Overwintering Plants: Your Questions Answered 

by Marissa L. Tomasic

Frost greets us each morning and the trees have dropped their leaves. Another season has come and gone as we find ourselves at the start of Winter. Many of our recent customers have been asking the right kinds of questions about their plants and what to do with them in the colder months. Let's review some of the common questions and subsequent methods of preserving plant life over the winter. 

"I have perennials in containers. Can I keep them on my porch?" 

The short answer is no. Perennials, although hardy in our zone, need the insulation that the ground provides for their root systems. Roots are much less cold-hardy than shoots. Taking them inside is not the answer however, as they need the dormant "rest" period that cold temperatures provide. Cold but consistent temperature is key, as repeated freezing and thawing will incur damage to the plant's roots. Ideally, perennials should be planted while the ground is still workable. If you aren't sure where you want to plant, you can always put the potted plant right in the ground and then easily find and move it in the Spring.  

If you absolutely do not want to put your plants in the ground, the next best overwintering method is to keep potted plants in an unheated, indoor space where temperatures will remain consistent and cold. Most perennials that are hardy in our zone have a root-hardiness temperature between 30° and 34°F.  If your storage area will experience low temperatures much below 30°F, consider placing pots in cardboard boxes and filling the boxes with hay or other insulating material. Light is not needed, as the plants will not be growing. Remove any dead foliage before storing to reduce the opportunity for fungus and molds to develop. 

Potted perennials should be watered before they are stored, but only the soil should be wet, not any of the remaining foliage. Soil should be kept from drying out completely, but too much moisture will cause root rot. Check the pots every two to three weeks and water as needed. Make sure there is adequate ventilation in your storage area. Lastly, keep an eye on rodent populations and set out traps or repellants as needed to prevent critters from munching on your perennial roots. 

"Can I buy a small evergreen for Christmas and keep it on my porch until I plant it in Spring?" 

You can do this with a select few evergreen varieties, as long as you keep the soil watered. Water provides insulation for roots. Water always has a temperature above 32°F, and when water freezes, it acts as insulation, protecting root systems from extreme temperature fluctuations. Evergreens that do best in containers are Boxwood, Dwarf Alberta Spruce and Dwarf Blue Spruce. If you wish to keep one of these specimens in a container outside, just make sure you keep the soil moist throughout the season. If the shrub is in a spot where it will be exposed to high winds or bright sun, it's a good idea to wrap the foliage in burlap or spray with an anti-desiccant, like Wilt-Pruf, to prevent winter burn. (Choose burlap instead of Wilt-Pruf for Blue Spruce - the former can turn the blue hued needles green temporarily.) 

"Can I spray my arborvitae with Wil-Pruf?"  

There is some belief among the general gardening public that Wilt-Pruf should only be used on broad-leaf evergreens, but this is simply untrue. Wilt-Pruf and other anti-desiccant products are labeled for use on all evergreens. (The manufacturer does warn that anti-desiccants can discolor Blue Spruce.)  

"I'm wrapping my shrubs in burlap; how may layers do I need?" 

Burlap or other protective coverings are a great way to protect evergreens from damaging winter burn. Simply wrap the shrub in one complete layer of burlap (two layers or more isn't necessary) and secure in place with twine. The burlap is certainly porous enough to allow for plenty of air circulation throughout the winter. 

After seeing the damage that a severe winter like last year's can cause for our trees and shrubs many people are trying to take steps to keep their plantings alive and healthy throughout the season. Long-range forecasts are predicting another very cold, very snowy winter again this year, so a bit of precaution now will pay off greatly in the Spring as far as minimizing winter damage and landscape casualties. If you have any other questions on protecting your landscape this winter, just give us a call.