In the Deep South and other mild winter regions of the country, spring has been in full fling for a couple of months now. Meanwhile, places like Madison, Wisconsin and Denver, Colorado are just starting to wake up to warm weather. Wherever you are gardening, it’s critical to understand the right time to plant individual crops ensure a successful harvest. A good grasp of the effects of temperature on different types of plants is integral to this understanding.
When it comes to annual vegetables, they can generally be divided between warm season and cool season crops. (See below for a complete list of warm and cool season crops according to their plant families). Warm season crops require frost-free weather to survive, and they generally need warm nights and long, hot days to thrive and produce well. On the other hand, cool season crops tolerate frosts and have evolved to do most of their growing in the mild weather of fall and spring. When the hot weather hits in early summer, most of these will attempt to set seed, signifying the end of their life cycle. If they are planted as fall crops, they will grow until the first hard freeze, though many species will survive mild winters and begin to grow again in early spring.
To know when to plant your warm season vegetables, it is helpful to know the average date of the last frost in spring and the first frost in fall (see the link below for a reference). It is a good idea to add a few weeks to the average date of last frost before planting heat-lovers like tomatoes, melons, and okra because they will not begin to grow until nighttime temperatures are consistently above 50 degrees. Another important piece of information is the time to maturity, which is usually listed on seed packets. This is just an estimate, which depends on the actual weather, soil conditions, pest problems, and other factors. Subtract the days to maturity from the average date of first frost to determine the latest date to plant a warm season crop. It is wise to subtract several more weeks to account for the unknown variables mentioned above and because the growth of warm season plants slows down with the shorter days and cooler nights at the end of the growing season.
The time to plant cool season crops in late winter or early spring is dependent on temperature and the ability to work the soil. A good rule of thumb is to plant one month before the average date of last frost. Earlier planting may be possible as long as the ground is not frozen or snow-covered. Heavy clay soils are likely to be very wet at this time of year, which will delay planting time until they have dried sufficiently to be tilled.
Cool season crops for fall are actually planted in late summer. Ironically, cool season crops do most of their growing during the warmest parts of the cool season, and are then ready to harvest when the nippy weather first hits. Again, the actual dates of planting depend on how early winter comes. A good rule of thumb is to plant fall crops about two months before the average date of first frost. Later plantings of certain crops, mainly greens, can succeed in mild winter areas because they will overwinter and come into maturity the following spring. In cold climates with really short growing seasons, cool weather crops get planted at the same time as warm weather crops and will grow throughout the summer, setting their seed in fall.
Timing is everything in the world of plants. The difference between cool season and warm season annuals is determined largely by day length. Cool season crops have evolved to set seed when day length is approaching its peak at the summer solstice. Warm season crops are programmed to set seed as the days are becoming shorter in fall. The dynamic pulse of the seasons gives a rhythm to a gardener’s life that is punctuated with the wonderful busyness of planting times and lulled by the long interludes of tending to the crops as they grow. It is important to recognize this rhythm and pace and let its wise cadence be our guide.
The easiest way to learn the difference is to study them according to the common plant families to which most annual vegetables belong.
Solanaceae – known as the nightshade family, includes many popular warm season crops, such as tomatoes, peppers, eggplant, and potatoes.
Leguminosae – the bean family is split between warm and cool season vegetables. All types of green beans (including there yellow and purple cousins) and field peas (everything from lima beans to black-eyed peas) are warm season crops, while fava beans and all types of sweet peas (snow peas, English peas, sugar snap peas) produce best in cool weather. Peanuts are also warm season legumes.
Curcurbitaceae – the squash family is full of heat-lovers, like cucumbers, melons, zucchini, pumpkins, and of course squashes.
Brassicaceae – this includes a long list of cool season crops, like broccoli, kale, collards, mustard, Brussels sprouts, cabbages, cauliflower, radishes, turnips, and rutabagas, plus tatsoi, bok choi, and most other Asian greens.
Chenopodiaceae – spinach, beets, and Swiss chard are all cool season vegetables in the goosefoot family
Asteraceae – the aster family includes many cool season crops, including all lettuces, chicory, endive, and artichokes
Apiaceae – carrots, celery, dill, cilantro, parsley, and parsnip are all cool season vegetables
Alliaceae – cool season vegetables including garlic, onions, shallots, chives, and leeks.
Miscellaneous – corn is a warm season crop in the Poaceae (grass) family; okra is a warm season crop in the Malvaceae (mallow) family; basil is an annual warm season herb in the Lamiaceae (mint) family.
*A list of average first and last frost dates for select American cities can be found at:
For a more detailed list, see:
Written by Brian Barth