It’s one of the most common questions I hear asked about dahlia growing the world over. Is it absolutely necessary to dig up tubers, or can I leave them in the soil over winter? They are the most sensational flowers with a long flowering season (mine are still going in early November), but they often demand special treatment.
However, having grown dahlias for many years, I can safely say that you can save yourself a little effort at this time of year, as there are many instances where you really don’t have to dig them up.
There are, of course, some caveats, but in most cases, as long as you follow a few simple steps to keep them safe and snug in the soil, overwintering dahlias this way really works. Tubers will survive and happily bloom again the following year. Here’s my tried and tested method for leaving dahlias in the soil over winter.
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What to do to protect dahlia tubers in the soil over winter
(Image credit: Getty Images/Jacky Parker Photography)
The reason we tend to dig up dahlias and store them out of the soil over winter is to keep them dry and frost-free, and prevent them from rotting over winter, as both conditions can damage and often kill off tubers.
The key thing to be aware of when leaving dahlia tubers in the soil is that they need to be kept warm, so you almost need to think of this as tucking them up in a cozy bed for the winter.
According to Maryland-based cottage garden designer Caroline Maurer, wait until the first frosts, as you would if you were planning to lift your tubers, and then cut down your dahlias to two inches above the soil.
Something Caroline suggests that I haven’t done in the past is to lay the stems on top of your beds, whether your dahlias are in the ground or in raised beds. The decaying foliage will act as an extra layer of insulation and will rot back into the soil, helping to add nutrients.
What I always do, and Caroline also suggests, is to cover your beds with two feet of straw. You may need quite a bit depending on the size of your beds, and you can purchase straw in bulk from Amazon.
You can also add grass clippings and fallen leaves to the straw, which will help the composting process and insulate the beds.
Lastly, Caroline advises covering the entire bed in a waterproof tarpaulin, (I like this heavy duty one from Amazon for its more natural brown shade). Doing this will help to keep the tubers dry, as sitting in soggy, waterlogged soil will cause the tubers to rot.
This might not be such a huge concern if growing dahlias in raised beds, which have really good drainage, but definitely something to consider if you are growing them in beds and have clay soil.
When you should dig up and store your dahlias
(Image credit: Alamy/Olga Seyfutdinova)
If you live in US hardiness zones 2 – 6 and experience particularly long, harsh winters with protracted frosts, it would be a better idea to dig up your dahlia tubers and store them over the winter. Otherwise, you would have to treat them as annuals and buy new tubers or plants the following year.
The other reason you might choose to dig them up anyway would be to divide dahlias and multiply plants for your yard or cutting patch.
I usually dig up a few chosen plants every year. I tend to pick the ones that have performed the best, so that I can divide the tubers, pot them up in the spring and plant out again once the risk of frost has passed. Doing this triples the number of plants I grow without spending a dime, so it’s well worth giving it a try.
Hopefully, knowing you can leave dahlias in the soil will save you a lot of time and effort this winter. There are a lot of growing tricks when it comes to getting more from these exquisite flowers, including pinching out plants to double your dahlias and picking the right fertilizer for dahlias, too.
Garden essentials when leaving dahlias in the soil over winter
You need quite a lot of straw when covering your tubers in beds for the winter, so this bulk 20lb buy is ideal for large areas.
Fiskars Bypass Pruning Shears
For cutting down this year’s plants, you need a reliable pair of pruning shears, and the Fiskars ones are always my go-to for a wide range of gardening jobs.
If you want to cover your straw with an extra layer of insulation, this is what I would opt for. Bark mulch is great for the soil as it locks in soil and nutrients.
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