Dahlias (Dahlia spp.) bloom in summer or early fall in most U.S. Department of Agriculture plant hardiness zones, but in warm climates dahlias flower in late winter or spring. Hardy in USDA zones 7 through 10, dahlias are planted in spring in most climates, but in USDA zones 9 and 10, they are often planted in fall. Shady growing spots can prevent dahlias from blooming, and overfertilization encourages healthy growth but discourages flowers. Drought can also reduce dahlia blooms.
Look at Light Levels
Sunny spots produce the best flowering in dahlias. Partially shaded sites can result in few dahlia flowers, and in full shade the plants grow poorly and may not flower at all. Remove overhanging foliage or other causes of shade, if possible. Alternatively, grow dahlias in an area of the garden that receives at least six hours of sunlight every day.
Avoid Excess Fertilizer
Overfertilizing dahlias results in weak stems and small flowers, or no flowers. Dahlias grow best when they receive low-nitrogen fertilizer, such as a 0-20-20, 5-10-10 or 10-20-20 formulation. These provide the phosphorus and potassium dahlias need for healthy roots and blooms, but little or no nitrogen, which encourages leafy growth. Organic fertilizers to avoid with dahlias include compost and fish fertilizer.
Don't fertilize non-blooming dahlias. The following year, when dahlia sprouts appear, evenly sprinkle 1/3 cup of 2-12-12 fertilizer around each plant. Water the fertilizer into the soil.
Meet Their Water Needs
Dahlias are fast-growing, succulent plants, and if they don't receive enough water they may not bloom. Dahlias are especially vulnerable to low water levels during the flowering period.
Water dahlias when the soil is dry to a depth of 1 inch, which is usually about twice a week in dry weather during the peak blooming time. Apply water until the soil is moist to the depth of the root systems. Spread a straw mulch 3 or 4 inches deep around the dahlia plants to conserve soil moisture, but don't pile mulch against the plants' leaves or stems.
Deadhead Faded Blooms
If your dahlias bloom a little then don't produce any more flowers, it may be because you didn't deadhead the plants. Deadheading means removing spent flowers, and this prevents plants from setting seed. When plants set seed, they often stop blooming.
Remove dahlia flowers when they wilt. Pinch the stems behind the flower heads, and remove whole flowers, not just the petals.