When it comes to winterizing dahlia tubers, there are now two schools of thought Is it necessary to dig up dahlias this time of the year or can they stay in the ground year round ?
Digging and storing dahlia tubers can be a lot of work, but leaving them in the ground often does them in. Extreme cold sometimes kills them but even in mild winters, the tubers often rot from our winter rain.
The traditional method is to lift them and then store in a cool dry, frost–free position, while the second – slightly more risky way – is to leave them where they are, but with the addition of extra insulation.
I leave my tubers in every winter, but I have raised beds with very good drainage. In addition, I mulch the tubers over winter with compost or leaves. But, if your soil tends to be moist or wet all winter, then you may find it necessary to winter the tubers indoors.
I almost always leave my tubers in the ground, and rarely lose one. That’s because after I cut the dahlia stems to the ground in fall, I cover them with a thick layer of evergreen-fern fronds. The fronds make great insulators, and just as importantly, they also repel water.
The best fronds for this job come from our native sword fern. Usually I layer the fronds at least 6 inches deep, and put a rock on top to make sure they don’t blow away.
I try to remove the fronds in early to mid-April and then keep the fingers crossed. With a bit of luck, new dahlia stalks emerge dry and happy.
This year I grew some of my dahlia tubers in containers for the following reasons:
1) Winter is not an issue. Containers can be easily stored out of the weather to avoid over saturation. There doesn’t seem to be any greater risk for cold damage to the tubers in containers than in the ground, at least in my rather mild area.
2) Soils in containers warm up faster than ground soil does. Growth begins earlier and you have a larger, more robust plant earlier in the season than one that has been left in the ground over winter or freshly planted in late spring.
3) Since dahlias in the ground are late to appear in spring, one runs the risk of damaging buried tubers with other gardening activities, specially if not clearly labeled. Not a problem with containerized plants.
4)I also find it easier to monitor for slugs with container grown plants and slugs can do considerable damage to emerging dahlia growth. I can also regulate fertilization better with the containers.
I usually use a 3G black nursery pot, sometimes a squat 5G. Depends on the ultimate size of the dahlia I’m growing
One bit of advice if you decide to dig your dahlias.Do not dig the dahlias too early. If you do they will be “green,” meaning they have not had time to harden off and prepare themselves for winter, so they may not survive storage.
Wait until two weeks after a killing frost to dig them. If we haven’t had a killing frost by mid-November, it would probably be safe to dig them then.
Allow the tops to be nipped by the frost. That shuts everything down. Then dig and clean them by hand, but don’t use water, and dust each one with powdered sulfur for insect and disease protection.
Store the dahlias in paper bags in a dark, cool location that will not freeze. Plant them again in the spring when the danger of frost is past.
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Yours truly, Polly – Organic Gardener