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Friday, May 4, 2012

Why An Early Spring Means Trouble

Laelia purpurata 'Adam's' x "alba"  flowering this week in the FOC*
We had an exceptionally mild winter this year followed by a very early spring. Atlanta's dogwoods flowered a month before the Dogwood Festival.

Greenhouse plants are not exempt from the effects of an early spring. I was only slightly surprised and a little dismayed this week to see the first flush of flowers on our bench of Laelia purpurata orchids. They typically flower for us in late May and June.

Why dismayed? In a normal year Atlanta's plants have to endure almost four months of daily daytime temperatures above 90º. Ninety degrees outside translates into 85º in the greenhouse when the humidity is high.  Laelia purpurata certainly doesn't mind the heat, but our soft-leaved intermediate growing orchids begin to show signs of heat stress after four months. Since the daytime temperatures are already above 90º, they have a long slog ahead. A few years of this and some of these orchids will simply disappear from our collection.

Just as worrying is the effect of the mild winter and prolonged summer on pest populations. Aphids and some thrip species originate outdoors, so a population boom outdoors spells trouble indoors or in a greenhouse where there are no natural predators.

Resident populations of scale and mealybugs can also explode. As temperatures rise the generation time (the length of time an insect needs to become reproductive) shortens. A long hot summer means many more generations of pests at work on a population of plants already weakened by heat stress.
Stanhopea tigrina flowering on May 1
Moreover, hot conditions limit our use of our favorite low toxicity pesticide, Neem oil, which can harm a plant that is heat and drought stressed. I would hate to have to resort to conventional pesticides because of the heat. This year we started our spray cycle with Neem oil a month early.

One weapon that I have at my disposal is the ability to manipulate the greenhouse environment. By programming the exhaust fans and evaporative cooler to run not just during the day, but also during the coolest part of the night (3 am to 6 am) I can drive down the night temperature to a reasonable level.

Even so, we're in for a long battle.

Sophronitis purpurata Van den Berg & Chase (2000)
Cattleya purpurata Van den Berg (2008)
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