The dahlia (dahlia pinnata) is a tuberous flowering plant native to Mexico, where it grows in nature in loose, volcanic soil. In cultivation, dahlias prefer a sandy soil with a slightly acidic pH of 6.2 to 6.5. Dahlias love the sunshine and do best when they receive a full day of sun. There are many dahlia varieties in an array of colors, with single and double blooms. Bloom sizes vary as well, from a petite 2 inches to 1 foot. Dahlias bloom in the summer and are hardy to USDA zones 7 to 11.
Remove weeds from within a 3-foot radius of the dahlias. Weeds compete with the plants for water and nutrients.
Water the dahlias once a week, deeply. Soaker hoses work well for this task as you can set them and come back in an hour and the plant should be thoroughly watered.
Fertilize the dahlia when it reaches a height of 1 foot. Scatter 1/2 cup of granular 5-10-5 fertilizer around the dahlia plants and spread in a 2-foot radius. Fertilize again during the first week of August. Always water the plants after fertilizing.
Add a 3-inch layer of mulch to the soil at the base of the dahlias. Peat moss will help keep the soil pH down and will allow the soil to stay moist and cool. Other mulch materials include bark, compost and lawn clippings.
Stake the dahlia as it grows. Use strips of old nylon stocking that will stretch to tie the plant to the stake and tie it every 6 inches as the plant grows. The last tie should be located 24 inches below the top of the plant.
Encourage the dahlia to produce a longer blooming period by pinching off dead flowers as you see them. To encourage larger flowers, pinch off all the buds on the side of each branch, starting when the plant is 1 foot in height. For a more compact plant, pinch off all new growth at the tips of the plant. Keep pinching new growth during the season.
Protect the dahlias from slugs and snails by applying snail bait to the soil two weeks after planting. Follow the product's label instructions as to the rate and timing of subsequent applications.
Spray the dahlia plants every 10 days with an insecticide and miticide. Horticulturists with the University of Minnesota Extension suggest that you begin this routine when the plants are 3- to 6-inches tall and that you spray the entire plant, even the undersides of the leaves, at the rate suggested on the package.
Over-winter your dahlias in the garden if you live in a region with mild winters. Cover the planting area with 6 to 12 inches of mulch prior to the first freeze. If you choose to dig up the tubers for the winter, cut the stems back to 6 inches, wait one week and then dig them up. Use a garden fork to dig into the soil 1 foot away from the plant and pry them gently from the ground. Use the hose to wash the soil from the tubers and allow them to dry for 24 hours. Lay them on a bed of sawdust in a cardboard box and store them in a cool (40 degrees F), dry area.