Dahlia tubers benefit from annual division to maintain plant vigor and increase stock. This process involves separating the large, overwintered clump into smaller, viable sections, each capable of producing a new plant. Dividing dormant tubers in the spring is often preferred because warming temperatures encourage the dormant growth buds, known as “eyes,” to swell and become visible. Successfully identifying these eyes before making any cuts is the single most important factor in ensuring the future growth of the dahlia. The following steps guide the preparation and execution of the division process in the spring.
Timing and Essential Pre-Division Checks
The ideal time for dividing dahlia tubers is late winter or early spring, typically four to six weeks before the last expected frost date. Bring the stored clumps into a slightly warmer environment, such as a basement or garage, to encourage them to rouse from dormancy. This gentle warming causes the small, pointed eyes located near the neck of the tuber to begin swelling and sprouting, making them easier to locate.
A tuber section is only capable of growing if it contains at least one visible eye, as the eye is the meristematic tissue from which the new stalk will emerge. Attempting to divide the clump too early often results in non-viable divisions that will fail to sprout. Before making any cuts, verify the presence of a distinct, often pink, green, or white sprout or bud near the attachment point to the old stem.
Gathering Supplies and Preparing the Tuber Clump
Effective division requires a few simple, clean tools to minimize the risk of spreading disease and ensure clean cuts. Gather sharp implements, such as a gardening knife, a box cutter, or small pruning shears, and sterilize them using a solution of 10% bleach or rubbing alcohol before beginning. You will also need permanent markers and labels to immediately identify the newly separated tubers.
Begin preparation by carefully removing any excess soil clinging to the overwintered tuber clump, often referred to as rinsing. Use a gentle stream of water to wash away the dirt, which helps reveal the structure of the crown and the delicate neck areas. Handle the clumps with care, as the point where the tuber connects to the main stem, known as the neck, is brittle and easily broken.
Once cleaned, inspect the entire clump for signs of rot, mold, or damage that may have occurred during storage. Any soft, mushy, or discolored sections should be removed and discarded immediately. This prevents the potential spread of fungal or bacterial pathogens to healthy tissue during the division process.
The Step-by-Step Division Process
The initial step is to orient the clump and locate the main stem, which is the woody remnant of last year’s growth. All viable tubers radiate outward from this central point, and the growth buds are clustered around the junction where the tuber meets the stem, often called the crown or neck. First, trim away any thin, thread-like roots and the bulk of the old stem, leaving only a short section, perhaps two to three inches long, attached to the crown.
Carefully inspect the neck of each individual tuber, confirming the presence of at least one visible eye before planning the cut. A successful division piece must contain a section of the crown material, the eye, and a healthy tuber attached to supply the energy for initial growth. Following the line of the old stem, use your sterilized cutting tool to begin separating the cluster into manageable, smaller sections, working around the perimeter of the clump.
When making the cut, aim to include a small sliver of the crown tissue with the desired tuber and its corresponding eye. The cut should be clean and deliberate, minimizing jagged edges which can invite disease. For tubers that are firmly attached or intertwined, use a strong, sharp knife to slice through the crown, rather than attempting to pull or twist the tubers apart.
If a long, thick tuber has multiple eyes along its neck, it is possible to cut this single tuber into two or more sections, provided each section retains a firm attachment to the crown and has its own viable eye. Conversely, any tubers that feel soft, appear shriveled, or have been completely separated from the crown tissue are non-viable and should be discarded. A tuber without an attachment to the crown and an eye cannot produce a plant.
Continue this process methodically, separating the entire clump until all viable tubers have been isolated into individual planting pieces. Immediately label each new division with the variety name using a permanent marker, as the newly cut surfaces will begin to dry quickly. Taking the time to ensure each piece meets the criteria—tuber, neck, and eye—maximizes the number of plants you can propagate.
Post-Division Handling and Immediate Planting
Once the division process is complete, the fresh, open cuts on the tubers require a brief period of curing before planting or storage. Curing involves allowing the cut surfaces to dry and callus over for 24 to 48 hours in a cool, dark, and well-ventilated area. This drying process forms a protective layer that significantly reduces the chance of fungal or bacterial pathogens entering the tissue.
After the cut surfaces have callused, the divided tubers are ready for planting or temporary storage. If you are still several weeks away from safely planting outdoors, the divisions can be potted up immediately into small containers filled with a sterile, moist potting mix. Planting them indoors gives the dahlias a head start on the growing season before they are transplanted outside after the last frost.
Alternatively, if the outdoor planting date is imminent, the cured tubers can be temporarily stored in slightly damp peat moss or vermiculite. Place the stored divisions in a cool location, ideally between 40 and 50 degrees Fahrenheit, until the soil temperatures are appropriate for outdoor planting. Proper post-division handling ensures the energy stored within the tuber is directed toward developing the new plant.