The two-zones-colder rule for plants that winter in pots

The zone on a plant tag describes a plant growing in the ground. Iowa State Extension points out that roots are far less cold hardy than the parts above ground, and that the mix in a pot left above ground can drop into the single digits.

Source: Iowa State Extension, overwintering unplanted trees, shrubs and perennials, checked October 7, 2026.

Your USDA zone is built from the average coldest winter night, in bands of 10°F. Two whole zones are 20°F, so the rule gives a potted plant a margin of about 20°F on the coldest night of an average winter. That's arithmetic on the zone bands, not a measured temperature for your pot.

Source: USDA Plant Hardiness Zone Map (2023), checked October 6, 2026.

• 2 zonescolder than your yard, for a pot that stays outIllinois Extension, overwintering potted plants, October 6, 2026
• 10°Fper whole USDA zoneUSDA Plant Hardiness Zone Map (2023), October 6, 2026
• 20°Fof margin on an average coldest night: two zones of 10°FArithmetic
• Single digitshow cold the mix in a pot above ground can get, in °FIowa State Extension, overwintering unplanted trees, shrubs and perennials, October 7, 2026

Zone bands from the 2023 USDA Plant Hardiness Zone Map; two zones colder from Illinois Extension. Drop the a or b from your zone before you look it up.

Source: USDA Plant Hardiness Zone Map (2023), checked October 6, 2026; Illinois Extension, overwintering potted plants, checked October 6, 2026.

1. 1

   Look up your zone

   Type your zip code into the zone finder. It returns your zone from the 2023 zip-code listing, letter and all.

   Source: PRISM 2023 USDA hardiness zone GIS datasets and zip-code listing, checked October 6, 2026.

2. 2

   Take two off the whole number

   Drop the letter and subtract two. Zone 7b becomes 7, and 7 minus 2 is 5. That's the zone the tag should reach.

3. 3

   Read the low end of the tag

   Find the zone range on the tag or the nursery label. If its lowest zone is at or below your number, the plant can stay out in its pot with the usual protection.

4. 4

   Choose a bigger pot

   Illinois Extension says large containers protect roots from cold better than small ones. If you're torn between two sizes for a plant that stays out, take the bigger one.

   Source: Illinois Extension, overwintering perennial patio containers, checked October 7, 2026.

5. 5

   Plan B if the tag falls short

   A plant that doesn't pass can still come through winter. Bury the pot in a bed or move it to an unheated garage, the plans in the bury, group or garage guide.

   Source: Illinois Extension, overwintering potted plants, checked October 6, 2026.

• The pot spends winter in an unheated garage or shed. Iowa State Extension suggests about 20 to 45°F for that kind of storage, which keeps the roots well above the outdoor lows.
• The pot is buried in a bed. Illinois Extension says the soil around a buried pot insulates it, much like a plant in the ground.
• Pots are grouped on the north or east side of a building and mulched heavily. Illinois Extension lists that as a way to protect pots left outside, so a plant right at the edge of the rule has better odds.
• Your zone may have moved. USDA said about half of the country shifted to the next warmer half zone on the 2023 map, so look your zip code up again if you last checked in 2012.

Source: Iowa State Extension, overwintering unplanted trees, shrubs and perennials, checked October 7, 2026; Illinois Extension, overwintering potted plants, checked October 6, 2026; USDA ARS news release, November 15, 2023, checked October 6, 2026.

A mild winter or two doesn't prove a plant passes. The zone is an average of the coldest nights, and USDA notes that any winter can be colder than that average.

Source: USDA ARS, how to use the zone maps, checked October 6, 2026.