Overwintering Container Plants on a Patio or Balcony

How to Overwinter Container Plants: Four Options

To overwinter container plants, protect the roots, because roots are far less cold hardy than stems and buds. There are four ways to do it. Grow plants rated two zones colder than yours and leave them out. Sink the pots into the ground. Group and insulate them where they stand. Or move dormant plants into an unheated garage or shed that stays between 20 and 45°F. Tender plants such as rosemary, bay and citrus are a separate case and come indoors before frost.

Start the work after the first hard frost, when the plant has lost most of its leaves. Protection applied earlier keeps a plant warm when it should be going dormant.

Why Roots in Pots Freeze

A plant in the ground has the whole soil mass around its roots. A plant in a pot has a few inches of potting mix and a thin wall. Iowa State University Extension notes that the temperature of the potting mix in a container left above ground may drop into the single digits in winter, and that single-digit temperatures may damage or destroy the root system. The top of the plant can look fine through all of it and then fail to leaf out in spring.

This is the reason behind the two zone rule. University of Illinois Extension advises selecting plants that are two zones different from your own if you want them to overwinter outdoors in containers. A gardener in zone 6 would choose shrubs and perennials rated to zone 4. A plant rated only to your own zone can still be kept, but it needs one of the protected methods below.

Pot size helps at the margin. A large volume of mix cools and warms more slowly than a small one, so combining several small pots into one large planter in fall gives the roots a steadier winter. Our guide to potting mix for containers covers what to fill it with.

Sink the Pot in the Ground

Where there is open soil, burying is the most reliable outdoor method, since soil is a good insulator. Dig a hole in a vegetable bed or a spare corner and sink the pot until it sits at the same level as the surrounding ground, then fill soil in around it as if you were planting. Iowa State recommends doing this within a few weeks after the first frost in the fall.

Finish with a loose cover. Place 6 to 12 inches of straw, pine needles or other loose mulch over the buried pots. In early spring, remove the straw after the extreme cold has passed but before the plants break dormancy, which is typically around mid-March in Iowa, and lift the pots several weeks before the last frost date.

Rabbits and rodents find mulched plants in winter. Iowa State suggests chicken wire fencing pinned to the ground and tall enough to block access after heavy snow.

Group and Insulate Pots on a Patio or Balcony

A balcony has no soil to dig, so the pots have to insulate one another. Move them against the building wall in a sheltered spot out of the wind and push them together into one tight cluster, with the smallest pots in the middle. Surround the cluster with straw, leaves or bark mulch, or with bags of potting mix stacked around the outside, and cover the soil surfaces. University of Nebraska-Lincoln Extension recommends a thick 6 to 12 inch layer of wood chips, leaves or straw over grouped containers.

For a single large tree or shrub that cannot be moved, Nebraska describes a wrapped cage. Stand a cylinder of wire fencing around the plant, tall enough to enclose it down to the base of the container, stuff it with straw or leaves, and wrap the outside. Cover the top of the soil with the same material. On a balcony, loose fill needs a wrap of burlap or netting to keep it from blowing away.

Two balcony cautions apply. Wet potting mix is heavy, so keep a cluster of large pots near the wall of the building instead of out at the railing. And lift pots onto feet or bricks so the drainage holes stay clear of ice on the slab.

Evergreens in pots keep their leaves and keep losing water through them. Give them a windbreak, such as a burlap screen, and pay closer attention to watering than you would for a leafless shrub.

Storing Pots in an Unheated Garage or Shed

Dormant trees, shrubs and perennials can spend winter in a moderately cold building. Iowa State and Illinois both give a range of 20 to 45°F. University of New Hampshire Extension narrows it for perennials, describing an unheated garage, shed or basement at 30 to 40°F as a perfect environment. For potted trees and shrubs, Nebraska advises the upper 30s or lower 40s and not below freezing.

Cold matters as much as protection. Many hardy plants need a cold dormancy to grow and bloom properly, so a heated basement or a warm attached garage is the wrong place. Put a thermometer next to the pots for the first few weeks and move them if the reading drifts above 45°F or sits below 20°F.

Stored plants still need water. Iowa State notes that it may be needed as often as once every two to three weeks if temperatures are above freezing. Check the mix with a finger and water lightly when it is dry.

Watering Through Winter

Roots continue to take up water after the top has gone dormant, and a pot that goes into winter dry is at greater risk. Keep outdoor pots well watered through fall. New Hampshire’s advice is to continue watering until the soil freezes, and through winter whenever the temperature is above 40°F.

Water in the morning on a mild day so the excess drains before night. Take saucers away for the season, because a pot standing in a tray of ice cannot drain. For plants that struggle with moisture at other times of year, see container plants wilting in wet soil.

Feeding stops much earlier than watering. Nebraska advises against fertilizing after mid-summer, so that growth hardens off before cold weather arrives.

Pots That Crack, and Tender Plants

Empty pots need protection too. Illinois Extension recommends removing the growing media from containers that are not in use and storing them upside down, because freeze and thaw cycles shorten their life. Porous clay and ceramic are the most likely to crack when wet mix freezes inside them, so plants that are staying outside are safer in plastic, resin, wood or metal.

Tender perennials and herbs do not belong in any of the four methods above. Rosemary, bay, citrus and similar plants need to come inside before frost. See overwintering rosemary, bay and tender herbs for light and temperature, and overwintering dahlias and tender bulbs for tubers that are lifted and stored dry.

Bringing Pots Out in Spring

Reverse the process in steps. Nebraska recommends unwrapping gradually, taking off the outer cover first and the stuffing later, and removing protection fully only once the danger of a late spring freeze is past. Buried pots come up once the danger of a hard frost has passed, or several weeks before the last frost date if they are still dormant, with the straw cover removed first around mid-March in a climate like Iowa’s.

Plants coming out of a dark garage should go to a shaded, sheltered spot for their first days outdoors, since new leaves that opened in low light scorch in full sun. Water thoroughly on the day they come out. If a plant is slow to leaf, slide it from the pot and look at the roots before discarding it. Firm, pale roots mean it is alive and late.