Choosing the right annuals and the correct varieties of them for your area is key to creating a colorful planter, window box, or garden bed. Growing them requires taking into account low light levels, cooler soil temperatures, and the occasional extreme weather of systems such as El Nino and La Nina.
Although many annuals, such as calendulas, geraniums, and sweet peas, can withstand a light frost, these tend to flower sporadically in all but the frost-free areas of the South and West. The key to long bloom is finding annuals that continue to flower during the short, cold days of December, January, and February. These are what I call true winter annuals. Based on my talks with gardening experts from Tampa to Seattle, I've selected the best varieties of eight of the most widely available and popular annuals. Unless otherwise noted, all of these will perform well throughout mild-winter areas.
Snapdragon (Antirrhinum majus). Varieties come in heights from dwarf 'Bells Mix' (8 to 10 inches), semidwarf 'Sonnet Mix' (15 to 20 inches), to tall 'Liberty Mix' (2 to 3 feet). 'Black Prince' (18 inches tall) performed well in University of Georgia trials. Plants are hardy into the 20s and will flower throughout the winter as long as day length exceeds 10 hours. An added value is snapdragons' fragrance. All taller varieties benefit from being pinched back at planting to force more flower stalks to form.
Ornamental cabbage and kale (Brassica oleracea). Although these are not flowering plants, it would be difficult to write about winter annuals without including them for their unique leaf shapes and colors. The colors intensify in cold weather, providing accents in the garden or in large containers, and they complement flowering annuals such as pansies and violas. They grow to about 12 inches wide and high, are hardy into the 20? Fs, and can grow in partial shade. 'Peacock Mix' flowering kale and 'Tokyo Mix' flowering cabbage are particularly attractive. Other appealing foliage plants that are becoming more available as winter transplants include 'Red Bor' and 'Red Russian' kale, 'Red Giant' mustard, and 'Bright Lights' Swiss chard.
Cyclamen (C. persicum). Florist's cyclamen is usually thought of as a holiday houseplant, not an outdoor winter annual. However, it can take light frost and will continue flowering as long as temperatures stay above 45? F. They add stunning crimson, purple, red, salmon, or white to a bed or container. While they're not commonly planted in the Southeast, they grow well in southern Texas, the low deserts of the Southwest, and coastal California. The 10- to 12-inch-tall plants combine well with other annuals, or can be mass-planted for dramatic effect. The Miracle and Laser series have the bonus of being fragrant. Cyclamen perform best when planted in a partly shaded area with good water drainage and air circulation.
Chinese pink (Dianthus chinensis). Chinese pink is generally underused as a winter annual, but newer hybrids such as the Diamond and Ideal series are more cold- and heat-tolerant than older varieties and will flower all winter from Georgia to California. One-inch-diameter flowers come in pink, red, white, and bicolors; petals can be deeply fringed or smooth. Some have a spicy fragrance. Many varieties grow to only 12 inches tall.
Stock (Matthiola incana). Newer varieties such as the Vintage series have a strong clovelike fragrance on the 18-inch-tall flower spikes of lavender, lilac, red, and white that have an extended bloom time. The grey-green foliage adds another value. For best flowering, provide good drainage and full sun, and choose transplants with flower buds already set. If plants are set out in cool soil before flower buds have formed, they may not flower.
Primrose (Primula). Of all the hundreds of species, selections, and hybrids, only a few are used successfully as winter annuals, mostly in coastal, central, and southern California and low-desert areas of the Southwest. Colors range from white to red and blue. Polyanthus (P. polyanthus) and English (P. acaulis) primroses perform well outdoors all winter. P. p. 'Crescendo Mix' and P. a. 'Supreme Mix' are widely adapted, with fragrance as an added benefit. In frost-free areas of the Southwest, California, and south Texas, consider the tender P. obconica 'Libre', which is free of primin--a compound in many primrose leaves that causes mild skin irritation. If you want to try growing primulas in the Southeast, a good choice is P. a. 'Pageant Mix', which performed well in trials at the University of Georgia.
Viola (V. cornuta). This pansy cousin is equally hardy and flowers more readily than pansies, although the flowers are smaller. The trend in violas, as with pansies, is toward single flower colors from white to red and away from the faced varieties such as V. tricolor 'Johnny Jump Ups'. Violas have great resistance to adverse weather and are very hardy. They even overwinter under the snow in my Vermont garden, flowering each spring. Some of the best varieties for uniform plant and flower size and consistent flowering habit are in the Penny and Sorbet series, such as 'Penny Blue', 'Penny Yellow', 'Sorbet Blueberry Cream', and 'Sorbet Lemon Chiffon'.
Pansy (V. wittrockiana). This is the king of winter annuals. Plants will continue to flower with temperatures into the 20? Fs and can survive into the teens. In rainy-winter areas such as the Pacific Northwest and the northern California coast, light-colored varieties seem to stand up better than dark-colored ones. Light blues and yellows such as 'Crystal Bowl Primrose' and 'Universal Plus Yellow' tend to rebloom faster, and their smaller flowers stand upright better, especially in the rain, than the giant varieties. However, in warmer areas such as southern California and the desert Southwest, larger-flowered kinds such as the Maxim Supreme series are best because warm weather tends to shrink flower size. Like violas, the gardening trend in pansy varieties is away from the faced kinds such as 'Majestic Giant' and toward solid-colored types such as 'Crystal Bowl'. Trials highlight these series: Accord, Baby Bingo, Crystal Bowl, and Universal Plus.
Even though the plants described above offer the most consistent winter color, you need not limit yourself just to these. In many places, even though cool-season annuals may not bloom throughout the winter or survive a hard freeze, they are worth trying because they make excellent combinations in containers and beds with the true winter annuals.
Some good choices to try are calendula, nemesia, petunia, sweet alyssum, and verbena. These tend to grow best in warmer areas (zones 9 and 10), but they may provide some late-fall or early-spring color in cooler areas if the weather isn't too severe. Also, plants such as dusty miller and ornamental grasses provide attractive foliage to complement the flowers.
Growing annuals in winter is a bit different from growing them in other seasons. Light levels are lower and plant growth slower. Plants such as pansies that normally would shun the bright sun are more tolerant of full sun. Although temperatures may rarely drop to freezing, the cooler air and generally more frequent rains tend to keep the soil moist longer.
In cool, moist soils you'll need to adjust your fertilizing schedule and take steps to prevent rot diseases. In containers, use a light soilless mix. In gardens, improve drainage with raised beds, especially if you live in a wet area with heavy soil. Cold, wet roots are a sure way to rot plantings. Remove summer mulches such as bark; don't till them into the soil. Incorporating high-carbon mulches will create a nitrogen deficiency in the plants.
At the nursery, choose plants (November is too late to sow seeds) that are already flowering or have obvious buds. Annuals that haven't set flowers by November are not likely to flower well all winter. Set out various combinations of plants as soon as you can, remembering that plants will not grow fast in winter so spacing can be a bit closer than normal. Fertilize sparingly with a fertilizer containing a nitrate form of nitrogen, such as calcium nitrate, during the season. Nitrogen fertilizers containing ammonium can cause leggy growth during brief warm spells, but when the soil cools below 45? F, the nitrogen becomes unavailable to plants. Reapply mulches in drier winter areas such as the desert Southwest. Now just sit back and enjoy a winter season of beautiful flowers.