Dahlias are flowering plants known for their massive variety, ranging from small pom-poms to dinner-plate-sized blooms in nearly every color imaginable. These flowers offer months of color, typically blooming from mid-summer until the first hard frost of autumn. Successfully growing them begins with understanding the proper methods and timing for planting their tuberous roots. This guide provides a step-by-step approach to starting dahlias and ensuring a robust, beautiful display in the garden.
Selecting the Starting Method and Timing
The most common way to start a dahlia is by planting dormant underground tubers, which are storage organs that produce the next season’s plant. Starting from tubers is a reliable method that ensures the resulting flower is genetically identical to the parent plant. Growing from seed is a less frequent approach, primarily used by gardeners seeking new, unique varieties, as the resulting flower will be a genetic mix.
Timing is critical when starting tubers because dahlias cannot tolerate frost. The safest time to plant tubers directly into the garden is after the last expected spring frost has passed. The soil temperature must have warmed to at least 60°F (15.5°C). This often coincides with the period when warm-season crops like tomatoes are typically planted outdoors.
Many gardeners choose to pre-sprout their tubers indoors to gain an early start on the growing season and achieve earlier blooms. This process involves planting the tubers in pots four to six weeks before the last predicted frost date. Pre-sprouting ensures the plant is more established when transplanted, which can lead to flowers several weeks sooner than direct outdoor planting.
Preparing the Planting Site
Dahlias thrive in a location that receives a minimum of six to eight hours of direct sun daily. Full sun exposure results in sturdier stems and more abundant blooms. In regions with hot summers, some afternoon shade can be beneficial. A sheltered position is also preferable, as the large blooms and tall stems of many varieties are vulnerable to strong winds.
The soil must be well-draining, as the tuberous roots will quickly rot in cold, soggy conditions. Dahlias prefer a slightly acidic to neutral soil pH, generally falling between 6.0 and 7.5. Physical soil preparation involves loosening the planting area to a depth of 10 to 12 inches to improve drainage and allow the tubers space to expand.
Amending the soil with organic matter, such as aged manure or garden compost, enhances structure and fertility. Incorporating these materials helps to lighten heavy clay soil and improves moisture retention in sandy soil. Avoid using high-nitrogen fertilizers at this stage, as too much nitrogen encourages excessive leafy growth at the expense of flower production.
Step-by-Step Planting Guide
Dig a hole four to six inches deep, making it wide enough to accommodate the tuber clump. Spacing between tubers should be determined by the specific variety, but a general range is 12 to 36 inches apart to allow for mature growth and air circulation. For varieties that require staking, insert the stake into the ground before planting the tuber to prevent accidentally piercing the root later.
Place the dahlia tuber horizontally in the prepared hole, ensuring the “eye,” the small bud from which the stem grows, faces upward toward the soil surface. Position the tuber clump so the neck, where the eye is located, is about two inches below the soil line. Carefully backfill the hole with soil, covering the tuber completely but leaving the soil level slightly depressed.
Avoid watering the tuber immediately after planting unless the soil is exceptionally dry. The tuber already contains enough moisture to begin sprouting. Adding water to cold soil increases the risk of rot before the roots have formed. Hold off on any regular watering until the first green shoots emerge above the soil surface, which typically takes between two and four weeks.
Initial Care and Support for New Growth
Once the first shoots appear, begin a regular watering schedule, providing a deep soak once or twice a week, depending on local rainfall and temperature. The goal is to keep the soil consistently moist, but not saturated, to encourage strong root development. As the plant grows taller, the need for water will increase, especially during hot, dry periods.
When the plants reach a height of 12 to 16 inches, they are ready for “pinching.” This involves snipping off the central growing tip just above a set of leaves. Pinching halts upward growth and forces the plant to create side shoots. This results in a much bushier plant structure and the production of more flowering stems throughout the season.
At this stage, begin feeding the dahlia with a fertilizer lower in nitrogen and higher in phosphorus and potassium, such as a 5-10-10 blend. This nutrient ratio promotes strong tuber formation and abundant flowering rather than just leafy stems. Taller varieties require support, so secure the growing stems to the pre-installed stake or support system with soft ties as they gain height.