Dahlias are popular garden plants, known for their vibrant blooms. They grow vigorously throughout warmer months. As the growing season concludes, proper end-of-season care is important for their return in subsequent years. A key part of this annual cycle involves cutting back the plants.
Understanding Why Dahlias Need Cutting Back
Cutting back dahlias prepares them for dormancy. Dahlias are tuberous plants that store energy in underground structures called tubers. As the weather cools and the plant’s top growth declines, cutting back foliage signals the plant to redirect energy and nutrients from dying stems and leaves back into these tubers. This process helps tubers mature and accumulate reserves to survive winter.
Removing decaying foliage also protects tubers from harsh winter conditions, pests, and diseases. Leaving it in place can create an environment conducive to rot and fungal infections. Removing this material minimizes these risks, ensuring tubers remain healthy during overwintering.
Identifying the Right Time for Cutting Back
The most important signal for cutting back dahlias is the first hard frost. This occurs when temperatures drop to 28°F (-2°C) or lower for several hours, causing foliage to turn black and wilt. This indicates the plant’s active growth has ceased and it has entered dormancy. Waiting for this allows tubers to continue maturing and storing energy from the plant’s top growth, ensuring they are robust enough to survive winter storage and produce strong plants the following season.
While hard frost is the primary indicator, regional climate variations influence timing. In areas with mild winters or no hard frost, look for other signs of natural decline, such as yellowing foliage, reduced flower production, and woody stems, typically as day length shortens. Regardless of climate, complete cutting back before the ground freezes solid, as frozen tubers will not survive.
Step-by-Step Guide to Cutting Back Dahlias
Once dahlia foliage is blackened by frost, cut back the plants. Use sharp, clean pruning shears or loppers. Cleaning tools prevents the spread of plant diseases. Cut main stems down to 4 to 6 inches above the ground. This remaining stem acts as a handle, making it easier to locate and lift tubers later.
Remove all remaining foliage from the cut stems and surrounding area. This eliminates potential hiding spots for pests and reduces disease risk. Leaving a portion of the hollow stem also allows excess water to drain, which can be beneficial before digging up tubers.
Preparing Dahlias for Winter Storage
After cutting back the plants, allow the tubers to remain in the ground for one to two weeks before digging them up. This brief period helps the tubers “cure” or toughen their skin, which improves their ability to store successfully over winter. When ready to dig, use a garden fork or pitchfork, inserting it about 12 inches away from the remaining stem to avoid damaging the tubers. Carefully loosen the soil around the entire clump and gently lift the tubers from the ground.
Once lifted, carefully remove excess soil from the tuber clumps. After cleaning, allow the tubers to dry for several days in a cool, dark, well-ventilated area, away from direct sunlight and freezing temperatures. This drying period, often called curing, helps prevent rot during storage.
Inspect each tuber clump for damage, rot, or disease, and trim unhealthy portions. Label each clump clearly with the variety name. Store the dried tubers in containers that allow air circulation, such as milk crates, plastic bins with holes, or cardboard boxes.
Ideal storage conditions involve a cool, dark, and frost-free environment with temperatures between 38-50°F (4-10°C) and a humidity level of 75%-85%. Common storage mediums include peat moss, vermiculite, or wood shavings, which help maintain appropriate humidity and prevent tubers from drying out or rotting. Check on the stored tubers periodically throughout the winter to remove any that show signs of rot or shriveling and adjust conditions if needed.