Are pecan trees in Georgia slow to develop foliage and flowers this year?

It seems pecan trees in Georgia are lagging behind this year. Well, what is going on exactly and what does this mean? The timing of pecan bud break and fruit set is determined by a combination of chill hours accumulated in the dormant season and heat units accumulated in spring.

If we have a cold winter and accumulate a high amount of chill hours, there are less heat units required to generate bud break. The warmer it is thereafter, the faster the foliage develops and the flowers form. On the other hand, if we have fewer chill hours, it will theoretically take more heat units to stimulate bud break and will also generate extreme variations in bud break within and between cultivars, which can throw off pollen shed and pistil receptivity, thus upsetting pollination.

So, what did our chilling and heat units look like this year and how does that compare to years past? Based on the University of Georgia (UGA) Weather monitoring service’s weather station located in Tifton, from mid-November to March 1 we accumulated 712 chill hours (hrs below 45 degrees F –some people use 39 degrees for pecan, but you get the general idea). Pecans generally need from 200-500 chill hours depending on cultivar, so it looks like we got plenty of chill this winter. This means it should take fewer heat units (HU) (hours above 65 degrees F) to generate bud break. However, we only received 233 HU in 2021 from March 1-April 25. While it takes fewer HUs to generate bud break, the amount we had this year lagged way behind and wasn’t quite enough for an early or really even a normal foliage progression following bud break.

Year	Chill Hours	Heat Units
2019	581	631
2020	543	353
2021	712	233

If we look at the very early bud break and crop development from last year, we had 543 chill hours and 353 heat units in 2020. In 2019 we had 581 chill hours and 631 HUs, so it would seem 2019 should have been early too, and it was a little, but it was much closer to normal than 2020. The difference was that in 2019, March was considerably cooler than 20 or 21, which delayed bud break a little. But, when bud break did occur, it warmed up fast and stayed warm throughout the spring allowing foliage development to proceed pretty rapidly. This can be observed from the following temperatures:

Year	Month	Mean High	Mean Low
Average	March	70.2	46.8
	April	77.7	53.8
2019	March	69	48
	April	77.5	55.9
2020	March	76.4	56.6
	April	77.7	55.3
2021	March	74	51.9
	April	73.4	52.2

 

Normally, once we reach mid-March our temperatures remain relatively warm with the exception of the usual cool snap around Easter. That was not the case in 2021. We had a cold winter with lots of chill hours, but our 2021 spring temperatures have remained in

consistent and relatively cool since mid March. The combination of these factors delayed bud break by perhaps a few days but it slowed the progression of foliage development after bud break considerably.

So, what does this mean for crop maturity? Well, we are off to a late start and usually when we are off to this late of a start, the crop will lag behind accordingly. This means you will likely have to spray later into the season to protect scab susceptible cultivars. Keep that in mind as the season progresses and take advantage of dry periods where you can, especially prior to nut sizing, in order to spread out the spray interval and reduce cost if possible.

– Lenny Wells, University of Georgia Cooperative Extension

Photo at top: Foliage and catkin development of Cape Fear pecan on April 1, 2021.