By Julie Christensen
Few flowers are as bright and cheery in the garden as asters. These perennial plants bloom from late summer to fall, when most other plants are dwindling. Asters are loved for their daisy-like flowers that come in a wide range of hues. The plants create a bright spot in the garden and also attract butterflies, birds and bees.
Found in USDA plant hardiness zones 3 through 8, asters grow best in areas with cool, mild summers. If you live in a dry, hot or overly humid area, you’ll probably struggle with asters. If, on the other hand, you live in a place with mild summer days and cool nights, such as the Pacific Northwest, your asters will thrive.
Asters can be planted from seed, but they’re slow to grow and germination is inconsistent. Most gardeners prefer to grow asters from nursery transplants. Choose a protected site that gets full sun to partial shade. Amend average garden soil with compost, peat moss and manure to improve drainage and provide the rich, moist medium asters prefer. Set asters out in late spring, after the last expected frost. Water frequently as the plant becomes established.
Established plants should be fertilized every spring with a balanced 10-10-10 fertilizer at a rate of ½ cup per plant. Broadcast the fertilizer around the plant, being careful not to get it on the plants’ leaves. At the same time, spread 1 inch of compost around the plants and top with 2 to 3 inches of wood chip mulch. The mulch helps conserve moisture and keeps the plants’ leaves dry. Mature plants are somewhat finicky about moisture conditions. If the soil is too wet or too dry, the plants drop their leaves and might not flower. Aim for soil that is consistently moist, but never soggy. Use soaker hoses or drip systems, rather than overhead sprinklers, which can spread disease.
Asters range considerably in size, depending on the variety. Dwarf varieties that grow only 8 inches tall are ideal for rock gardens or in the front of the border. Large varieties can stretch 8 feet tall. These varieties often need staking, particularly in windy areas, which is why it’s a good idea to place them in a protected spot. Divide the plants every two to three years if they seem overcrowded or growth slows. Deadhead spent flowers to encourage more blooms and cut the plants back in late fall or early spring.
Caring for Asters
Asters suffer from several diseases, especially in hot, humid climates. Powdery mildew causes a white growth to form on the leaves. Space plants so air circulates freely and use soaker hoses. Apply fungicides labeled for treating powdery mildew. Leaf spots, rust and stem cankers are also common. Plant disease-resistant varieties and clean up any plant debris. Remove diseased plants promptly so the disease doesn’t spread.
As far as insect pests, you’ll find the usual suspects: mites, aphids, slugs, snails and nematodes. Aphids and mites often move on without any help from you, but if infestations are severe, spray the plants with a stream of water or spray both the tops and bottoms of the leaves with insecticidal oil. To combat slugs and snails, set a wooden board in the garden. The slugs and snails like to hide in this moist, dark area. Turn over the board and handpick the pests. Drop them in a bucket of soapy water. You can also use slug and snail traps, but read the packaging carefully. Some of these products are toxic to humans and pets.
There are more than 250 species of asters, ranging from the common perennials to annuals, and from climbing vines and shrub-like plants. In hot climates, your best bet is to grow asters as biennials or annuals, replacing them frequently.
Try King George (Aster amellus), a perennial variety with deep-blue blooms, or Silver Spray, which has white blooms tinged with pink.
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Julie Christensen learned about gardening on her grandfather’s farm and mother’s vegetable garden in southern Idaho. Today, she lives and gardens on the high plains of Colorado. When she’s not digging in the dirt, Julie writes about food, education, parenting and gardening.