Nutrient solution temperature can build up surprisingly fast and become excessive under hot lights and in systems with limited root zones. Research shows that even a short duration of root zone heat buildup can have harsh effects that cannot be compensated by having a low daily temperature average. Just a few minutes a day of root zone temperatures above 86˚F can retard the growth of some heat-sensitive crops like lettuce and parsley.
While regular checks ensure roots don’t get cooked, growers can more actively manipulate root zone temperatures to fool many plants into handling higher-than-optimal air temperatures. Root zone chilling is a well-known technique used by many commercial growers in warm or tropical climates for cool-season crops like butter head lettuce, herbs, and other vegetables. Research shows chilling the nutrient solution down to as low as 61-64˚F allows these crops to grow and produce well at ambient air temperatures of 82-97˚F, which are above optimal. Other researchers report that chilling the nutrient solution of lettuce crops reduces the occurrence of the fungal root disease Pythium aphanidermatum. Without chilling, the root zone could warm up to the level of the air, creating numerous problems like slow growth, lack of heart formation, bolting, tip burn, and low marketable yields. However, trials show that root zone chilling must be applied soon after early crop establishment and maintained for the life of the crop for maximum effect.
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