and what were flowering conditions like?
It is also important to consider how much winter chill was accumulated, whether pollination conditions were favourable, which thinning products were used and at what rates, and how much hand thinning was required afterward. Every season leaves valuable clues, and by recording and reviewing these observations, growers can better understand the factors that influenced orchard performance. Don’t be tempted to rely on memory. The more detailed the records, the more informed future decisions become, helping to improve yield, fruit quality, and overall orchard productivity.
POMEWEST chemical thinning
Table 1. Commonly used thinning products and their best use tips.
Bloom thinning options
Ethephon
• Requires warm conditions.
• Avoid application around frost events.
• Can improve return bloom.
• May reduce tree vigour.
• Unsuitable for young trees
ATS (Ammonium Thiosulfate)
• Applied early during flowering.
• Does not require heat.
• Avoid conditions where flowers may be re-wetted after spraying.
NAA (Naphthaleneacetic acid)
• Generally applied at full bloom or shortly afterwards.
• Works best in warmer conditions.
• Can create pygmy fruit issues in some varieties such as Fuji.
Fruitlet thinning options
BA (Benzyladenine)
• Performs best in warm weather.
• Can improve fruit size and pressure in addition to thinning effects.
Metamitron
• Works well when carbohydrate production is low.
• Performs better during cloudy or shaded conditions.
• Can be aggressive if rates are too high.
Accede (1-ACC)
• Emerging option that growers may wish to trial and evaluate within their own systems.
Crop load management is a year-round process
Jonathan encouraged growers to view crop load management as a season-long process rather than a single spray application. It starts with setting targets and pruning appropriately, then assessing flowering, implementing a thinning programme, carrying out any necessary hand thinning and reviewing harvest results. Each stage provides information that can improve decisions in the following year.
The four factors that drive thinning success
AgFirst identified four key factors that determine thinning success: the tree, the weather, the chemistry being used and the quality of the spray application. Variety, tree vigour and crop load influence how a tree responds. Weather conditions such as temperature, humidity and frost risk can dramatically alter results. Product choice, application timing and spray coverage also play critical roles. Outcomes result from the interaction of all four factors rather than any one factor alone.
Why some blocks thin easily and others don’t
The discussion highlighted why some blocks are easier to thin than others. Young, vigorous trees with dense canopies and weaker pollination often respond more readily to thinning treatments. In contrast, mature orchards with strong pollination and heavy-setting varieties can be more difficult to manage. This is why a “one-size-fits-all” programme rarely delivers consistent results across an orchard.
Choosing the right tool for the job
AgFirst emphasised that growers should understand how each product works and avoid unnecessary changes simply for the sake of trying something new. Consistency and understanding generally produce better outcomes than constantly changing programmes. Below is a list of commonly used thinning products and tips on how to get the most from them.
Weather often makes the final decision
One of the strongest messages from the day was that weather frequently determines success or failure. Factors to watch include:
• Day and night temperatures
• Relative humidity
• Wind speed
• Rain forecasts
• Frost risk
• Previous winter chill accumulation
The presenters suggested growers should focus on achieving the best weather window rather than chasing an exact growth stage. If conditions are unsuitable, it may be better to switch products or alter timing rather than proceed regardless.
Keep counting and keep walking
The session concluded with a reminder that good orchard management requires regular observation. Bud counts, flower counts, fruitlet assessments and post-thinning measurements provide valuable feedback throughout the season. Walking blocks regularly and recording results allows growers to make informed adjustments and continually improve performance.
The bottom line
The message from the presentation was clear: successful chemical thinning is not luck. The best results come from setting clear targets, understanding tree physiology, monitoring weather, applying products correctly and carefully measuring outcomes.
For apple and pear growers, the opportunity is to turn chemical thinning from an unpredictable art into a repeatable management practice.
The orchardists who collect good data, stay close to their blocks and continually review results are most likely to achieve the fruit size, quality and profitability they are aiming for.
“Know what good looks like, make a plan, and keep your boots on the ground.” That was perhaps the simplest and most valuable take-home message from the day. The full presentation can be found on the APAL Future Orchards website.
More information
Future Orchards® is the apple and pear industry’s highly regarded extension program, which is delivered by APAL. The Future Orchards — adoption and innovation in apples and pears (AP25004) project is funded by Hort Innovation, using the apple and pear research and development levy and contributions from the Australian Government. Hort Innovation is the grower-owned, not-for-profit research and development corporation for Australian horticulture.
Staff & Committee
Pomewest Committee
Jason Jarvis chair e: jason@primeorganics.com.au
Murray Collins e: murray@collinsbros.com.au
Wayne Ghilarducci e: wayne@strathspey.com.au
Harvey Giblett e: harvey.giblett@newtonorchards.com.au
Mark Martella e: mwmartella@gmail.com
Mark Scott e: markpscott@bigpond.com
Nardia Stacy executive manager e: nardia@pomewest.net.au
Susie Murphy White project manager e: susan.murphy-white@dpird.wa.gov.au
Jennifer Riseley project officer e: jen.riseley@dpird.wa.gov.au
Budgeted income 2024–25
Project $
General Account Budget (including FFS, interest, project grant funding) 547,434
Biosecurity Account Budget (FFS, general account transfers & interest) 56,490
Budgeted expenditure
Project $
Grants & Subsidies 10,000
NOS Project 20,000
Quality Testing Project 32,000
Promotion and Events 28,500
Event Sponsorships 6,000
Annual Meetings and Communications 15,200
Admin, including salaries and Office Costs 372,144
APC Admin Charge@ 11% of FFS budgeted Income 46,201
Total 530,045
Budgeted expenditure biosecurity account
Project $
Codling Moth 35,500
APC Admin Charge@ 11% of FFS budgeted Income 5,441
Total 40,941
APC fee-for-service charge
Pome fruit effective from 1 January 2015
Type of fruit $/kg
Fresh fruit — apples, pears, Nashi, other 0.015
Processing fruit 0.005
Biosecurity FFS for fresh fruit 0.002
Biosecurity FFS for processing fruit 0.001