Some gardeners choose to leave their Dahlia tubers in the ground over the winter. In certain regions, this can be very risky but if you have decided to follow this path, make sure that your Dahlias are growing in very well drained soil and apply 6"-12" inches of mulch to the planting area before the ground freezes as insulation for the tubers. If you leave the tubers in the ground, you run risk of damage by moles or voles, plus if the ground receives lots of rain, then freezes you could lose your tubers to rot. If your dahlias are planted next to your house in a well drained area the heat from the house should keep them from freezing so they can be left in the ground.

Here, in the Pacific Northwest, the best plan is to dig the tubers and store them in a cool, dry place for the winter.
After the dahlias experience a light to medium frost they turn dark colored and the blooms have blemishes from the frost. At this point, all the dahlias should be cut down to 4” to 6” in height from the ground. Leave them for 2 weeks because we generally have another nice spell of weather and during this time the dahlias begin to put out new growth or what’s called the “eyes.” The tubers will be easiest to divide if they are harvested after this two week period.

There are TWO recommended methods of digging tubers:
One: Use a garden fork to prevent damage, dig a circle about 12 inches around the plant stub, and lift the clump carefully out of the ground. (Be careful not to damage the tender new tubers) If the neck of a dahlia tuber is broken, normally, it will NOT grow.
Or, Two: Use a shovel, shove the blade into the ground about 6-8” from the stalk of the dahlia plant. You will cut part of the tuber and root off with the shovel. This does not cause any harm because a dahlia tuber does not require the entire tuber in order to grow. Dahlia tubers grow to all different sizes—little and big, long and short, fat and skinny. Size does not matter for tubers.

I, personally, use a potato fork and dig down, lifting one side, going down the entire row. Then, go back up the row on the other side, lifting the tubers from the ground and placing them on the top of the garden soil until I have two rows completely dug. Then, use a gentle spray from your hose to clean and remove the remaining soil from the clump. Allow the clump to dry for a day in a cool dry place. You are now ready to divide the clump.

You can also store the clump(s) and do your dividing in the spring. The stems do become very hard which makes it difficult to cut the tubers apart. You also run the risk of mold and often the stems of dahlias will rot down the stem into the tubers and they will not survive. To be on the safe side, I always dividing in the Fall. To produce a new plant, each tuber must have an eye (the new growth bud) which appears at the point where the tuber connects to the main stalk. (Each tuber on the clump will not necessarily have an eye.)
Digging, dividing & storing dahlia tubers.
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