Broadfork™

How to

Harden off seedlings

The seedlings are three inches tall under the lights, and the garden is two weeks away. Take them outside before you plant them, a little more each day.

Grown in Ohio, zone 6a. Your frost dates move the calendar; the signals don't. Broadfork goes by where your garden is, not its zone: frost dates for your ZIP code in the US, or the ones you type in anywhere else.

Six minutes to read. The steps stand on their own.

1 to 2 weekshow long
plant-out daycount back from
45 °Fnot below

Before you start#

A seedling raised indoors has never felt wind, full sun or a cold night. Set it straight into the garden and its leaves scorch, brown or turn white, and it can wilt and die. The ones that live take longer to settle in and flower. Outside a few hours at a time, the plant thickens its skin, stiffens its stems and puts on root.

Start when the seedlings are three to six inches tall or have four to six leaves, one to two weeks before the day you plan to plant out. Count back from that day, not from the frost date.

Seedlings you buy are usually hardened already. The ones you need this for are your own, and most of all the tender crops: tomatoes, peppers, cucumbers, squash. Cabbage, broccoli and lettuce are tougher and go out sooner. Carrots, beets and radishes are sown where they grow and skip this altogether.

Have a spot picked: a cold frame is the easy way, and a porch, the shade under a shrub or a tree, or a wagon you can wheel in and out all do the job.

The steps#

  1. Day one: two or three hours, in shade, out of the wind. Pick a mild afternoon. Bring them in before the night cools.
  2. Each day, a little longer and a little more sun. Add an hour or two, and move the trays toward the light. By the second week they stand in full sun most of the day.
  3. Water less often, and check twice a day. Pots dry fast in sun and wind. Water when the surface is dry to the touch, never let them wilt, and hold the fertilizer.
  4. Skip a day for wind, a storm or a frost. A windy afternoon snaps stems. If one is coming, leave them in.
  5. The last few nights, leave them out, or don't. The data splits here: one way, the trays stay out overnight once nights stay mild; the other, they come in every night to the end. Cucumbers and melons stay out only if the night holds above 50 °F.
  6. Plant out on a cloudy day or late afternoon. Water the trays an hour before, plant, and water again to settle the soil around the roots.
hours outside each day day 1 day 14 2 to 3 hours 10 to 12 hours shade, out of the wind more sun each day full sun last few nights out, if mild plant out
  1. day 1Two or three hours in the afternoon, in shade, out of the wind. In before night.
  2. each dayA little longer and a little more sun.
  3. by the endTen to twelve hours, most of the day in full sun.
  4. last nightsOut overnight, if the nights hold above the floor for the crop.
  5. thenPlant out on a cloudy day or late afternoon.
Fourteen days, drawn to the published numbers. Two or three hours to start, ten to twelve by the end; shade first, then sun; nights indoors until the last few, if those nights stay mild. A week is the short version of the same shape.

Watch the thermometer#

Don't put tender seedlings out below 45 °F. Some of the data draws the line at 50.

Tomatoes are fussier: bring them in on days and nights under 60 °F, or the cold sets them back and the first fruit come out rough and scarred. Cucumbers, squash and melons lose yield below 55 °F.

The hardy crops take more, but not forever: cabbage and broccoli held too long below 40 °F can flower early. Freezing hurts even the hardy ones until they're hardened; after it, cabbage, broccoli and kale shrug off a light frost. On tomatoes the data disagrees: by one account a hardened plant usually rides out a light frost, and by another a light frost kills them. Don't test it.

32 °F 40 45 50 55 60 frost warm cabbage, broccoli flower early if held below this tender seedlings stay in below this; some data says 50 cucumbers, squash, melons lose yield below this tomatoes come in below this day or night freezing hurts anything not yet hardened
  1. 32 °FFreezing hurts anything not yet hardened.
  2. 40Cabbage and broccoli flower early if held below this.
  3. 45Tender seedlings stay in below this; some data says 50.
  4. 55Cucumbers, squash and melons lose yield below this.
  5. 60Tomatoes come in below this, day or night.
The floors. One scale, frost to warm, with each crop standing at its line. The number to remember is 45 °F; tomatoes and the cucumber family want more.

If it goes wrong#

Leaves sunburned, white or brown
Moved too fast. Back into shade for a few days, then on again, slower.
A few lower leaves yellow
Normal. So is a wilt on the first full-sun day; they come back within a day.
Stems snapped
Wind. Keep them behind something until they've stiffened.
Wilted dry
A seedling that dries out for a few hours is usually lost. Water, shade, and hope.
Growth stopped cold
Overdone. Cauliflower buttons, cabbage bolts, cucumbers and melons stall. The point is to slow growth, not stop it.
Rough, scarred tomatoes
Chilled below 60 °F while hardening. Keep the next batch warmer.
From the beds in OhioKeep an eye on the trays for pests once they're outside. Brassica pests find seedlings fast and lay eggs so tiny you'll miss them, and those go straight into the garden as worms. Same with aphids on tomatoes and peppers.

After they're in#

Set most seedlings no deeper than they sat in the tray. Tomatoes wait for the frost danger to pass and the soil to reach about 60 °F.

If a cold night or a hard wind follows, cover them: a pot, a box, a milk jug, a row cover. Take it off in the morning. A plant that wilts the first days gets the same cover against the sun.

In Broadfork The plant-out day on a planting's card is the day this guide counts back from.

The exceptions, by crop#

Every number on this page comes from a published extension source.

Sources for this guide: Purdue Extension, Ohio State University Extension, University of Minnesota Extension, Alabama Cooperative Extension System, Iowa State University Extension and Outreach, NC State Extension, University of Missouri Extension, Utah State University Extension, Clemson Cooperative Extension, Colorado State University Extension, Penn State Extension, UMass Extension, University of Georgia Extension, University of Illinois Extension, University of Maine Cooperative Extension, University of Maryland Extension, University of New Hampshire Extension.

The documents behind this guide (29)

Updated 11/10/2026 · Checked against our sources 11/12/2026

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