The smaller- flowering types, which are usually about three feet tall, should be spaced two feet apart. The taller, larger-flowered dahlias should be spaced three feet apart. Dig the planting hole slightly larger than the root ball of the plant and incorporate some compost or sphagnum peat moss into the soil. Plant dahlias so the crowns are just above soil level. Tall, large-flowered cultivars will require support.
Dahlias tend to have shallow roots so be careful when cultivating.
About the end of June, mulch plants with well-aged compost, aged manure, peat, or straw. Water plants regularly if rain is insufficient but avoid wetting the foliage. As plants grow, a monthly feeding will encourage large blooms. If using a granular fertilizer, water it in thoroughly after application. Do not fertilize dahlias after mid-August.