Chemical thinning – is it an art or a science?
Chemical thinning success involves a matrix of factors driven by varietal tendencies, block, targets, weather, the chemistry and accuracy in application. The outcome is rarely driven by the chemistry alone.
Good data and planning helps take the guesswork out of chemical thinning, making the move from luck to educated decision-making. That being said if you’re sleeping soundly over chemical thinning, you likely haven’t been brave enough!
The key to streamlined, low-stress chemical thinning is to have a clear plan and then adjust to the conditions. Understand the targets for each block and know what the triggers are that indicate you have to adjust your plan.
The sooner the crop load is brought down to optimum levels, the higher the chance of capturing the full potential of the crop (yield, fruit size, colour and quality). Getting the crop to target as early as possible also has knock-on benefits due to improved carbohydrate efficiencies and allowing for early resting sites, which in turn supports return bloom. The overall goal is to improve fruit quality, while reducing the costs of hand thinning – driving on-orchard profitability.
Targets
Understanding block-specific targets and tendencies is stage one of chemical thinning decision-making. This starts with understanding what you are trying to achieve and how this differs from previous seasons.
Understand what you’re starting with, the difference to previous seasons, and the block’s natural propensity to thin. Factors to consider include:
- bud/flower load/pollination conditions
- presence or absence of other pollen sources (cross-pollination)
- bee activity
- vigour status/rootstock
- wood texture and any shading influence
- biennial tendencies
- any other PGRs used that could affect efficacy of thinners, for example, vigour management tools.
Reflecting on last season’s chemical thinning can provide valuable insight into what you can repeat or change and improve on. Block-specific targets may include increasing canopy fill, improving fruit quality, reducing the influence of biennial bearing tendencies, or reducing labour requirements at the hand thinning stage.
When determining these season-by-season target adjustments, it is important you have a good understanding of the market requirements for your varieties, such as fruit size, internal fruit quality, and colour specifications. Keeping these market expectations in the back of your mind helps keep blocks on track to achieve profitability right from the start of the season.
To understand what is happening in your blocks, nothing beats boots on the ground. Walk, walk, walk. Where are your pockets of variability, stress factors, or areas that may respond differently at chemical thinning? Variability in tree vigour, soil moisture and floral stages can significantly impact the thinning response within a block.
History and recording
History is built through good data collection, including application and observation records. These include key dates of the varying floral and fruitlet stages, bud and cluster counts and application records.
Key dates/observations
At the very minimum collect green tip and full bloom dates for every block. Other key dates may include pink/popcorn, king bloom, full bloom, king set and petal fall. These all provide context when benchmarking timing between seasons and determining crop stage at the time of application, and provide confidence when assessing outcomes. Commenting on the strength and bloom characteristics (for example, snowball, weak, off, on, spread) also provides good context when reflecting on thinning outcomes.
Collecting key dates associated with the floral stages not only helps with chemical thinning decision-making but has downstream benefits in supporting dormancy breaker timing comparisons, harvest management, and provides good seasonal context. Bud counts and flower cluster counts can also be valuable data that will provide confidence when making the final decision.
Figure 1: King bloom open on spur sites after an even budbreak.
Application records
Recording specific information about the application itself is key to understanding the trees’ response to the ‘recipe’. This data should include:
- application date, time and weather conditions (cloudy, sunny, wind speed, daily high temperatures)
- product choice and rate
- water rates, nozzle configurations and target zone (top half, top third, top two thirds, whole tree).
Efficacy observations
Finally, documenting commentary on outcomes – whether you were happy with the results or not – provides useful context when setting your plan each season. This helps build a record of what effect a particular application had under the conditions in that season.
Collecting good information, block history and outcome observations are key to successful planning. If it ain’t broke, don’t fix it.
The chemistry
Different chemistries target different floral or crop stages. Chemical thinners are often broken into two broad categories – primary thinners and secondary thinners – which generally relates to the window in which they are applied; primary being at the blossom stage (prior to or during fruit set) and secondary (after the blossom period).
Primary thinners include active ingredients that target varying floral stages (right from popcorn/balloon, through to open flowers). Primaries are a great tool to reduce the initial number of fruiting sites.
Figure 2: Good response from primary thinner application, leaving kings set only. No secondary thinners needed.
Secondary thinners include active ingredients that actively thin fruitlets, with different chemistries targeting different fruitlet sizes. Secondaries are most commonly used to break bunches, helping establish more even fruit distribution through the tree. Across the different products the active ingredients have varying modes of action, from plant growth regulators and bloom desiccants to photosynthetic inhibitors.
Figure 3: Response from secondary thinner application: BA:NAA brew in action.
The recipe for each block may be one or a combination of multiple primary thinners, followed by one or multiple applications of secondary thinners depending on the desired thinning effect. Other combinations of thinners, such as tank mixes of benzyladenine (6-BA) (a secondary thinner) and naphthaleneacetic acid (NAA) (a primary thinner) targeted at fruit stage can increase the thinning effect when mixed together.
Product/chemistry selection should be determined by the crop stage you are wanting to thin and the weather windows you have available. Aside from being a primary or a secondary thinner, each product has different target crop stages and optimal weather requirements. Are you targeting open flowers, pink/popcorn flowers, or fruitlets? What weather conditions does the product need to be effective? If crop stage and weather windows don’t align, then adjust your plan.
