When to Cover Plants for Frost: Temperatures, Timing and City Dates

Cover tender plants when the forecast low is 36°F or colder on a clear, calm night, and always at 32°F or colder. Put the cover on in the late afternoon, let it reach the ground, and take it off the next morning once the air is above freezing. Below about 28°F a cover usually can’t save tender plants, so pick what you can and bring pots inside.^7,3

That leaves a short window. In a typical US ZIP code, the median first 32°F night and the median first 28°F hard freeze are only 11 days apart in NOAA’s 1991–2020 climate normals.¹ For what separates a frost from a freeze, and what each National Weather Service alert means, see frost vs. freeze.

What temperature to cover plants

Frost can form before the thermometer reaches 32°F, because leaves lose heat to a clear sky faster than the air does. The National Weather Service says frost often forms when the standard thermometer reads in the mid-30s,³ and its Memphis office uses a 36°F low as its estimate of a frost night.⁵

Sources: NWS glossary,² Iowa State^7,8 and UF/IFAS.¹² Below 28°F, says Iowa State’s frost guide,

covering is typically no longer effective at preventing cold damage

Duration matters too: covered plants can be damaged above 28°F when a freeze lasts 5 hours or more.⁷ Asked for a “magic number,” a UF/IFAS agent answered:

It’s more about how long it stays below freezing.

Which plants need covering

Purdue and Iowa State sort vegetables into three groups (they differ on a few, such as cauliflower).^9,8

Annual flowers and tropical plants belong with the tender group. Trees, shrubs, roses and established perennials need cold to go dormant and don’t need fall covers.⁷ Potted plants are the exception: container soil gets almost as cold as the air.¹³

Late frosts in spring

The same temperatures apply in spring, but fewer plants need help. Iowa State says the ones worth covering are vegetables, annuals and tropical plants set out early. The new shoots of tulips, daffodils and most perennials tolerate the low 30s and upper 20s°F.⁷

Blossoms and new leaves on fruit trees and shrubs can be damaged, but Iowa State calls covering them impractical: the plant survives, and only that year’s flowers or fruit suffer.⁷ The surest protection is timing. Iowa State advises waiting until after the typical frost-free date to plant tomatoes, peppers and green beans;⁷ our guide on when to plant tomatoes gives dates by city.

Frost advisory, freeze watch and freeze warning

The National Weather Service uses three alerts for plant-killing cold:^4,2

• Frost advisory: widespread frost expected, usually with lows in the mid-30s (33–36°F, per Iowa State).⁷ Cover tender plants.
• Freeze watch: widespread freezing temperatures possible in the next 24 to 36 hours. Get covers and weights ready.
• Freeze warning: below 32°F for an extended period over a large area, frost or not. Cover what you can save and harvest the rest.

These alerts run only during the growing season. NWS Binghamton says they normally end after a freeze in a forecast zone or two weeks after the median first fall freeze, and Northeast offices tested fixed end dates about 10 days after that median in fall 2023.⁶ Late in fall, watch the forecast low yourself. The frost dates tool explains frost vs. freeze dates.

How long covering helps: fall frost dates by city

Covers pay off between the first frosty nights and the first hard freeze. We measured that stretch for every US ZIP code from the nearest NOAA station.¹

A 36°F night comes a median 10 days before the first 32°F night, and the hard freeze 11 days after that.

Dates are medians except the “1 year in 10” column: a 10% chance of the first 32°F night before that date. For other places, look up your ZIP code’s frost and hard-freeze dates.

Across 37,766 ZIP codes, half get 10 to 13 days from first freeze to hard freeze; only 2.5% get 3 weeks or more. Spring is similar: the last 32°F night comes a median 12 days after the last 28°F night.

Early hard freezes happen too: in a typical ZIP code there is a 1-in-10 chance of a 28°F night by about 4 days before the median first-frost date.¹ As of early October, about a third of US ZIP codes have passed their 1-in-10 first-frost date, Minneapolis, Madison, Denver and Boise among them. The same 28°F date sets the deadline in our overseeding guide.

Why covers can’t stretch the season further

A cover only holds in the soil’s stored heat. Colorado State University Extension notes that a cold front can bring an advective freeze far below critical levels, where covers may not help. Clear, calm radiation frosts at the ends of the season are:¹¹

typically only a few degrees below critical levels, making crop protection worthwhile

Use covers for the first light frosts, not a hard freeze.

How to cover plants for frost

1. 1

   Water during the day

   Water dry beds a day or so ahead. Moist soil stores more heat, and drought-stressed plants suffer more cold injury; don’t leave it waterlogged.^13,12

2. 2

   Set up supports

   Use stakes, wire loops, PVC pipe or patio furniture to hold the cover off the leaves.⁷ Leaves can freeze where they touch a cover, and plastic burns them.^11,13

3. 3

   Cover in the late afternoon

   Get covers on before the soil loses the day’s heat. Drape them to the ground, since the warmth comes from the soil,¹³ and weigh the edges down with bricks, rocks or soil.¹⁰ Never tie a cover around the stem; extension agents call that a “lollipop.”¹²

   

4. 4

   Add warmth on colder nights

   Plastic over fabric holds in more heat, and water-filled milk jugs under the cover release it slowly. Holiday lights help only if they give off heat; some newer types don’t. Mind fire and shock risks.¹³

What to cover plants with

Frost cloth is row cover, rated by weight. The University of Maryland Extension gives these ranges:¹⁰

Open plastic by day: an hour of sun can push the air under a closed cover past 130°F.^11,12

When to uncover plants after frost

Uncover the next morning once the air is above 32°F.^7,12 Sheets and blankets must come off by day so the sun can rewarm the soil; replace them in the late afternoon if more cold is due. Light fabric row cover can stay on through a run of cold days.^11,10

If frost hits before you cover

Damaged leaves wilt and turn brown or mushy within a day or two. UF/IFAS advises waiting for new growth and not pruning perennials or shrubs until spring, since dead stems insulate the plant.¹²

Next: our frost and freeze guides, vegetable guides, tomato planting guide and fall garlic planting guide. The methodology page explains our frost dates.