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Understanding Cool-Season and Warm-Season Annuals

Get more out of Iowa's gardening climate by planting both cool- and warm-season annuals

The name says it all.

Cool-season annual flowers and edibles thrive during the portions of the growing season that people find chilly.

Warm-season annual flowers and edibles are more like we humans—almost warm-blooded, so to speak. They won't tolerate any frost and have to be planted after all danger of frost is passed. They need warm soil and warm air to take off, so their seeds shouldn't be planted directly in the soil until late April, May, or even early June.

In a place like Iowa, where we have so much cool weather in spring and fall, it really pays to understand the difference between those annuals that like warm weather and those that like cool weather. That way, in the chilly days of spring and frosty days of fall, you can enjoy more plants longer!

Want to know more? Read on:

Cool-Season Annuals

They thrive even when nighttime lows hit freezing. Most demand plenty of moisture (though they don't like soggy soil). When daytime temperatures regularly start getting to 80° and 85°F, cool-season annuals start to brown and fade and their flowers shrivel and blooming stops.

When and how to start cool-season annuals is slightly tricky. When in doubt, check the seed packet. Many, such as California poppy, godetia, larkspur, violas, and bachelor's buttons do best if you just direct-sow them on the ground (just scatter them!) in March and April in Iowa, as soon as the snow melts.

Others, such as pansies, snapdragons, and sweet alyssum, are easiest to grow from established plants. Pansies are the most cold-tolerant and can even take a dusting of snow.

Garden, snap, and flowering sweet peas, another type of cool-season annual, like to be planted directly in the ground. Soak the large seeds in water for a few hours and then plant outdoors in April, about the time the lilac leaves are the size of a mouse's ear, according to folk wisdom. Keep evenly moist as needed.

Warm-Season Annuals

Warm-season annuals are a little less complicated. These are the annuals that are standbys of the American summer flower garden: Impatiens, marigolds, petunias, geraniums, annual salvia, celosia, zinnias, and more. Many are natives of the tropics, and so it follows that they like it warm. Frost kills them. And even if there's no frost, planting warm-season annuals too early in wet, cold soil makes them sulk and do nothing. Plus, for these warmth-lovers, it's a standing invitation for diseases and pests. 

Plant warm-season seedlings outside after all danger of frost has passed. This is May 10 for southern Iowa; May 15 for northern Iowa.

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