Weather conditions over bloom and early fruit set are the key drivers behind product choice, and understanding product requirements is key to success. For example, some thinners work better with cloud cover/carbohydrate lows (such as metamitron, sold as Brevis® in Australia), while others require temperature. Temperature-responsive thinners include ethephon, NAA and 6-BA products. All three of these products have a significantly greater effect as temperatures increase above 18°C. They also target different floral/fruitlet stages, for example, ethephon: popcorn/balloon blossom, 6-BA: 10–12mm fruitlet). NAA is relatively complex in what it does and has a multi-use window (bloom through to 12mm); it is commonly applied at bloom stage but not restricted to that timing.
The chemical rate, water rate and combination of products also significantly impacts the thinning efficacy and outcome. Read the label and understand the individual product requirements.
To complicate things further, surfactant choice and rate must align with the product choice and can have a significant impact on the plants’ thinning response. Water quality (particularly pH) also needs to be considered when selecting chemical thinning products.
The product and its suitability to different climates can be regionally specific. Does your climate limit the use of any product, ruling it ineffective or unpredictable? The options can also be variety specific and, aside from the thinning response, it’s important to understand what other positives and negatives the chosen recipe may have on the tree. Some products have beneficial secondary impacts such as helping manage a biennial block in its ‘on’ year; it could be beneficial to use products linked to enhanced return bloom, for example, 6-BA or ethephon, if conditions permit. However, there can also be negative side effects, for example NAA can increase the risk of pygmy fruit, so avoid using this product on varieties prone to pygmy, such as Fuji and PremA129 (marketed as Dazzle™).
While a number of chemical thinning products have been registered for many years, it is an area of continual development, with metamitron (sold as Brevis) and 1-ACC (sold as Accede®) now entering the market as additional choices.
When using any product, it’s important to understand the rules around how this product ties in to your specific growing environment and how to optimise thinning outcomes.
Weather
Successful chemical thinning is largely determined by climatic conditions at the time of application. Having access to multiple weather forecasts that you can trust is critical to confident decision-making for chemical thinning. How does your site’s microclimate differ from your local weather station? It’s important to factor in these differences when confirming the suitable weather windows.
The level of stress the plant is under at the time of application also has an influence on thinning efficacy. Stress factors may include recent frost events, excessive soil moisture or even just an excessive bloom.
Wind also distorts spray coverage and speeds up drying time which determines uptake into the plant. Humidity, cloud cover and temperature at the time of application can also alter drying time and therefore result in uneven thinning outcomes. Aside from drying time and plant uptake, some products also see varied outcomes as a result of temperature in the lead up to, on the day of and in the days subsequent to application. Cloud cover can also impact carbohydrate efficiencies – all of which can impact efficacy in thinning depending on product selection.
Figure 4: A block heading into flowering period with severe water logging means that the chemical thinning plan needs to be adjusted to account for plant stress.
Bloom desiccants, such as ammonium thiosulphate (ATS), are non-temperature dependent; however, the risk of rewetting with subsequent rain showers can lead to overthinning or a phytotoxic response – understanding the weather window available is a critical success factor.
Hail nets are another variable that can influence chemical thinning outcomes as they alter the microclimate and bee activity. Nets are known to alter block humidity, temperatures and drying time as well as shading when compared to an unnetted area. Hail nets can also influence pollination conditions. All of these factors should be taken into consideration when managing the chemical thinning decision matrix.
Application
Accuracy in application is critical and the final step to conducting a successful chemical thinning program. If your product choice, rate, crop stage, weather and planning all line up, but then you fail to apply the product as planned, the outcomes can vary significantly.
Ensure sprayers are calibrated correctly (including nozzle configuration, accuracy and consistency in output) to achieve the correct water rates and hit the target zone. For many applications on large trees, the applications may only be to the top of the tree as blossom or fruitlets may drop naturally due to greater shading. Operators should have clear instructions and be supervised. You’ve got one chance to get it right!
Figure 5: It doesn’t get much better than this – kings set evenly through the tree.
Conclusion
The goal for every chemical thinning job is to achieve consistent fruit distribution through the tree and get the crop to the block target as quickly and efficiently as possible to improve fruit quality and crop outcomes. Doing this with confidence means making good decisions with the information that is in front of you.
The best outcomes come from having a plan based on your understanding of the variables that impact the thinning response. Use block targets, history and knowledge of chemistry to back up your decision-making. Match the chemistry to the crop stage and weather window. Walk your blocks, watch the weather and apply with accuracy. Be prepared to pivot if conditions change or the weather window you have planned for doesn’t present itself.
For many reading this article, you may be frustrated that we don’t discuss specific chemical rates for each variety. As this article attempts to illustrate, there are many factors that influence a chemical thinning result; each orchard manager must upskill as much as possible with how each chemistry works. Often, it’s wise to employ a consultant or agronomist to assist with the season block plan and also the pivot options required, if the weather or crop dictates.
The art of chemical thinning is one of the most critical management inputs of the year. Be prepared with your plan and be willing to execute it seven days per week to be able to target the crop stages in the optimum window.
If a program consistently delivers good results, do not change it simply for the sake of change. Successful chemical thinning is rarely about any one factor. Make a plan, be brave, but back your decisions with good data. Chemical thinning is part art and part science – the science outlines the plan, but the art is driven by experience and a grower’s ability to adapt.
Further reading
