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Overwintering Dahlias - What I do
The dahlia is a plant that originates in Mexico - there it grows wild and the Aztec name was acocoxóchitl.
It is suited to warmer climates and in most parts of the UK needs to be protected from the frosts of winter. There are a couple of main methods of keeping them away from frost. Some people lift the tubers and store them in sheds, cellars or garages. However, if I take anything indoors it tends to die or be eaten by mice, so this is what I do instead.
That Aztec name acocoxóchitl gives the clue to why I think most dahlias freeze in the winter. The word translates as 'hollow stemmed plant' or 'water carrier' - and the stems were used to carry water on journeys.
In the garden - if you chop the tops off your dahlias to neaten them up over the winter, you are creating hollow straws for rain water to be channelled straight down to the tuber underground, pretty much guaranteeing that it will freeze over the winter and turn to mush.
Instead I do nothing for the first 3 weeks after frost - I just leave the tattered mush of a plant to gently collapse, closing up the tops of the stems as it does so.
Then I remove any broken stems - the ones that snap and fall over - and pour a dry mulch of some sort over the tuber. This is usually spent compost from my tomato plants and garden pots. The rest of the plant remains intact - flowers and collapsed stems stopping the water from getting into the stems. I check the mulch after heavy rain to make sure it hasn't been washed away, and top up if needed.
Last year I added a second mulch of sheep fleeces to half the dahlias to stop the need for topping up. Many fleeces have no commercial value at the moment, something that seems ridiculous, and farmers are often glad to have them used. Wool fleeces make a great insulated and slug repellent covering in the garden*.
To keep the dahlias cosy I simply stuffed the fleeces carefully around the bases and then left them alone until late March/April when the soil begins to warm again.
This does mean that you have a garden that looks a little as though there are rather scruffy sheep lying all over it - and that may bother you. Last year both lots of dahlias came through the winter fine here - the ones with the fleece mulch and the ones with the old compost mulch - but though we had a lot of rain (and I did need to top up the compost mulch a couple of times), it wasn't a particularly frozen winter.
* I have been offered a large bag of fleeces by a neighbour, so I am intending to cover all my fallow vegetable beds with them over winter, possibly over a layer of manure and grass clippings to make a kind of sheet mulch. They are also wonderful for planting hedges and trees into as they keep the ground weed free and gradually rot down. The ones I put on the beds for the winter will probably be reused in some tree planting we have planned for the Spring.