Choosing the Right Annuals for Your Garden
To grow annuals, you don’t need to worry about your precise climate zone and temperature extremes as much as you do with permanent plants, such as perennials, trees, and shrubs. The first thing you need to know about any annual that you want to add to your garden is whether it’s a warm-season or a cool-season annual. The difference is vital to planting annuals at the right time of year in your area.
Cool season and warm season are, of course, relative terms. Where summers are cool (such as along the foggy California coast or other overcast climates) you can grow cool-season annuals all summer. Where winters are warm and nearly frost-free (such as in low-elevation Arizona) fall through spring is an ideal time to grow cool-season annuals, such as Iceland poppies and stock, and even some warm-season annuals, such as petunias. In fact, winter and early spring make up the main flower-growing season in Arizona — summer there is too hot to grow any annuals except the most heat-tolerant warm-season varieties.
Cool-season annuals are those that perform best when temperatures are mild — about 70°F (21°C) — days are short, and soil is cool. In most parts of the United States and Canada, these conditions are typical in early spring and early fall. Temperatures may be similarly mild all season in mountain regions or in regions to the far north (or the far south, in the Southern Hemisphere). In some coastal regions, temperatures stay mild year-round. Cool-season annuals can stand varying amounts of frost; some types, in fact, are quite hardy and are actually perennials that live through the winter in many areas. The enemies are hot weather and long days, which cause cool-season annuals to produce fewer blooms and ultimately die. Examples of cool-season favorites are calendulas, pansies, and snapdragons.
In the typical cold-winter/hot-summer climate, the time to plant cool-season annuals is early spring — from four to eight weeks before the typical last frost or as soon as you can work the ground (dig and turn over the soil). Their season ends with the arrival of hot weather, when you can replace them with warm-season annuals. Where summers rarely heat up, many cool-season annuals can thrive all summer right alongside warm-season annuals that don’t demand hot weather.
Warm-season annuals are those that thrive in hot summer weather. Most are tender, and freezing temperatures damage — or destroy — them. Examples are celosias, marigolds, vinca rosea (also called Madagascar periwinkle), and zinnias. Plant these heat-seekers after soil and air temperatures begin to warm up and expect them to reach their peak in midsummer.
The magic date for planting warm-season annuals depends on your climate. Suppose, for example, that you live in the most typical climate, the one that predominates over most of the northern United States, Canada, and northern Europe. This climate typically has cold winters (usually with snow) and warm, often humid summers. In this climate, you can generally grow warm-season annuals from late spring through late summer or early fall. The basic rule for planting is to wait until the danger of frost has passed and the weather has warmed up a bit. Note that some warm-season annuals need more heat than others.
The growing season is the typical number of days between spring’s last frost and fall’s first frost. Generally, the farther north, the shorter the growing season. Growing-season length can be a factor when you’re choosing annuals, especially from seed catalogs, which list the number of days to bloom. Days to bloom is an important number for annuals. It’s usually listed right on the seed packet or in the seed catalog, sometimes right after the plant name. Specifically, this figure refers to the average number of days a plant requires after you plant its seed for the flower to bloom. Your goal is to determine whether a plant’s days-to-bloom average fits comfortably within your growing season.