Photos by Lainie Kertesz, Johnny's Territory Sales Representative
Part 2. Getting Your Cool-Weather Crops Through Summer's Heat
Just as human productivity wanes at the height of summer, so, too, does that of many vegetable crops.
Vegetables vary in their sensitivity to heat and humidity, 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 developing heat-tolerant varieties in response to warming summer temperatures.
Before seeding, growers can refer to the germination guides we provide for each crop, to determine whether the soil is cool enough for optimal germination. Surprisingly, many of the vegetables we think of as cool-weather crops will germinate at very warm temperatures: Brassicas in general germinate very well in a very warm greenhouse, so shade cloth is not generally needed. Cabbage and Cauliflower will germinate at 100°F (37.8°C), Carrots and Onions at 95°F (35°C), Turnips at 105°F (40.6°C). But they won't thrive if the temperature remains that high, because there are other growth stages that are more sensitive to heat.
Varieties, too, vary in heat tolerance relative to one another. In this article we recommend varieties of cool-weather crops that outperform their peers in warm-weather trials. Growers can apply this knowledge in planning the crop production cycle.
Our heat-tolerant varieties include Imperial, which is the best and Green Magic, also a superior performer. Varieties that fare well in moderate heat include Gypsy. In periods of extreme heat and/or humidity, however, such as those experienced in the Southeast, Deep South, and Florida, none perform well unless transplanted after the hottest part of summer has past.
Brussels Sprouts, Cabbage, & Chinese Cabbage
All Brussels sprouts, cabbage, and Chinese cabbage varieties perform reasonably well in areas with moderate summer heat. In areas of extreme heat and/or humidity, however, such as the US Southeast, Deep South, and Florida, these brassica crops will perform well only if transplanted after the hottest part of summer has passed.
Beyond germination stage, cauliflower detests hot weather. In areas with moderate summer heat, white varieties that do reasonably well include Snow Crown and Amazing; for color, choose Cheddar, Graffiti, and Vitaverde. In extreme heat and/or humidity (the Southeast, Deep South, and Florida), none perform well unless transplanted after the hottest part of summer has past.
We offer a diverse selection of greens with some measure of heat tolerance, provided the grower follows the basic production, harvest, and post-harvest practices recommended in Part I. Here are some recommendations for success.
For proper spinach germination irrigation is a must, to keep soil temperatures cool. Spinach really likes cooler weather to germinate and grow, though slower-growing varieties can be chosen for late-spring and summer sowing. Varieties such as Corvair, Space, Emperor, and Tyee are recommended for this slot. Flamingo can also be used if an arrowhead leaf shape is desired, as this is one of the slower-bolting Asian types. (Please note that although one of the slower-bolting of these types, it is still quick to bolt!)
Spinach should always be harvested promptly, before it begins to bolt, or stems will become stringy and flavors will be off. The benefits of irrigation previously described apply to spinach.
All the basic cultural practices described in Part 1, including irrigation, shade cloth, and seeding in cooler locations, should be used with lettuce as well.
Of the other Head Lettuces, we offer a number that have earned our Heat-tolerant Symbol by performing better in heat than others: Tropicana, Green Star, New Red Fire, Panisse, Salvius, and Adriana.
Carrots are grown year-round, but summer heat causes increased bitterness and decreased sweetness. Romance is our best variety for the heat of summer.
When it's hot and dry, you can also grow shorter, smaller carrot types that mature relatively quickly, such as the Parisian Market variety Atlas.
Growing carrots in the shadier, cooler part of the farm is critical for bringing them successfully through the heat. Sufficiently irrigating carrots from seeding to harvest can help keep soil cool, and flavor better. Roots that are well irrigated will be milder and less bitter.
Choosing Varieties for Heat Tolerance —