CULTURAL TIP: HOW'S YOUR FRUIT SET FOR NEXT YEAR'S CROP?
In the northern growing regions there has been a considerable fruit set failure, probably caused by very cool temperatures before and during the main period of bloom. While flower development was greatly delayed, when the flowers finally opened the bloom seemed to progress normally; however fruit set remains very poor. For some groves the delay in flowering did not greatly affect fruit set but has resulted in smaller fruit than normal as the fruit are younger than they would otherwise be at this time of year. For other groves which were healthy and well nourished the excessive growth now present on trees has become a serious problem.
Cold weather at flowering can reduce fruit set through its affect on the main pollinators, honey bees. Managed honey bees are usually the best pollinators for avocado trees, as the grower can easily introduce very large numbers of bees to the orchard. For very good pollination a large numbers of bees are needed to visit flowers for the most effective transfer of pollen. When weather conditions are cold honey bees can do a poor job of pollination as bee activity is dependent on temperature. As a general rule of thumb when the temperature is below about 63 F few bees will leave the hive and those bees that do rapidly become moribund and inactive. Under cool to cold temperatures avocado flowers will continue to open, unfortunately though the female phase becomes increasingly shorter with the flowers inclined to shed more pollen than be receptive to pollen. A shortage of bees and a lack of female phase flowers is often a recipe for a poor fruit set. Under the weather conditions experienced this spring in the northern growing regions this may be the explanation for the very poor fruit set.
The poor fruit set has left growers with a serious problem of trees that are now growing very excessively, tempting the avocado grower to prune the trees back hard. After all there is no fruit on the trees. Unfortunately, pruning the trees back now when they are growing strongly will not cause the trees to stop growing, but actually have the opposite effect and cause the trees to grow taller and longer. Without knowing the exact circumstances of the trees it is difficult to give advice specific to the grove, however, in general, where trees are likely to grow very strongly due to a failed fruit set, the grower should fertilize the trees lightly, if at all, and limit irrigation without overly drought stressing the trees. Pulling or bending branches down so the growth is more horizontal than vertical, can sometimes be useful but is often not practical due to the large amount of labor needed. Girdling at the right time may be the best option for very healthy trees to force the strongly growing shoots into flowering mode for the next year. Once the trees have fruit on them again a judicious pruning of the tree can be undertaken after the fruit have been harvested to begin the process of reducing tree size. All-in-all recovering the trees to a more acceptable size while maintaining good production in a conventional management system could take more than two years.
