The China aster seastar mix (Callistephus chinensis) blooms from mid-summer to frost and produces 3 1/2- to 4-inch blooms in shades of pink, blue, white, salmon and rose. Each flower appears atop a 2-foot stem and resembles a pompom of dense, needle-like petals that form into a swirl, adding visual interest and texture to the garden bed. These annuals reach heights of 24 inches and spread to a width of 12-to-18 inches. Known for their bright color in the fall when many annuals have ceased blooming, aster seastar is right at home in the flower bed throughout the fall.
Seastar mix asters grow best in full sun to partial shade. Full sun typically means 6-to-8 hours of direct sunlight a day, while partial sun refers to 4-to-6 hours. Although they will grow with less light, growth and blooming will be inhibited. Placing containers in a sunny or partially sunny location. Choosing a garden spot in the sun ensures your asters will receive enough light to perform photosynthesis and produce healthy foliage and blooms.
Asters grown in containers perform well in a mixture of equal parts peat moss, perlite and garden or potting soil. Many prefer potting soil to garden loam because it is sterile and weed-free. This mixture allows roots to grow freely, provides good aeration to the roots and promotes drainage. Asters grown in the ground prefer humus-rich, well-drained soil with a soil pH between 6.0 and 7.0.
Asters prefer 1 inch of rain a week but might require supplemental watering if rainfall is inadequate. As a rule, water your asters deeply to saturate the soil to the root level once or twice a week or whenever the soil feels dry to the touch 1 inch below the soil level. Asters in containers may need more frequent watering because soil dries quickly in containers. Water until water runs freely through drainage holes in the bottom of the pot and repeat when the soil feels dry to the touch. Plants in containers might require daily watering during hot, dry weather.
Asters benefit from an application of water-soluble fertilizer designed for flowering plants, such as 15-30-15, once or twice during the growing season. A mixture of 1 tablespoon of the fertilizer granules to 1 gallon of water can be safely applied as often as every seven-to-14 days if your asters show signs of slow growth. Those grown in containers often require frequent fertilizer because the nutrients leach through the bottom of the container with each watering. Use the health and growth of your asters as your guide to frequency of fertilizer application.
Deadheading old blooms as soon as they begin to fade keeps your seastar mix asters looking good and prolongs blooming. Removing the spent blooms channels the plant's energy into producing new flowers instead of wasting energy on seed development.
Keeping weeds under control improves the appearance and health of your asters. Mulch applied around the base of seastar asters creates an effective barrier against weeds and keeps the soil cool and moist. Alternatively, hand-picking weeds as soon as they emerge prevents weeds from overtaking your garden bed and robbing your plants of valuable nutrients and water.
Insect Pests and Disease
Both aphids and spider mites may attack asters but typically do not cause severe damage. However, some plant diseases, such as aster yellows, aster wilt and stem rot can cause damage to seastars. Keeping weeds under control and removing plant debris from the garden, as well as planting your asters in a new location each year reduces the risk of insect pests and disease. Digging and destroying diseased plants also helps to control the spread of insects and disease. The Oregon State University Extension Service recommends mixing 3 tablespoons of dish detergent per gallon of water and spraying the foliage on your plants to kill both aphids and spider mites.
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