Dahlias, known for their vibrant colors and diverse forms, bloom prolifically from mid-summer until the first hard frost. These plants grow from underground storage organs called tubers, which provide the initial energy for robust growth. Growing dahlias successfully requires understanding the environmental needs of the dormant tuber and following specific care instructions. This guide outlines the steps necessary to grow a successful display of dahlias, from preparing the planting site to storing the tubers for the following year.
Preparing Tubers and Planting Site
A strong dahlia plant starts with a healthy tuber that should be firm to the touch; avoid specimens that feel mushy or shriveled. The most important feature is the presence of an “eye,” or growth bud, located on the crown where the tuber connects to the previous year’s stem. A tuber without an eye will not sprout, so it must possess at least one viable growth point to produce a successful stalk.
Dahlias require a location that receives a minimum of six to eight hours of direct sunlight daily to support heavy blooming. Planting should occur only after the last expected frost date, when the soil temperature has warmed to approximately 60°F (15°C). The soil must be well-draining, as tubers are highly susceptible to rot in cold, waterlogged conditions. For heavy clay soils, incorporate organic matter like compost or aged steer manure into the top 12 inches to improve porosity and drainage. Aim for a slightly acidic soil pH between 6.5 and 7.0 for optimal growth.
Detailed Tuber Planting Instructions
Once the threat of frost has passed and the soil is prepared, dig a shallow hole about four to six inches deep. Lay the tuber horizontally on its side, positioning the eye, or growth bud, facing upward or slightly sideways. If the tuber has already sprouted, ensure the sprout points upward before gently covering the tuber with soil.
Spacing is important for air circulation and plant health, with standard varieties needing 12 to 18 inches of space between them. Initial watering must be avoided immediately after planting unless the soil is exceptionally dry. The dormant tuber contains enough moisture and nutrients to initiate growth, and adding water before the sprout emerges significantly increases the risk of the tuber rotting in the cool, moist soil.
Summer Care and Maintenance
Once the dahlia plant has sprouted and reached a height of about 12 to 16 inches, pinch the central shoot. This involves removing the top growth just above a set of leaves, forcing the plant to branch out and resulting in a bushier structure with more blooms. Staking is necessary for varieties expected to grow over three feet tall, and installing the stake at the time of planting prevents accidental damage to the developing tuber later in the season.
Watering should be deep and consistent once the plant is established and actively growing, requiring one to two inches of water per week in the absence of rain. Deep watering two to three times a week is more beneficial than frequent shallow watering, as it encourages the roots to grow downward. Dahlias are heavy feeders but respond best to a low-nitrogen fertilizer, such as a formula with a 5-10-10 ratio, which encourages flower production rather than excessive leafy growth. Fertilization should begin about 30 days after planting and be repeated every three to four weeks throughout the growing season, stopping by early September. Regular deadheading, which is the removal of spent flowers, signals the plant to continue producing new blooms until the season concludes.
End-of-Season Digging and Storage
In regions where the ground freezes, dahlia tubers must be lifted and stored to survive the winter. The process begins after the first hard frost, which causes the above-ground foliage to turn black and die back. Leave the tubers in the ground for an additional one to two weeks after the frost, allowing the plant’s energy to fully transfer to the tubers for better storage preparation.
To dig the tubers, cut the main stalks down to about four to six inches above the soil line. Carefully use a garden fork to loosen the soil in a wide circle around the plant, lifting the entire clump gently to avoid breaking the fragile tubers. After removing excess soil, the clumps should be allowed to dry in a cool, dark, and frost-free location for a few days. Final storage must maintain a consistent temperature between 40°F and 50°F. Tubers can be packed in a medium like peat moss, wood shavings, or vermiculite, and stored in cardboard boxes or plastic tubs, requiring periodic checks throughout the winter for signs of rot or excessive shriveling.