Garden mums (Crysanthemum x morifolium or Dendranthema x grandiflorum) are late-blooming, and brighten up dreary fall days. Plant nursery seedlings in spring or full-grown plants in late summer for an autumn display. Their clouds of pungent flowers don't come naturally, though. Modern garden mums are typically hybrids -- bred for show rather than longevity -- and require grooming throughout the summer to mount their fall displays.
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Garden chrysanthemums are traditionally hardy in U.S. Department of Agriculture plant hardiness zones 5 through 9. In USDA zones 5 through 6, though, they are half-hardy and may last more than one year only with optimum weather. In the warmer USDA zones 7 through 9, they grow as true perennials, returning year after year.
Mums need short days to cue their flowering, to initiate the flowering process in July. Their photosensitive biological clocks trigger blooms when days become shorter than nights in August or September, depending on the location.
Planting and Pinching
Whether you plant nursery-grown mum seedlings or divide existing plants, young chrysanthemum plants bear little resemblance to the bushy globes of fall. Plant them 12 to 15 inches apart in holes one-and-a-half times the size of their root balls in well drained soil. Mums tolerate full sun in northern growing zones, but in southern gardens, some afternoon shade helps keep them hydrated.
When the plants reach 6 inches tall, pinch branches back about one-third of their length with your thumb and forefinger. New branches will form at the thick nodes where leaves grow. Continue pinching new branches to encourage shrubbiness until July 4 in northern areas and until mid-July in the south when flower buds start to set.
Food and Drink
Mums need at least 1 inch of water per week throughout summer -- more often if they begin to wilt in hot weather. Water plants in the morning to allow foliage to dry thoroughly. A drip hose avoids water waste and runoff.
Feed mums monthly, beginning when you plant them. Apply 1/4 pound of all-purpose, granular garden fertilizer 13-13-13 per 10 square feet of soil surface in beds. For individual plants, scatter 2 tablespoons of a slow-release, granular 13-13-13 fertilizer 2 feet around the plants. Work fertilizer into the soil lightly, and then water the soil deeply. Stop feeding spring-planted mums in mid-July so they can transition from foliage growth to flowering.
Alantolactone, an oil in mum leaves and flowers, is a skin irritant. If your skin is sensitive, do not pinch or handle mum plant parts. Instead, wear gloves and clip mums' branches with sterilized hand shears.
Pests and Diseases
Pungent mums repel many pests, and good air circulation and sanitation fend off fungal diseases.
- If aphids or spider mites persist, disperse them with a strong spray of water. Use insecticidal soap spray for persistent infestations. Acephate sprays are also effective, but pose dangers to bees.
- Treat leaf spot, powdery mildew, ray blight and ray speck with fungicides containing myclobutanil, as granules, mixed 1 ounce per gallon of water or according to directions.
- Treat rust, ray blight and ray speck with mancozeb, available as a spray or mixed, 2 to 5 teaspoons per gallon of water.
- Use chlorothalonil fungicide, mixed 1 ounce per gallon of water or premixed spray, on gray mold.
- Wilt and virus diseases such as mosaic and chrysanthemum smut virus are invariably fatal. Remove infected plants immediately and clean up any debris promptly.
Use fungicide and insecticide sprays in calm conditions and according to package directions. Wear protective clothing, and wash with soap and water after using these chemicals.