Some crops need a cool start

Chicago Tribune

15-03-2025

Before too long, it will be time to start planting vegetables — at least some vegetables.
'Some crops, such as carrots, radishes and most leafy greens, actually prefer to grow in cooler soil before the heat of summer,' said Sharon Yiesla, plant knowledge specialist in the Plant Clinic at The Morton Arboretum in Lisle. Other vegetables, such as tomatoes, peppers and melons, need to wait at least until May, when the soil is thoroughly warm and any danger of frost is past.
Cool-season crops can be started from seed indoors in early spring and the seedlings transplanted outdoors in April in the Chicago region, once the soil is thoroughly thawed and drained. 'You can start them from seed yourself or purchase transplants,' Yiesla said. 'You also can sow seeds of many cool-season vegetables directly into the garden soil, although it will take longer to get a crop.'
Most cool-season vegetables sprout grow best in spring temperatures in the 50s, and the foliage can tolerate a night or two of freezing temperatures about 32 degrees, or even a sprinkling of snow. 'That's a good thing in the Chicago area, where our spring weather is extremely erratic,' she said.
To hedge your bets against both cold spells and hot spells, space out each crop by sowing the seed or setting out transplants in batches over a period of two or three weeks. Make sure you choose a full-sun site for all vegetables, well away from any source of shade such as tree branches overhead.
Cool-season vegetables include carrots, radishes, parsnips, turnips, beets, asparagus, peas, onions, garlic, kale, spinach, lettuce, potatoes, cabbage, cauliflower, Brussels sprouts and broccoli. 'Most leafy greens will turn bitter if they grow in hot weather,' she said. 'Time them so you can harvest before July when the real heat sets in.'
Warm-season vegetables, on the other hand, thrive in the heat of summer and can't tolerate either having their seeds in cool spring soil or having their leaves nipped by cold weather. These crops include tomatoes, peppers, melons, beans, cucumbers, pumpkins and other hard-shelled squash, zucchini and summer squash, eggplant, sweet potatoes and sweet corn.
Because the warm part of the Chicago-area growing season is so short, warm-season crops are usually started from seed indoors and grown for several weeks before they are transplanted outdoors. 'Plan your garden so you can plant them out in mid-May to early June,' Yiesla said.
Some relatively cold-tolerant herbs such as parsley, cilantro, chervil, chives, dill and oregano, can be planted outdoors in mid-spring. Others, such as basil and thyme, are more tender and need warmer weather. 'Plant your basil when you plant your tomatoes,' she said.
Not all varieties of any vegetable are the same. Some have been bred to be more tolerant of cold or heat, or will take more or less time to reach maturity. 'Read the seed packets or the catalog descriptions carefully,' Yiesla said. 'They will give you the specific information you need to plan when to plant each vegetable variety.'
Planting times will usually be given in reference to the average expected date of the last frost. In most of the Chicago area, that is about May 15. 'That's just an average, though, and we have sometimes had frosts in late May,' she said. Play it safe by consulting the long-term weather forecast before you set out warm-season vegetables or other tender plants.
Cool- and warm-season vegetables have different needs because they originally came from different climates. Kale, for example, comes from the Mediterranean region, where the winter is much like a cool Chicago spring. Tomatoes are native to Central America, which never sees a freeze. Potatoes come from the high, cool Andes mountains of Peru.
'Their genes tell plants when to sprout and grow,' Yiesla said. 'We need to plan our gardening around their basic natures.'