Dahlias, with their abundant, extravagant blooms, are the centerpiece of many Canadian summer gardens. Dahlias are best known for their large, sometimes dinner plate-sized blooms. Unfortunately, many gardeners, especially in cooler Canadian climates, stay away from dahlias because they think they're hard to grow. Dahlias, for sure, are not hardy in Canada, but their successful culture is still not difficult.
Dahlia grows from a tuber, a type of swollen root tissue, that is planted in spring. As weather warms up, it grows abundant leaves, soon followed by flower buds. Flowers can be quite large--up to 9 inches in diameter. In its native regions, dahlias go dormant in winter, re-emerging when spring temperatures return. In Canada, the tubers are dug up to protect them from freezing cold, then replanted in spring.
Plant dahlias in spring as soon as all danger of frost has past. In the warmer parts of Ontario, Quebec, British Colombia and the Maritimes, May 1 is usually safe. Plant in full sun in moist, well-drained soil. Mix in plenty of compost or composted manure. Mix a tbsp or so of bone meal into the planting hole. Plant the tubers so their crowns are just above the soil. Follow the recommended spacing for the varieties you choose.
Water dahlias when the soil becomes dry. Fertilize at planting with 5-10-20 fertilizer, then each month afterwards. Replenish mulches as they become depleted to conserve water and shade roots. Stake the plants with plant stakes and garden twine to keep them upright. Remove spent blooms to encourage new flowers.
Dahlias are prone to aphids and slugs early in the season. Mites appear later in the season. Insecticidal soap takes care of insect problems with minimal toxicity. Slug repellents or diatomaceous earth (DE), a natural product, are effective against slugs. Use according to label directions. Dahlias that develop root rot or viral infections should be removed and discarded. Root rot is an indication that the soil is too wet or that fertilization is too high.
Lift dahlias before the first frost. In cooler Canadian gardens, dig up tubers in late September. In warmer climates dig them up in the second or third week of October. Avoid damaging the tubers and roots. Shake off the dirt and cut the stems back to 4 inches. After letting the stems dry, store the tubers in a wooden or cardboard box filled with peat moss, leaving the stems exposed. Check the tubers occasionally and sprinkle water on tubers that have begun to shrivel.