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Hot and Cold: Thermoperiods

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tomatoes on vineThe temperature of the air and ground surrounding a plant’s parts affect the way it grows and produces.  An important factor is whether the plant itself likes a warm or cool growing season.

Tomatoes, for example, straggle along until the days are consistently warm while lettuces get a charge out of lengthy, nippy, early spring weather.  Here are some thoughts to consider:

Tomato plants remain stumpy, and for the most part don’t set fruit in cool weather.  Lettuces bolt to make flowers, and turn bitter to the taste when the heat that makes tomatoes shout “Wahoo!” hits.  Mums are happy campers when the daytime temperatures are around 60 degrees, and here’s a surprise, those potted cyclamens–the ones that appear in the stores around Christmas–like it chilly.

Horticulture professionals call the range of temperatures in 24 hours, a day’s thermoperiod.  Plants perform best when daytime temperatures are 10-16 degrees higher than nighttime temperatures.

Crudely speaking, our green friends cook up sugars during the day then take a breather to exchange gases or respire after the sun goes down.  They inhale carbon dioxide and exhale oxygen.  The oxygen is leftover from making sugar during photosynthesis.  If hot weather causes a plant to breathe faster than it cooks, it can run out of gas, so to speak.  This imbalance between photosynthesis and respiration impairs health and growth.

North Central Texas vegetable gardeners see the phenomenon clearly in July and August when many vegetable plants struggle with wilting, and stop flowering and consequently setting fruits.

Low temperatures can also put the brakes on photosynthesis which also hampers growth and fruiting.  Plants are not one-size-fits-all where weather is concerned, some flourish in warm weather, and others like cold weather.  The gardener’s job is to figure out what’s what.

The USDA (United States Department of Agriculture) map of “hardiness zones” helps us determine which plants will best withstand the range of air and soil temperatures where we live.  Texas has 4 temperature zones with sub-zones stretching from the high plains up north to the subtropical coast and citrus tree-friendly Rio Grande valley.

Peach trees, for example, require a certain number of cold weather hours each year in order to flower and set fruit.  We may plant any variety of peach trees we like, but doing our hardiness zone homework will prevent a lot of production disappointment later.  Did you know different varieties have different cooling requirements?  Your county extension agent can help you figure out what will work best in your location.

Flower gardeners learn their lilies need 6 weeks of 33 degree weather a year before they will bloom.  Those gorgeous Asiatic lilies are hard to grow here, but day lilies love the area.  Non-hardy plants can be injured by too low or too high temperatures which restrict water intake.

Premature budding often follows an out-of-season warm spell, after which the plant may get slammed by normal seasonable cold weather.  On the other hand, at one time or another most gardeners experience a late spring killer frost.

April 15 is the late frost date for our latitude.  If you plant warm weather vegetables before that, have a frost guard plan ready.  That date, however, is not foolproof.  Last year we got hit with an unusual cold snap in the second half of April.  I jumped the gun then planted tomatoes 3 times last year.

Contact Noelle Hood at noellemhood@gmail.com

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