KIRKSVILLE, Mo. - Radishes, carrots, broccoli and cauliflower all grow best in cool weather. That’s why they’re called cool-season crops. If they’re not planted early enough, they won’t have enough time to grow before Missouri’s heat arrives.

Too often, inexperienced gardeners will plant cool-season crops in May.

“They’re planting at the same time I’m harvesting,” said Jennifer Schutter, horticulture specialist for University of Missouri Extension. “That’s a problem.”

By the time we get to May, it’s 70-80 degrees on most days. Cole crops, such as broccoli, cabbage and cauliflower, don’t like warm weather and they’re not going to form a good, harvestable head, Schutter said. If you want a productive head of broccoli, cauliflower or cabbage, you need to plant a transplant in late March, or early April at the latest.

Keep in mind that planting in March does carry the risk of frost damage in Missouri, Schutter says. The Show-Me State can have hard freezes in May.

“Those crops can tolerate 27, 28, 29 degrees. But once we get down below 25 degrees, there’s probably going to be some damage, and if we get down to 17 degrees, it probably will kill them,” she said.

Be prepared to cover the plants if a hard freeze hits. During cold spells you can use 5-gallon buckets or row cover, or throw straw on top of them, Schutter says.

The cole crops aren’t the only cool-season vegetables that need to be planted early. Snow peas, carrots, spinach, lettuce and other greens need to be direct-seeded into the soil at the end of March or the early part of April. Seed potatoes also should be planted at this time.

As a gardener, you’ll need to decide how much you want to grow.

“Think about what is your end use,” Schutter said. “Do you just want to grow a little bit for that season to use fresh, or do you want to grow enough to freeze and eat through the winter months?”

By the time Missouri’s summer heat begins in earnest, the cool-season vegetables will start to turn yellow, and that’s when succession planting starts.

“You are growing a crop and then pulling it out when it’s finished and planting another one,” Schutter said. “You can follow up with any warm-season crop such as tomatoes, squash or a warm-season crop of your choice.”

There is one exception: broccoli.

“Harvest the broccoli head in May, but leave the broccoli plants because they will produce side shoots, and as it gets cooler in the fall you’ll get a lot of side shoots,” Schutter said.

You can get a second chance at cool-season vegetables by planting them again in the fall. This can be a bit tricky because you’ll need to plant them in late August, when it’s still hot in Missouri.

“I’ll plant spinach, lettuce, peas, but first I’ll wet the soil with cold water to cool it down,” Schutter said. “Then I’ll keep watering them with cold water until temperatures start to come down. Refrigerating the seed for two, three days in advance also helps with germination in late summer.”

Schutter recommends an MU Extension publication that helps new gardeners understand planting dates no matter where they live in Missouri.

“It’s called the Vegetable Planting Calendar. It is divided into regions of Missouri such as north Missouri, central Missouri, south Missouri,” she said.

The Vegetable Planting Calendar is available for free download at extension.missouri.edu/g6201.