In the Garden:
Lower South
November, 2011
Regional Report

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Now that our nights are growing longer fall asters have begun their annual blooming season.

The Case of the Missing Fall Blooms

Strolling about the garden this week I was very impressed with a fall aster that is absolutely loaded with blooms. Bees and beneficial insects including hoverflies (syrphid flies) were busy at work on the small lavender blooms. Nearby some Mexican mint marigold (Tagetes ludica) and Mexican bush sage (Salvia leucantha) were joining the asters on center stage and the yellow bells (Tecoma stans) were still going strong, attracting some bumblebees to their nectar.

Here in the South fall brings a renewal of blooms to our summer-scorched landscapes. After a dry summer some scattered rains are helping renew growth, blooms, and the spirits of gardeners who have endured one of the hottest, driest, longest summers on record.

It isn't just the break in temperatures and meager rains that have brought the blooms. The season has its own triggers to signal some plants to bloom. Traditional wisdom has been that it is the short days that signal fall blooming plants to take center stage.

While we still refer to these plants as "short day" plants, meaning they are induced to bloom with the arrival of short days in the fall season, they are in fact "long night" plants. It is not the daylength that causes them to form bloom buds and begin their flowering season but rather the length of darkness.

So for example if you let a fall bloomer enter the short days and long nights of fall, but during the long night you turn on a light for a few minutes each evening, the plant won't bloom normally. Rather than experiencing a long night, it experiences two short nights separated by the brief night lighting period.

Why does this matter? Well for one thing, it affects how we induce bloom in plants like poinsettia and holiday cactus. Beginning around early October we cover these plants from about 5 p.m. to 8 a.m. to signal them that the nights have grown longer, thus setting the stage for a colorful holiday season.

Another reason the distinction between short day and long night matters is with some of our landscape plants. Plants like fall aster, Mexican mint marigold, goldenrod, Mexican bush sage, mountain sage (Salvia regla) and copper canyon daisy (Tagetes lemonnii) that wait until fall and early winter to bloom can be adversely affected by security lights around the home.

I came face to face with this phenomenon one fall when a fall aster and a mountain sage were not starting to bloom at the expected time. For a while I scratched my head as to what was going on. Then when the first few sporadic bloom appeared the answer to the mystery became clear.

We had a very bright security light on the building where I worked and this light was preventing the plants from getting the message that fall was upon us. The problem became evident when the first blooms began to emerge on the "shady" side of the aster, opposite the night light. Likewise the mountain sage started to bloom in a section of the bush where a nearby tree trunk cast a security light shadow across the plant.

If you've had problems getting a fall bloomer to flower, this may be part of the reason why. If you don't have a good mix of fall blooming plants in the landscape, this is a great season to plant some. By adding plants that bloom in each of the four seasons you can add year round interest to your landscape.


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