Here it is early November and dahlias are still blooming! Dahlias provide such great color in late summer and fall, it is no wonder they are so popular with gardeners. They are easy to grow but do need some extra care in preparation for winter. Generally, with proper pruning and mulching, they will survive our winters in the ground. However, if you have a special one, or are gardening in a colder zone than Portland, I would not take a chance on leaving it in the ground. Regardless of whether you leave the tubers in the ground or dig them, wait until we have had a frost.
If you are going to dig your dahlia tubers, cut the stems off to about 4 inches above the ground and very carefully dig the tubers out of the ground. If you are a first-time dahlia grower, you will probably be very surprised at how large the clump of tubers will be after planting just one tuber in the spring. Hose the soil off the tubers and let them dry for several days. Put the tubers in a cardboard box or other container and completely cover them with peat moss or perlite. Store the container in a cool dark place like a garage. It is a good idea to check them a couple of times during the winter to be sure that none of the tubers are rotting and if they are, simply cut out the rotted tuber. Wait until spring and all danger of frost is over before planting your tubers in the ground.
If you are going to leave the tubers in the ground, with some simple steps, they will usually survive our winters. Often the winter damage can be more from poor drainage rather than the cold. Dahlias have hollow stems and water must be prevented from getting into the stem. When we have freezing temperatures, the water will freeze and rupture the cells, killing the plant.
One method is to put a plastic baggie over the stems to keep water from getting into the hollow stems. Use a rubber band or plant tie to keep the baggie in place. Some gardeners will use a sheet of bubble wrap and securely lay it over the tubers. The idea here is to prevent any water from getting into the stem.
Another method is to cut the hollow stems back to a point where they are not hollow. This cut will be below the soil and will probably be close to the actual tuber. Without the hollow stem to be concerned with, this can lessen the chance that water will be an issue provided the plant has good drainage and no standing water around it. Adding pumice or perlite can help with drainage.
With either of these methods, once the cuts have been made, replace the soil back over the top and apply a mulch of garden compost of 4-5 inches over the tubers. This should act as sufficient insulation for winter protection in our Willamette Valley area.
While dahlias may require a little bit of extra work at this time of year, for all the color they provide, I think the time required is well spent.