Q. Last year my favorite large container planter cracked over the winter. What can I do this year to keep the same thing from happening?
A. The short answer is, of course, move to Florida. However, the more accurate and useful answer may not be exactly the information you'd hoped for.
Ceramic and terra cotta planters absorb moisture and when temperatures fall below freezing, the moisture will expand and, with nowhere to go but out, the exterior of the planters will begin to spall; that is, chip or flake away. As temperatures fluctuate in the winter, the freezing and thawing create an accordion effect and the planter walls will deteriorate more and more.
When temperature changes are rapid and there is sufficient moisture to expand the planting medium, the planter may crack completely.
Plastic and composite planters will hold up better during winters, but given sufficient moisture and cold enough temperature, they may crack as well. Wood containers allow a measure of expansion, so there is less chance of damage to them. Concrete planters are the safest bet to retain their structural integrity.
Considering that in our Zone 5 area winters we experience many ups and downs from mild to minimal temperatures, a gardener's best avenue for protecting planters is to remove the soil, clean the planters and store them in the garage or other protected area for the duration. Enclosing them in a large plastic lawn bag will keep moisture out and discourage Mr. and Mrs. Rodent from saying, "Harry, what a nice place! Let's move in." Removing wads of mouse debris from your planters takes the edge off spring planting.
Given our winter conditions, it's difficult to keep any perennials alive over the winter, except for the hardiest plants in very large containers, such as half-barrel planters. Even in winter, small trees and shrubs and the fleshy roots of perennials need a quantum of moisture. Sufficient moisture for plants often means damage to planters. Storing one's planters is often the most practical solution.
If your favorite planters are such that they cannot be removed without great difficulty, wrapping them in plastic will provide a measure of protection, but not a guarantee that when you unwrap them in the spring they'll be in the lovely, butterfly and daisy festooned condition you wrapped so diligently.
-- Matt Steichmann
• Provided by Master Gardeners through the Master Gardener Answer Desk, Friendship Park Conservatory, Des Plaines, and University of Illinois Extension, North Cook Branch Office, Arlington Heights. Call (847) 298-3502 on Wednesdays or email email@example.com. Visit web.extension.illinois.edu/mg.