Is The Weather Causing Fruit Drop and Excessive Seeds On Maple Trees?
GARDEN TIPS – written by
Marianne C. Ophardt
WSU Extension Faculty
for the Tri-City Herald, Kennewick, WA Written June 25, 2015
Earlier this month some gardeners noticed that a number of small apples were dropping off their trees and wondered why it was happening. There are three different types of “fruit-drop” that backyard orchardists may observe.
Fruit drop very early in the season is linked to pollination. Apples and pears require cross-pollination for fruit to form. Cross-pollination is the exchange of pollen from the flowers of one variety to the flowers of a different variety of the same type of fruit.
Cross-pollination is needed for fruit development to occur in many tree fruit, like apples and pears. If adequate cross-pollination does not occur, fruit may start to develop but then drop from the tree. This typically happens soon after the flower petals drop. It occurs because there are not enough viable seed within the fruit producing the plant growth regulating chemicals needed for fruit development.
Lack of pollination can be the result of not having a compatible variety nearby to enable cross-pollination, frost, a deficit of bees and other pollinators, or weather conditions that deter bee activity during bloom, such as rain or strong winds.
Apple or pear fruit may also be observed dropping in early summer. This is called “June-drop.” The drop is usually due to the crop load of the tree. June-drop is a way for the tree to thin itself because it can not support all the fruit that were pollinated and developing on the tree.
This self-thinning allows more of the tree’s carbohydrate resources to go into the development of fruit left on the tree. Backyard fruit growers can avoid an excessive June-drop by thinning or removing extra fruit early in the season, allowing only one fruit per cluster to develop and spacing these an average of six inches apart on the branches. This results in the development of larger fruit instead of many small fruit or considerable fruit loss from June-drop. June-drop may be extraordinarily heavy if late spring weather is hot.
If fruit drop occurs close to harvest, it is called “pre-harvest fruit drop.” This may be caused by a heavy fruit load, high temperatures, wormy fruit, or drought stress.
Local gardeners are also noticing another phenomenon this year, the production of an excessive amount of seeds on maples and other trees this spring. I was once told that trees produce copious amounts seed like this when stress triggers them to “think” they are dying and driving them to procreate. This is only a partially correct untechnical explanation.
Abnormally large seed crops may be due to heat or drought stress that occurred the previous year, but it may also be due to spring weather the current year. Maples do flower quite early in the spring and their flowers are subject to spring frost damage. Mild spring weather with no killing frosts allows for good pollination and the development of more seeds than in most years.
Scientists have also discovered that some types of trees normally bear heavier seed crops every other year or every few years. There is even a phenomena called “masting” where some trees, such as oaks, produce massive seed crops on cycles of three to twelve years. This occurs over large regions and is thought to have evolutionary significance within forest ecosystems.
When it comes the abundance of seeds this spring on local maples and other trees, it could be due to the mild spring weather, stress last summer, or cyclical seed bearing… or maybe all three.