Overwintering Container Plants on a Balcony: Zones and Covers

To overwinter container plants on a balcony, choose plants rated two zones colder than your own, because roots in a pot face the winter air. Group pots against the wall, water before the mix freezes, wrap pots or cover plants with frost cloth, and bring clay or ceramic pots and tender plants indoors.

This guide is part of what to grow on a balcony.

Hardiness zones for plants in pots

Hardiness zones assume a plant growing in the ground. The USDA Plant Hardiness Zone Map is based on the average annual extreme minimum winter temperature, in 10°F zones and 5°F half zones, and you can look up your zone by ZIP code. Penn State Extension explains that roots in a container lack the earth’s insulation and can reach the temperature of the winter air, so it suggests going two zones colder with plants in containers.

A plant rated for your zone is therefore not enough in a pot. In zone 6, for example, a pot that stays out all winter needs a plant rated to zone 4.

Pot size helps. Penn State says larger containers insulate roots better, and Illinois Extension says larger, thicker or insulated pots do a better job of protecting overwintering roots.

Why balconies are hard on plants in winter

Balconies leave pots open to wind on several sides. The Royal Horticultural Society says balconies are vulnerable to strong winds and suggests slatted or mesh screens to filter them, and Penn State Extension adds that evergreens benefit from a screen to block the wind. Our guide to balcony privacy screens covers screens you can fix without drilling.

Penn State suggests grouping pots close together in a protected location.

Clay and ceramic pots in winter

Clay and ceramic pots are the most likely to fail in a freeze. Penn State says it is best not to leave clay, ceramic or glazed pots exposed to the elements, since they may crack. Move them under shelter, or move the plant into a sturdier pot for the winter. Our guide to ceramic and terracotta planters covers their winter care in more detail, and self-watering planters explains what makers say about freezing reservoirs.

Frost cloth and plant covers

Frost cloth is sold as row cover: a white, lightweight, non-woven fabric made from spun-bonded polyester or polypropylene, as University of Maryland Extension describes it. Heavier grades hold more warmth but let through less light.

The figures above are from University of Maryland Extension. Colorado State University Extension gives these rules for any cover:

• Hold it off the leaves. Plants freeze where a cover touches them, so support it on a frame or hoops.
• Open it on sunny days. CSU says a closed cover must be opened at least a slit during the day, because the air under it can pass 130°F after an hour of sun.
• Fasten it for wind. CSU warns that clothespins do not hold in the wind. Tie the cover to its frame or the pot, inside the rail.

CSU also says an old sheet works as long as the fabric stays dry.

Wrapping pots and roots for winter

Wrapping the pot protects the roots, which are the part most at risk in a container. Penn State suggests covering pots with straw, shredded leaves, evergreen boughs or a thermal blanket, or wrapping them in bubble wrap. On a balcony, bubble wrap or a blanket tied around the pot is the tidier choice, since loose leaves blow away.

Mini greenhouses on a balcony

A mini greenhouse is a plastic cover on a frame, and CSU puts plastic on a frame at 3°F to more than 6°F of protection. The same CSU rules apply as for any cover: open it on sunny days, and fasten it so wind cannot lift it.

Moving container plants indoors

Moving container plants indoors is the surest protection for tender plants. Penn State says pots can also go into an unheated garage, shed or basement; check the moisture occasionally and do not overwater. Rosemary is a common example: Illinois Extension says it is not reliably hardy in zone 6 or colder and should come indoors before frost. See herbs in pots for other herbs.

Some plants are not worth the effort. Iowa State University Extension suggests treating strawberries in pots as annuals rather than overwintering them.

Spring care after winter covers

Spring care starts with the covers. Penn State says to remove them gradually rather than all at once. When you repot in spring, potting mix for containers explains how to refresh old mix, and vegetables in containers covers what to plant next.