The Q&A Archives: Overwintering Perennials in Containers Outdoors

Question: I am growing perennials in containers, and most of them are hardy for this zone 6 climate. I want to overwinter these outdoors in their containers. How do I do this? I will be using sawdust for mulch. I plan to place the containers on the ground all together, side-by-side, and mound mulch around them. Should I cover the tops of the containers too? I am concerned about rot, especially with plants that have a basal rosette, such as digitalis, lobelia, etc. Is there any need to cover the tops of the plants if they are considered hardy? Also I have many 2-month-old perennials that I want to survive. How should these be treated over the winter? Lastly I am growing a lot of liatris and asclepias, both of which don't like wet feet. Should any plastic cover be put over them to decrease the moisture that they would otherwise receive during our wet fall, winter, and spring months? Here again, the downside would be lack of ventilation.

Answer: If these plants are for your garden, you might consider planting them in a holding bed as your first choice. Generally, place them in a sheltered spot, then surround them with insulating material, such as mulch or sawdust, up to the rims. Covering the crowns with any type of material that will pack when wet will invite rot. If you have a cold frame, place the pots inside and cover the frame with a polytunnel and possibly an insulating "blanket" of lightweight spun bonded material (allows air and water to pass through it) for added protection and a jump start in spring. A coldframe is usually suggested for very small plants. In any case, check periodically for rodent activity as well as fungal infection. For those plants requiring perfect drainage, in my experience the secret is in the drainage capacity of the potting mix used rather than in covering or not covering the plants. You might wish to consult with your local county extension office to see if they know how commercial growers in your area winter their perennials as well. Good luck with your project!

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