I live in midtown Toronto Can I leave my dahlia bulbs in the ground with compost and mulch over the winter?
Thank you for contacting Toronto Master Gardeners with your question about overwintering dahlias.
In researching your question, the advice is that dahlias (Dahlia spp.) need to be dug up and stored as tubers for the winter in plant hardiness zones such as in Toronto. Leaving them in the ground leads to them freezing and rotting.
A number of websites walk the reader through the steps involved in this process. In particular, these two articles, Winter Storage for Dahlia Tubers from Cornell University and Overwintering Dahlias from the Peterborough and Area Master Gardeners, are both clear and easy-to-follow.
Authors all seem to agree that storing dahlia tubers over the winter is about finding the right balance of temperature and humidity. Temperatures should be between 7-10 degrees C (45-50 degrees F) and relative humidity of 75-85%. If it is too dry, as is often the case in Ontario, the tubers will shrivel, but if it is too wet, they will rot.
Once winter is coming to an end, this article How to Grow Dahlias from the Royal Horticultural Society provides guidelines about how to grow your dahlias indoors from the tubers and how to transition them outside once spring temperatures are warm and there is no longer any chance of frost (mid-to-late May in Toronto).
All the best success with this dahlia project!