As winter settles in and the colder months approach, gardeners are faced with the tricky decision about how to look after dahlias to ensure they make it through the chill and come back strong in the spring. Over the years, I’ve found a method that works well for me: leaving my dahlias in the ground but covering them with a thick layer of straw mulch. Let me walk you through how I care for my dahlias during the winter and share some tips on how to ensure they survive the cold months.
Leaving Dahlias in the Ground: Why I Choose This Method
When it comes to overwintering dahlias, there are two main approaches: digging up the tubers and storing them indoors or leaving them in the ground. For a long time, I used to dig them up, but over time I’ve learned that leaving them in the ground can be just as effective, as long as you take the right precautions.
A few years ago, I dug up all my favourite dahlias from my main flower patch and carefully stored them in a shed at the farm. I left about 10 plants in the ground that I wasn't too bothered about. Just before Christmas, we had a week of consistently below freezing weather, and unfortunately the dahlias in the shed all froze, which completely killed them. In the spring, I was surprised to see the dahlias that I had left in the ground came up with no problems; I think the layer of snow that covered them had insulated them against the cold.
Apart from the reduced workload of leaving the dahlias in the ground, I do find that they come up earlier than any tubers that are planted directly in late spring. The plants can also grow bigger and have more blooms as they have not been disturbed by being dug up.
Covering Dahlias with Straw Mulch
The key to leaving dahlias in the ground through winter is ensuring that they stay insulated and protected from the freezing temperatures - otherwise they go to mush (imagine what happens if you put a sweet potato in the freezer!). After the first frost, I cut back the stems of my dahlias to a few inches above the soil level. Leaving a bit of stem is helpful as it helps to trap warmer air above the crown of the plant.
Then, I cover the entire planting area with a thick layer of straw mulch. I usually apply about 6 to 8 inches of mulch. This layer serves several purposes:
- Insulation: The straw helps regulate the temperature around the tubers, keeping them warmer than the surrounding soil and preventing freezing.
- Moisture Retention: It keeps moisture in the soil, which is important for the health of the tubers during the winter months.
- Organic Matter: The straw breaks down eventually and adds nutrients to the soil, improving it's structure.
I make sure to spread the straw evenly over the entire bed, covering every part of the dahlias’ root system. I also add a tarpaulin over the top to reduce the rain falling on the ground and stop the straw blowing away.
Conditions Needed for Storing Dahlias in the Ground
While covering my dahlias with straw mulch works for me, it’s important to ensure that the soil is well-drained and that the conditions aren’t too wet, as this can cause the tubers to rot. Before I even think about leaving my dahlias in the ground for the winter, I would make sure the planting site has good drainage. In future, if I had a particularly rainy season I might choose to dig up the tubers instead. Luckily my patch is on a slope so water drains away quite well.
Good drainage is crucial because if water accumulates around the tubers, it can cause them to rot, even with mulch on top. Consider your soil type; clay holds moisture like nothing else so could increase the chance of the tubers rotting. As I am planting in no-dig raised beds on a slope, I am fairly confident that keeping them in the ground is a good gamble.
If you don't have easy access to straw, other mulches could work; you could try putting dried leaves or a layer of compost over the dahlias. If you live in an area with frequent snow or rain, make sure the mulch doesn’t become soggy, as this will negate the insulation benefit; this is why I am adding tarpaulin over most of my dahlias.
When Spring Arrives
Once the weather warms up, there is always a sense of anticipation to see if the dahlias have made it through the winter. It is fairly simple to pull back some of the soil to see whether there is a lovely, firm tuber underneath or a pile of mush. This spring, I was concerned that my dahlias were not coming up at all. I pulled back the straw to see that the plants were throwing up loads of shoots, but these were being munched by slugs. I removed the straw to deter the slugs and soon enough, the shoots were growing strong.
If the weather has been particularly wet or the tubers show signs of rot, I may need to replace a few, but overall, this method of overwintering dahlias has worked beautifully for me. Every few years, I will lift some of the dahlias as this allows me to divide them; tubers left in the ground for too long can become congested which leas to lots of green growth but fewer blooms.