Storing Dahlia Tubers Over Winter: Vermiculite vs Glad Wrap
Hello friends,
Back in July, I shared how I dug up, divided, and stored the seedling dahlias from my dahlia breeding project for winter. This year, I decided to try two different storage methods I'd learnt about during the Floret Flower Farming Workshop this year: storing tubers in vermiculite, and wrapping individual tubers in Glad Wrap.
Throughout winter and into spring, I periodically checked on my stored dahlia tubers, looking for signs of rot and moisture loss, as well as watching for the first signs of sprouting as the weather warmed up. Now that my dahlias are planted out in the garden, it's time to look back at how the two methods compared and what I learnt from trying them.
Watching for the First Signs of Growth:
In late winter, while checking on my stored dahlia tubers, I discovered that a couple of those wrapped in Glad Wrap had already begun to sprout. The tubers were in excellent condition, with no visible signs of shrivelling or rot. As they could no longer remain in storage once they began growing, I planted them in pots filled with potting mix and moved them into the glasshouse. At this stage, all the tubers stored in vermiculite were still dormant, although some were beginning to show signs of moisture loss.
As winter gave way to spring, I continued checking all the stored dahlia tubers weekly. More of those wrapped in Glad Wrap began to sprout, so I gradually planted them into pots of potting mix and moved them into the glasshouse. The tubers stored in vermiculite were slower to wake up, with sprouting only beginning later in the season.
By the time I was ready to plant the dahlia seedlings out into the garden in mid-October, most of the tubers stored in Glad Wrap were already growing in pots in the glasshouse, while most of those stored in vermiculite were only just beginning to wake up. In the photo below, you can see the difference between two tubers from the same dahlia seedling, 2025-33. The tuber stored in vermiculite is on the left and is a little more shrivelled, while the tuber stored in Glad Wrap is on the right. It has begun to sprout and remains in excellent condition.
From my observations, the tubers stored in Glad Wrap appeared to break dormancy earlier than those stored in vermiculite, giving them a head start on the new growing season.
Checking the tubers over winter
When it came to checking the two storage methods, I found the Glad Wrap method much easier to manage. I could see the tubers through the plastic without having to unwrap them, and I could gently feel them to check that they were still firm. It was a quick and easy way to keep an eye on their condition.
The vermiculite method, on the other hand, was much messier. To check each tuber, I had to tip the vermiculite out of the paper bag, find the tuber, and then carefully put everything back into storage. The vermiculite got everywhere, making quite a mess in the garage, and I'm still finding bits of it on the floor even now!
How well did the tubers keep?
The two methods also differed in how well they preserved the tubers over winter. Provided the tubers were thoroughly dry after dividing, those wrapped in Glad Wrap retained their moisture well, and I found no signs of rot among them.
In comparison, the tubers stored in vermiculite lost more moisture, and a couple developed rot. Based on this season's results, Glad Wrap provided better protection against moisture loss and rot in my collection of seedling dahlias.
Cost, storage space and identification
Another consideration was the cost and storage space required for each method. For me, the Glad Wrap method was essentially free, as I had acquired a huge roll of commercial Glad Wrap from Moore Wilsons in Wellington years ago, and I doubt I'll ever use it all in my life time.
I could write each seedling's name directly on the wrap and place a plant tag inside beside the tuber, making it easy to identify each one. I was also able to stack the individually wrapped tubers together in a plastic container, making the method very space-efficient.
The vermiculite method, on the other hand, required many bags of vermiculite to store all my dahlia seedlings. This made it a considerably more expensive option, and the bags took up much more room than the compact containers of wrapped tubers.
Overall, I found Glad Wrap easier to manage, more economical and more space-efficient. The tubers also retained their moisture better and began sprouting earlier in spring. For my dahlia collection, it was the more practical option in several important ways.
The Environmental Consideration
There is also an environmental consideration when comparing the two methods. Glad Wrap is a single-use plastic, while vermiculite is a naturally occurring mineral that is mined and heat-treated before being used in gardening. Although it could potentially be reused, it is not without its own environmental impact.
I’d be reluctant to reuse vermiculite from one season to the next because I wouldn’t want to risk carrying over rot or potential diseases, such as leaf and crown gall, to another batch of dahlia tubers. Using fresh vermiculite each year would add to the ongoing cost and the amount of material required.
After planting out my seedlings, I sent the cleaned used Glad Wrap to soft plastic recycling rather than putting it in the rubbish. While recycling provides an alternative to landfill, it doesn’t eliminate the environmental impact of using plastic.
Both methods have their advantages and drawbacks, and this is another factor to consider alongside cost, storage space and how well the tubers keep over winter.
What I've learnt from this trial
Comparing these two methods has given me a much clearer idea of what works best for storing my dahlia seedlings over winter. This season, Glad Wrap worked best for my collection, but it was useful to compare the methods for myself rather than simply relying on what I'd heard from other growers.
As this was only one season's trial, I'll be interested to see whether I get similar results when I lift and store the tubers again next autumn. For now, I'm looking forward to watching my seedlings grow and flower over the coming months, and continuing to develop my dahlia breeding project.
Have you tried either of these methods for storing dahlia tubers over winter? I'd love to hear about your experiences and what works well in your garden.
Have a wonderful day,
Julie-Ann
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