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Cool-Season and Warm-Season Vegetables
Understand the differences that divide cool-season vegetables from warm-season vegetables:
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- There are two major categories of vegetables: cool-season and warm-season.
- The planting times and conditions for cool-season and warm-season vegetables are different.
- Different regions and climates must plant cool-season and warm-season vegetables at different times.
- Soil and air temperatures at planting and during flowering are important for good yields.
- The Vegetable Plant Guide names the season classification category for each vegetable and all of its special planting and care needs.
Matching Vegetables to Your Region’s Conditions
Dealing with regional conditions means understanding the habits of vegetable plants and plant them in conditions where they will thrive.
There’s more to the story of cool- and warm-season vegetables than their groups may suggest. Only a few vegetables perform well if planted in soils cooler than 40°F (4°C)—temperatures commonly found inside a household refrigerator.
These include endive, lettuce, onion, parsnip, and spinach, and it might be said that they are the cool-season champs.
In a similar manner, there are only a few heat-loving species that germinate best in very warm soil of 75–95°F (24–35°C). Among them are amaranth, cantaloupe, eggplant, southern pea, pepper, sweet potato, watermelon, and New Zealand spinach.
While all the cool-season vegetables sprout when the soil is 40–65°F (4–18°C), most of the remainder will germinate seed when the soil warms above 60°F (16°C).
(Nearly all vegetables grow when soil temperatures range from 70–80°F (21–27°C)—a level that can be judged roughly as equal to the point when it is possible to sit comfortably on the ground for an extended period of time without becoming cold.)
The second aspect of cool- versus warm-season vegetables pertains to the plant’s requirement for an air temperature range necessary to flower and set fruit.
If the temperature falls outside that range for many hours, these vegetables will drop their blossoms, fail to successfully pollinate, and will be fruitless.
Some plants that need warmth to flower but must avoid excessive heat to set fruit are eggplant, melon, pumpkin, squash, and tomato.
In very hot climates such as those found in tropical or desert regions you may find it necessary to partially shade these plants to reduce the temperature of their surroundings in order to grow fruit, and those in mild coastal or high mountain regions with generally low summer temperatures may wish for crops of melons and other produce that require warmth to set fruit.
In addition to germination and fruit-setting temperature ranges, a number of vegetables also are susceptible to a condition called “bolting” that occurs when the sunlight hours become long as the season progresses.
Leafy vegetables such as cabbage, lettuce, and spinach prematurely form seed stalks and flower as the days lengthen. There is little that can be done to prevent bolting except for blanching—wrapping them with porous paper to block sunlight.
Always plant leaf vegetables early in the season so that they can be harvested before they bolt, place them in shady areas of your garden to reduce the amount of light they receive, or cover them with shade fabric when days lengthen in summer.
Keeping these facts in mind will help you grow both cool-season and warm-season vegetables in your garden.
Cool-Season Vegetables
Cool-season vegetables are adapted for and grow best in most climates during early- to mid-spring and mid- to late autumn. In mild-winter climates and reverse-season climates, they are a mainstay of winter gardens [See: Regions and Garden Seasons].
All of the cool-season vegetables tolerate cool temperatures and mild frosts. Some cool-season vegetables are hardy to even lower temperatures.
Warm-Season Vegetables
Warm-season vegetables are adapted for heat and grow best during late spring, summer, and early autumn. In reverse-season climates, they frequently are the only vegetables planted during these times [See: Regions and Garden Seasons].
All the warm-season vegetables perform best when daytime highs are in the range of 80°–95°F (27°–35°C). Higher temperatures may slow or stop flowering and fruit formation, even damage plant tissue and cause sun scald or permanent stunting.
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