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"Is it hot enough for you?" You've probably heard that question more than a few times during the dog days of summer. If you're like most growers, your main concern is not for yourself (you can get out of the heat!) but for your crops. Most vegetables, even those we think of as heat-loving vegetables, don't do well when temperatures soar into the 90s and stay there. In this article, we'll focus on production, harvest and postharvest solutions to summer's heat.
Vegetables vary in their sensitivity to heat and in the stage of growth at which heat can be most damaging. For some, the heat-sensitive stage is seed germination; for others, it's flower bud development, fruit set, or some other period. Understanding these stages is important to plant breeders trying to develop heat-tolerant varieties in response to warming summer temperatures. Growers can use the information in scheduling their plantings to avoid the most damaging summer heat.
Germination temperatures are well established. For every crop, there is a minimum, a maximum, and an optimum range for germination. You can find those temperatures as a graph in the Johnny's catalog. You've probably used the minimum temperatures to schedule plantings in spring when the soil is just warming up. Now is the time to keep an eye on the maximum germination temperatures, to be sure your soil is cool enough to get your fall crops up and growing. Soil temperatures can be reduced by irrigating, mulching, and shading the beds for several days before you plant.
Surprisingly, many of the vegetables we think of as cool-weather crops will germinate at extremely high temperatures. Cabbage and cauliflower will germinate at 100F (37.8C), carrots and onions at 95F (35C), turnips at 105F (40.6C). But they won't thrive if the temperature remains that high, because there are other growth stages that are more sensitive to heat.
For example, Cornell University research found that the critical period for heat sensitivity in broccoli lasts only 10 days, during the time when the growing tip of the plant shifts from vegetative growth to flower bud initiation. That occurs about 10 days before the appearance of a tiny crown in the center of the plant, or about three to four weeks after plants are set out. Temperatures above 95F (35C) for more than four days during that critical period will result in uneven, poorly shaped heads. A grower can use this information to figure out the best time to plant fall broccoli. Download report on heat stress and heat tolerance in broccoli.
Varieties also vary in heat tolerance relative to one another. Whereas one variety of lettuce will get bitter when summer arrives, the Summer Crisps keep their sweetness. Some head lettuces do better in heat than others: Green Star, Vulcan, Red Sails, and Panisse. Romaines with better heat tolerance include Jericho and Coastal Star.
Even hot-weather crops vary in their heat tolerance. Whereas some tomato varieties stop flowering in 90F-plus heat, other varieties keep on producing. 'Valley Girl', for one, is both heat and cold tolerant. Johnny's makes it easy to choose heat-tolerant varieties: Just look for the sizzling sun symbol next to variety names.