Winter garden care: mulch, cold frames and the 28°F hard-freeze date

Last updated October 28, 2026 · Sources: cited below

Winter garden care is mostly about timing. Mulch goes on too early and plants stay soft; a cold frame left shut on a sunny day cooks its lettuce; leafy crops left unprotected stop at the first hard freeze. The dates that drive all of this are on your city page: the median first fall frost (32°F) and, more important for winter, the median first hard freeze (28°F). This guide explains what to do around each, with the dates for 417 U.S. cities on this site.

Frost vs. hard freeze: the 28°F date

South Dakota State University Extension defines a freeze as air at 32°F or below, and a “hard” or “killing” freeze as 28°F or lower, which can kill most non-dormant plants.¹ Iowa State adds a practical point: sheets, blankets or row cover help through frost and light freezes, but once temperatures drop below about 28°F, covering is typically no longer effective; cool-season vegetables such as spinach, kale, collards and lettuce often survive down to 28°F or sometimes 26°F.² So the 28°F date is roughly when unprotected leafy crops are finished and a cold frame or harvest becomes the plan.

On our data, the median first 28°F freeze comes 12 days after the median first 32°F frost (median across 417 cities with both dates, NOAA 1991–2020 normals). By the end of October, half the years have already seen a hard freeze in 91 of 417 cities; by the end of November, in 263. Here are some examples:

The spring column is the other half of the calendar: it tells you roughly when it becomes safe to pull winter mulch back and open cold frames for good. Look up your own city on the city list or the ZIP zone finder.

Winter mulch: wait for the soil to freeze

Winter mulch on perennials is not there to keep the soil warm — it is there to keep it frozen. Alternating freezing and thawing lifts (“heaves”) shallow-rooted plants out of the ground and breaks their roots, and mid-winter warm spells can coax plants out of dormancy too early.³

• When: after the soil has begun to freeze, not during fall cleanup. The University of Minnesota warns that mulch applied before the soil freezes can encourage growth under the mulch that weakens plants; it can even go on top of snow.³ Iowa State puts that point at “typically by late November in much of Iowa”.⁴ On your city page, a few weeks after the 28°F hard-freeze date is a reasonable signal to start checking the soil.
• How much: Iowa State suggests about four inches over the crowns of perennials that tend to heave, and 6–8 inches for tender or late-planted perennials;⁴ Minnesota suggests five or six inches of leaves, compost, corn stalks or straw for perennials and bulbs.³
• What: loose, weed-free straw and pine needles. Iowa State calls leaves a poor winter mulch for strawberries and herbaceous perennials because they mat down.⁵ Wood chips and bark are better kept between plants year-round.³
• Removal: pull mulch back in early spring as the top layer thaws — mid-March in much of Iowa — to avoid crown rot, but keep it nearby in case of a late freeze.³⁴

In zones 8 and warmer, the soil may never freeze, and heaving is rarely an issue; ordinary year-round mulch is enough for hardy plants.

Cold frames: winter greens without a greenhouse

A cold frame is a bottomless box with a clear lid. The sun warms the soil inside during the day, and the soil gives the heat back at night. Iowa State says a frame extends the season by two to four weeks in fall and spring, and that a fall frame closed on frosty nights can keep cool-season crops producing well into November in most Iowa seasons.⁶ South Dakota State lists lettuce, spinach, arugula, kale, Asian greens, brassicas and most root crops as good cold-frame crops.⁷

• Venting is the main job. Iowa State recommends raising the lid when the inside air passes 85°F and closing it before sunset.⁶ Virginia Cooperative Extension says venting matters most on clear, sunny days above 45°F outside.⁸ A cheap thermometer inside the frame takes the guesswork out.
• For cold snaps, throw blankets or burlap sacks of leaves over the frame at night, or stack straw bales around it.⁸
• In deep winter, keep the lid closed more often, add a light mulch around the plants, and brush snow off the glazing so light gets in.⁷
• Timing: sow fall crops so they are nearly full-size before days get short; in a cold frame they mostly hold rather than grow in midwinter. Our fall planting guide shows how to count back from your first frost.

A month-by-month example: Akron, Ohio (zone 6b)

• Around Oct 28 (median first frost): harvest the last tomatoes, peppers and beans; close cold frames at night.
• Around Nov 7 (median first 28°F freeze): finish harvesting leafy crops that are not under cover; cover or harvest the rest when a hard freeze is forecast.
• A few weeks later, once the top inch or two of soil stays frozen: mulch perennials, bulbs and strawberries with straw.
• Winter: vent the cold frame on sunny days; keep snow off the lid.
• Around Apr 13 (median last 28°F freeze in spring): start pulling mulch back gradually; keep it handy for late freezes until the last frost date.

Shift these steps to your own dates: every city page lists both 32°F and 28°F dates, and our guide to frost-date probabilities explains why a median is not a guarantee. Gardens in low spots can run colder than the official station — see microclimates.

More guides: How to read your hardiness zone · Frost dates explained · Why the 2023 hardiness zone map looks warmer · Microclimates · Succession and fall planting