by Joe Traynor, 12/1/11

It’s a given among almond growers: the weather during bloom determines the almond crop; poor weather, poor crop, good weather, good crop. How then, to explain the record 2011 almond crop: a phenomenal all-time record of 2600#/acre state average!?

For the 2011 almond bloom, Bee Hours (the number of hours with temperatures above 50 degrees, wind less than 15 mph and no rain) was the lowest in years. Growers hunkered down for a poor nut set, yet by April the trees were loaded with nuts. Why?

One explanation has been that mild weather during the previous two growing seasons allowed trees to build up carbohydrate reserves and these stored carbohydrates greatly increased retention of pollinated nuts. Sounds plausible, but I’d like to see some data.

A better explanation, offered by many, is that a strung-out bloom extended the time necessary for bees to complete the pollination job – the transfer of pollen from one variety to another. This explanation certainly makes sense. A strung-out bloom also provides another major benefit: better nut retention after pollination.

Fertilized ovules from newly pollinated almond flowers undergo an intense period of extremely rapid growth immediately after petal fall. When bloom is compressed, the number of nutlets expanding simultaneously puts an overwhelming demand on tree reserves. Trees are unable to feed all their “babies” at the same time. Under such conditions, pollinated nutlets abort (drop to the ground) to relieve the drain on tree reserves, and to allow at least some of the crop to reach maturity. Peach trees, closely related to almonds, incur a similar drop of pollinated flowers after petal fall, but peach growers welcome this drop as it reduces their thinning costs.

Almond growers like to see at least 50% of their almond flowers set a nut, a figure necessary to attain 3000#+/acre yields. An extended bloom makes this goal more attainable by providing bees more time to do their job and by providing trees a better opportunity to retain more nuts. Growers should certainly hope for Ideal bee weather during almond bloom, but if the blooming period is compressed, even perfect bee weather may not offset excessive post-bloom nut drop caused by too many nutlets growing too fast at the same time. When the blooming period is compressed, ideal bee weather cannot compensate for excessive post-pollination nutlet drop.

Winter chilling – the number of hours below 45 degrees – determines the length of the blooming period; whether the bloom will be compressed or strung-out. Cold winters give a compressed bloom, warm winters an extended bloom. Almonds have the lowest chilling requirement of any deciduous tree crop – 250 to 300 hours, compared to over 800 hours for pistachios and over 1,000 hours for cherries. Recorded chilling hours were more than ample for almonds during the winter of 2010-2011, so why was the bloom strung-out?

Unfortunately, chilling hours, as currently determined by most, don’t account for fog (or lack of fog). Branch (or bud) temperatures on a 50 degree day with no fog can be 20 degrees higher than branch temperatures on a foggy day. (Cherry and pistachio growers should consider white-washing their trees during the winter to minimize limb warming on sunny days). Sunny winter days offset the chilling hours accumulated at night and during cold, foggy days. December-January weather this past winter was noticeable for a minimal number of foggy days. A strung-out bloom was the result.

Unlike cherry or pistachio growers, almond growers should hope for a winter with below normal chilling, and for only enough foggy days to allow for efficient shaking of mummies. Almond growers should not fret if chilling hours are below normal; they should fret if they are above normal. Perhaps almond growers should also embrace global warming.

This article appeared in the December 2011 issue of Pacific Nut Producer