Dahlia (S)
Not Hardy
In areas with short seasons, tubers can be potted up 4-6 weeks before last frost date with the buds just at the soil surface. Keep pots warm and moist and in a bright spot. Plant potted tubers outside after last frost. Usually Dahlias are planted directly outside 2 weeks before last frost date. Lay tubers flat in holes 6 inches deep and cover with 3 inches of soil, filling in the hole later as the shoots grow. Plant 1-3 feet apart depending on the mature size of the dahlia.
Plant dahlias in full sun, including at least two hours of morning sun. Of all the plants grown from bulbs and tubers, dahlias are the least tolerant of shade. In areas with very hot summers, however, dahlias would prefer some afternoon shade, but full morning sun. Dahlias do not tolerate very hot or dry conditions, so water regularly in very dry conditions. Cultivate the area where dahlias are to be planted quite deeply, at least 12 inches to encourage a large, deep root system. Dahlias require abundant organic matter, very rich soil. Mix a granular bulb fertilizer into the soil right under the tubers, where the roots will grow. After planting, apply water-soluble 10-52-10 fertilizer once a week for three weeks, and then switch to 20-20-20 once a week during the growing season. Dahlias require at least 3cm of water per week. At maturity, they may need 6cm of water per day. Apply water directly to the roots, and avoid splashing the foliage.
Mulch dahlias heavily to conserve water. To grow stockier plants, encourage branching by pinching off the tops of the stems. To grow the biggest possible Dinner Plate or Cactus-type flowers, pinch off all but the terminal (topmost central) bud on each stalk.
A few days after frost blackens the plants, lift the tubers and let them dry off for a couple of days in a shady place. Dahlia stems may have water in them so hang them upside down to drain if you can. Dust tubers with sulfur dust to prevent rotting, then lay them in shallow boxes or crates and cover with slightly moist peat, sand or vermiculite. You can also wrap tubers in several layers of newspaper. Store them in a cold 35 to 45°F (2-7°C) but frost-free place to prevent tubers from sprouting. Check your tubers periodically throughout the winter. If roots are shriveling, mist them lightly with water in a spray bottle. Remove any soft or rotten roots and add dry peat moss if they are too moist. If they are rooting, it is not cold enough.
