Minnesota & Seasons

Hardening off: why new plants sulk (and how to prevent it)

The short answer: give new plants about a week of gradually more sun and wind before planting them out. Plants grown in a greenhouse or under lights have never felt full sun, wind or a cold night. Put them straight into the garden and many will get bleached, scorched leaves or stall for weeks.

Planting this week? Our Is it safe to plant yet? page checks the forecast for East Bethel against tomatoes, peppers, annuals and more.

What goes wrong

• Sunburn: white or papery patches on the leaves facing the sun, often within a day. Greenhouse glass and plastic filter out some UV light, so the leaves haven't built up any protection.
• Windburn: torn, brown-edged or limp leaves after a breezy day.
• Cold stall: tomatoes, peppers, basil and coleus that turn purple-tinged or just sit still for weeks after a night in the 40s.

A sunburned plant usually recovers; the new leaves come in fine. But you lose a week or two of growth, and a badly burned seedling can die.

A simple schedule

• Days 1–2: 1–2 hours outside in shade or dappled light, out of the wind.
• Days 3–4: a half day outside, with an hour or two of morning sun.
• Days 5–6: most of the day outside, including some afternoon sun.
• Day 7 and on: all day and overnight, as long as nights stay above 50°F for warm-season plants. Then plant.

Cloudy, calm days are a gift during hardening off; you can move faster. On a hot, windy day, back off. Keep the pots watered, because they dry out much faster outside.

Which plants need it most

Anything grown from seed indoors, and anything bought from a greenhouse early in the season. Tomatoes, peppers, eggplant, basil, coleus, impatiens and begonias are the touchiest. Pansies, snapdragons and cool-season vegetables like cabbage and lettuce can handle a light frost once hardened off.

When to plant out in Minnesota

Hardy plants like pansies and cool-season vegetables can go out in late April or early May. Tender annuals and warm-season vegetables should wait until the danger of frost is past and nights are consistently above 50°F, usually around Memorial Day in East Bethel (see when to plant tomatoes and the planting calendar). Plant on a cloudy day or in the evening, and water in well.