Watch for common pests and diseases.
Borers may be present if Dahlia stems show breakage and wilting. Keep weeds away from the planting, and cut off and destroy any larvae-infested stems. Use a insecticidal soap or Neem to kill aphids.
A bacterial or fungal agent may be present if stems rot at the soil line or plants suddenly wilt and die. Remove and destroy any affected plant parts. The following year, avoid this problem by planting in well-drained, light soil and do not overwater.
To deter leafhoppers, spray plants with a mix of one tablespoon isopropyl alcohol to one pint of insecticidal soap, 3 times per day at 3-to-5-day intervals.
Spider mites are a danger during hot, dry weather. The solution is to spray leaves with Neem or a forceful jet of cold water, particularly on the undersides.
For powdery mildew, which appears as a whitish coating on the leaves, spray with wettable sulfur or other appropriate fungicide. (Give plants more space for better air circulation the following year.) Keep mulch several inches away from the plant stems.
Dahlias make beautiful cut flowers.
Pinch out the center shoot just above the third set of leaves to achieve nice stems for cutting and bushier, compact plants. Tip to get the most of your cut dahlias: Place them in very hot water (160°F) until it cools.
Dahlias require end-of-season care.
Dig the tubers to store for the winter, but wait until a few days after the foliage is blackened by frost. (If you live in a frost-free area, dig by mid-November.) Cut the stalk to 4–6″ tall, rinse off the soil, and allow the clump to air dry under cover for twenty-four hours. Line cardboard boxes or terra-cotta pots with newspaper and layer tubers with barely moist sawdust, sand, or peat. Note: Don’t store in plastic.
Keep the boxes cool (40–50°F) and dry for the winter in a dark spot. Quickly check for rot or shriveling on a monthly basis and, if shriveling occurs, mist the packing material lightly with water. Remove all old foliage from the garden area.
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