Dinner plate dahlias, defined by their massive blooms often exceeding eight inches in diameter, are technically classified as tender perennials. This means their ability to return year after year is entirely dependent on the climate conditions where they are planted. The plant’s status shifts from perennial to annual based on the severity of winter temperatures in a specific location.
Understanding Tender Perennials
A tender perennial is a plant that grows and flowers for more than two years but cannot tolerate freezing winter temperatures. Unlike true perennials, dahlias originate from warmer climates, specifically the mountainous regions of Mexico and Central America. This native environment did not necessitate the development of deep freeze tolerance, making the plant’s underground storage organs vulnerable.
The distinction between a perennial and a tender perennial is mapped out by the USDA Hardiness Zones. In regions designated as Zone 8 and warmer, the dahlia’s fleshy, tuberous roots can typically be left in the ground to survive the winter with minimal protection. These warmer zones rarely experience the prolonged, deep soil freezes that would destroy the tubers.
In contrast, gardeners in Zone 7 and colder must treat the dinner plate dahlia as an annual unless they intervene. The dahlia’s tuber is composed largely of water and starches, and when the surrounding soil freezes solid, ice crystals rupture the plant’s cells. This cellular damage rapidly leads to the decay and death of the root system, preventing the plant from regrowing the following spring.
Essential Care During the Growing Season
Successful cultivation of these large-flowered dahlias begins with proper placement during the active growing season. Dinner plate dahlias require a minimum of six to eight hours of direct sunlight each day to support their vigorous growth and massive blooms. Inadequate light can result in weak stems and fewer flowers.
The tuber should be planted in well-draining soil once the danger of frost has passed and the ground temperature has reached about 60°F. Soil that remains soggy after watering or rain can cause the tubers to rot, even during the summer. Due to the sheer size and weight of the mature flowers, which can measure up to 12 inches across, support is often required.
Staking the plants early in the season prevents the heavy blooms from causing the stems to snap or flop over later. A consistent watering schedule is necessary, especially during dry spells, as the large plants consume a significant amount of moisture. Removing spent flowers, a process known as deadheading, encourages the plant to put its energy into producing new buds rather than setting seed, extending the bloom period until the first frost.
Preparing and Storing Tubers for Winter
For gardeners in colder climates, preparing dahlias for winter begins immediately after the first killing frost. The frost causes the foliage to blacken and die back, signaling that the plant has entered dormancy. Waiting for this natural signal is beneficial because the plant continues to mature the tubers even after a light frost has damaged the flowers.
The stems should be cut down to about four to six inches above the soil line before carefully digging up the tuber clump. Use a garden fork or shovel to lift the tubers gently, starting a safe distance away from the main stem to avoid accidental damage to the brittle roots. Damaged tubers are highly susceptible to rot during storage, compromising their viability for the next season.
After the tubers are lifted, excess soil should be gently removed, and the clumps are often allowed to “cure” in a protected, cool, and dry area for a few days. This curing process helps the skin of the tuber dry and toughen, providing a layer of defense against disease and moisture loss during storage. Once cured, the tubers are ready for their long winter rest in a frost-free location.
Ideal storage conditions involve a consistent temperature between 40°F and 50°F, which keeps the tubers dormant without risking freezing. The tubers must be packed in a storage medium that prevents them from drying out completely while also allowing for air circulation. Materials commonly used to surround the tubers inside a vented box or paper bag include:
- Slightly dampened peat moss.
- Vermiculite.
- Wood shavings.
Throughout the winter, periodically check the stored tubers for signs of shriveling (indicating they are too dry) or for mold and soft spots (signs of rot from too much moisture).