4 Nov 2006 - Dahlias - To Lift or not to Lift?

Every garden magazine that I pick up at the moment is debating whether or not dahlias can be left in the ground over winter. The consensus seems to be that you can but that it depends where you garden and the soil that you garden in?

As we grow quite a lot of dahlias, about 30 plants on the allotment, we decided a few years ago to leave them in the ground for winter. We just didn't have anywhere dry to store that number of tubers. This experiment was very successful and one that we have repeated over the years. We have lost one or two plants over this time but probably no more than we would have lost if we had lifted them. Indeed previously when we did lift them every year we actually lost more tubers.

This weekend 4 November as expected we found that the frost during the week had turned the scene below,

to this.

1. Firstly we cut all the dead growth down to the ground. The straw that you can see surrounding the plants was left over from last year. The straw was left on the ground to act as a mulch. Only straw covering the new growth was removed. Most of the straw had decomposed and would as a result improve the soil structure.
   
   

   

   
   
   
2. Then  a thick layer of straw was placed over the dahlia plants. The straw was pulled apart to allow air around the plants and among the blanket of straw.
   
   
   

   

   
   
   
3. As our soil is very heavy clay and one of the problems could be that the tubers become so wet that they rot, the final task was to cover the whole thing with thick black plastic.
   
   
   

   

We have tried various plastic covering including the bubble wrap that is often used to insulate greenhouses. In fact one year we used a different type of plastic on each of the three dahlia beds. Thinner plastic disintegrated over the winter. Black plastic also seemed to keep the plants warmer. When we uncovered the plants that had been covered with the strong black plastic we found that the tubers had made more growth than those covered by bubble wrap. We also found that the conditions under the black plastic were so favourable that one year a whole colony on bumble bees have made nests under the straw. Fortunately they didn't cause us any problems.