Fall is not the Time for Flowering

Why do trees or flowering shrubs like Forsythia or Lilac sometimes bloom a little in the fall? The reason is that they have an internal counting system to tell them when to bloom optimally. They want to bloom in spring and set seeds so their babies can go forth and multiply. The plant starts counting up cold units to tell it when it has had a winter. If you have enough cold days at the end of summer but the days are still warm enough to keep the plant growing, the “cold counter” may add up to enough chilling units to bloom.

This is also why we cannot plant apple trees in the tropics and expect them to do well. Some plants have a need for a chilling period. If they do not get a dormant cold period, they will not set flowers properly or flower at all. Most temperate zone fruit trees are like this as well as many ornamental flowering shrubs.

Some tropical plants go through a necessary dormant period too. Since they grow were it is warm, these plants count the season of the year by the wet and dry cycles. They know there is an optimal time of the year when their seeds or fruits will be distributed best. This may depend on the weather or it may depend on when some animal is available to eat and spread their seeds. The typical model plant you may be familiar with is the Amaryllis. Sold in its dormant, resting stage as a dried bulb, you can plant it, add water, and it switches to its active growing and flowering stage. We all enjoy the spectacular flowering display.

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