Dahlias are among the most popular garden flowers, celebrated for their dramatic blooms that provide vibrant color from mid-summer until the first frost. Maximizing the long flowering season and ensuring the survival of the plant’s underground tubers requires careful management of the plant’s structure throughout the year. Pruning is a fundamental practice that directs the plant’s energy, encouraging new growth during the active season and preparing it for winter dormancy. Understanding the difference between in-season maintenance and the final fall cutback is key to success with these tender perennials.
Seasonal Maintenance Pruning
Pruning throughout the summer growing season focuses on shaping the plant and increasing the total number of blooms. A technique called ‘pinching back’ is performed early in the season when the young dahlia plant reaches approximately 12 inches tall and has at least four pairs of leaves on the central stalk. This involves removing the main growth tip, which temporarily halts upward growth driven by the plant hormone auxin. The plant then redirects energy to the lateral buds below, causing them to develop into multiple side shoots. This results in a bushier, stronger plant with several stems, increasing the number of potential flowers.
The second form of in-season pruning is ‘deadheading,’ which involves removing spent flowers before they set seed. Allowing flowers to produce seeds signals the plant that its reproductive cycle is complete, causing it to slow or cease flower production. Deadheading diverts this energy back into forming new blooms, extending the flowering period into autumn. To deadhead properly, cut the spent bloom down to the nearest set of healthy leaves or lateral bud, where a new stem and flower will emerge.
The Critical Timing for Fall Cutback
The end-of-season cutback is a specific operation necessary for preparing dahlias for winter, especially in colder climates where tubers must be lifted. The best time to perform this cutback is immediately following the first hard frost. This occurs when temperatures drop below 32°F (0°C), causing the water inside the foliage to freeze and expand. The result is a distinct visual cue: the entire upper portion of the plant, including the leaves and stems, will turn black and wilt.
This blackened foliage signals that the plant’s active growth cycle is over and it has entered dormancy. The freezing event triggers the plant to transfer its stored energy and starches from the stems down into the tuber clump. Cutting the plant back before this transfer is complete is counterproductive, as it reduces the energy reserves needed for the tubers to survive winter storage. Once the foliage is fully blackened and the energy has moved, the stalks should be cut back to a height of 4 to 6 inches above the soil line.
Leaving a short section of the main stalk attached to the tuber clump serves two important purposes. First, the remaining stem acts as a convenient handle for lifting the root ball from the soil. Second, this stub visually marks the location of the tuber crown, where the new growing buds, or “eyes,” will form for the following season. The correct timing ensures that the tuber is ready for winter preparation.
Post-Cutback Care and Preparation
The process immediately following the final fall cutback focuses on preparing the tubers for storage. After the stalks are cut to the 4 to 6-inch stubs, the tuber clump must be carefully lifted from the ground using a garden fork to avoid damaging the roots. Damage to the tubers creates entry points for rot and mold, which can destroy the plant during dormancy. A light shake of the clump will remove the bulk of the soil clinging to the roots.
Once lifted, the cut stems and tubers benefit from a short “curing” period, allowing them to dry in a sheltered, frost-free location for a few hours up to one or two days. This initial drying helps the tuber skin firm up and allows the cut surface of the stem to dry out, reducing the likelihood of decay during storage. After curing, any remaining soil should be gently washed off with a hose, and the tubers must be allowed to dry completely before they are stored.
Thorough drying before storage is necessary to prevent mold and fungus. Before the tubers are packed away, it is beneficial to label the remaining stem of each clump with the variety name. This action prevents misidentification the following spring, ensuring the gardener knows exactly which color and form of dahlia they are preparing to replant.