You probably know chrysanthemums (Chrysanthemum spp.) -- or mums -- as familiar plants covered with colorful flowers that brighten the fall landscape. Although often seen in plant nurseries as container-grown specimens ready to flower in early fall, they are hardy perennial plants. The variety commonly called hardy garden mum (Chrysanthemum morifolium), for example, grows as a perennial in U.S. Department of Agriculture plant hardiness zones 5 through 9. When planted in spring within their growing range and given the right kind of summer care, mums become well-established and usually return each spring to bring their bright colors to the fall garden.
Watering and Feeding
If you plant young mums in spring or overwintered mums showing new growth, then keep them well-watered during late spring and into the summer months. Regular supplies of moisture is especially important for them during hot, dry weather. So water the plants deeply whenever the top 1 inch of their soil feels dry to the touch. During dry spells, a good rule of thumb is to water once each week, but water at each plant's base to keep foliage dry and discourage the growth of fungus. Adding 2 or 3 inches of organic mulch such as shredded bark on the soil surface under the plants helps conserve soil moisture, but keep the mulch back a bit from each plant's center to help prevent fungal diseases.
Feeding, or fertilizing, mums during their growing season -- late spring through summer -- also helps them put out strong growth and set lots of flower buds. Use a water-soluble, high-phosphorous fertilizer formula such as 5-10-5 to boost blooming, diluting the fertilizer by mixing 1 tablespoon of it in 1 gallon of water. Apply the fertilizer solution once each month during the growing season for the best results, using it instead of a regular watering.
Pinching Back for Bushiness
If you plant young mum plants in the garden in spring or grow mums from the previous season, then pinch off the tips on their new stems when they are about 6 inches long, using your fingertips or shears that you wipe with rubbing alcohol after each cut to prevent the spread of plant diseases. When new side shoots -- laterals -- sprout at each trimmed tip, also pinch them back when they're 6 to 8 inches long. Continue trimming back new shoots as they appear until midsummer. Then let the plants grow without further cutting. This approach produces compact but bushy mums with masses of flower buds.
Mums are usually trouble-free and simple to grow, but they may develop a fungal disease if grown in overly wet conditions. You could see white powder from powdery mildew on their leaves, or gray mold could cause grayish deposits on the leaves and stems. Keeping plants well-spaced for good air circulation and watering each plant at its base helps avoid these fungal problems. If you see signs of fungus, though, it's best to remove the affected parts of the plants, cutting into healthy tissue behind the damaged parts. Discard all plant debris on a regular basis because it can harbor fungus. If a fungal problem is severe, remove mulch from under the plants to help the plants dry quickly after rain or watering.
Mums are usually pest-free, but small, mahogany-tinted insects called chrysanthemum aphids might become a problem, sucking plant juices and slowing plant growth. A chrysanthemum aphid nymph is 2/100 to 4/100 inch long; an adult without wings is 6/100 inch long, and an adult with wings can grow almost 1/10 inch long. If you see these pests on a mum, spray the plant with insecticidal soap, diluted at a rate of 5 tablespoons of insecticidal soap concentrate per 1 gallon of water, and repeat the treatment every two weeks as needed.