Apple Thinning Update (04/24/17): Henderson County, NC

— Written By and last updated by

Apple fruit set

Apple Carbohydrate Balance Model for Edneyville, NC (Lewis Creek): April 24, 2017

Using NEWA (, I ran the Apple Carbohydrate Balance Model for Edneyville (Lewis Creek). For the purpose of this article, I ran the model for only one site in Henderson County. Since there are six NEWA stations in Henderson County, I am still trying to determine the best way to present/discuss model output to all growers in the region. Please let me know if you have any suggestions. The following dates were used as inputs in the model: Green Tip Date: March 7th; Full Bloom Date: April 10th.

If you’d like to run the model on your own, the link at the NEWA site is on the drop-down menu under “Crop Management”. Choose a site nearest your orchard location and enter your Green Tip and Full Bloom date. Then hit the green Calculate button.

Model output for Edneyville, NC

The Apple Carbohydrate Balance Model was developed by researchers at Cornell University, uses temperature and light data to predict the daily carbohydrate balance of a model apple tree, and has been used to help inform thinning decisions. Apple trees are running on stored carbohydrate reserves up until around bloom, so the daily carbohydrate balance up to this time is probably not important. Bloom is when stored reserves run out, and current photosynthesis starts to play an increasingly important role in supplying the carbohydrates required for growth of spur leaves, shoots and flowers/fruit. The carbohydrate balance is not going to have a significant effect on the activity of thinning sprays at bloom or petal fall. However, from around 8 mm fruit diameter on, the carbohydrate balance will influence thinning activity.

Using the information from the Apple Carbohydrate Balance Model

This model was not developed to account for all factors that influence the efficacy of a thinner application; it was only developed to estimate the carbohydrate status of the tree. For example, the model output suggests that if you were to thin today (4/24/17), you should “Apply the standard chemical thinner rate”. I guarantee that there is not one grower in Henderson County who is out making thinner applications today. Chemical thinners are relatively ineffective with temperatures in the 50’s…. and they do not work very well when applied during a 1-2” rain event. In short, the carbon balance model is a valuable tool, but other factors need to be considered when making chemical thinning decision. The following article from Penn State Extension provides some additional commentary on the use of the carbon balance model for thinning decisions:

A positive carbohydrate balance makes it more difficult to thin whereas increasing carbohydrate deficits are predicted to increase the response to chemical thinning sprays. For example, you might respond to a predicted carboThinning Index and Recommendationshydrate surplus or mild deficit by either adopting a more aggressive approach to chemical thinning, or by waiting until more ideal conditions for thinning develop. Alternatively, if the model is predicting a severe carbohydrate deficit then you might consider reducing the rate of chemical thinner or perhaps not applying a thinner at that time for risk of over-thinning.