Can dahlias be overwintered in the ground in Denver?
I have a large dahlia L-shaped (16'x20') garden enclosed by a 48" wall on the east and the house on the north (see photo). It gets wonderful sun throughout the day and is protected by all but the strongest winds. I've had great luck growing a variety, but mostly dinner plate dahlias. Last year I dug up all the bulbs & replanted in the spring. A few did not come back, but they were not quite a lovely as the previous year, when first planted. However, I forgot one set of dinner plate tubers in the ground planted in the back - west facing yard, with no protection on any side. Miraculously it came back and gave me amazing blooms. So my question is: next year I am planting all new tubers in April/ May in my front patio garden. Could I protect the tubers in the ground next fall, with enough mulch or other means, to keep them IN THE GROUND and allow them to overwinter? If so, is there anything in particular you could suggest for protection?
Arapahoe County Colorado flowers: annuals and herbaceous perennials
Thank you for the great photographs. You certainly did have beautiful dahlias last summer!
The reason it is recommended to dig up dahlias in the autumn is that the Denver area falls into the USDA climate zone 5 which can experience minimum winter temperatures of -20 degrees. While we've been lucky enough to have several mild winters back to back the last few years (probably the reason your bulbs survived), there is no guarantee that we won't have a -20 degree cold spell this coming winter. At that temperature any dahlia bulbs left in the ground would be frozen and wouldn't survive. Mulch would probably not provide enough protection if it gets very cold.
If you decide to gamble and leave the bulbs in place, it would be a good idea to give them water if we have prolonged periods of warmer than average weather with no precipitation. Lack of water could be the cause of some of your bulbs not surviving last winter.
I recommend that you dig up your bulbs and place them in a cool, dry place like a basement over the winter. Check every few months to make sure they are not drying out too much. If they are very dry, you can spritz them with a light spray of water. Plant them next spring after danger of frost has passed.
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