How to Harden Off Seedlings

Harden off over about two weeks: start with an hour outdoors in shade and shelter, add roughly an hour a day, bring them in every night for the first week, and only leave them out overnight in the second week once night temperatures are reliably mild. A cold frame does nearly all of this work for you -- prop the lid open a little further each day -- and floating row cover protects them once they are planted out.

A seedling raised indoors has grown in conditions no garden offers: steady warmth, no wind, no direct sun, and humidity far higher than outside. Its leaves have a thin cuticle, its stems have not been flexed, and its roots have never had to cope with a drying soil surface.

Hardening off is a controlled toughening. Over two weeks the plant thickens its leaf cuticle, builds stem strength in response to wind, and adjusts to real light levels. Skip it and the plant is put into full sun and wind with none of those defences, which is why unhardened transplants can be dead within a day.

• Direct sun, far more than cold. Leaves grown under a grow light scorch in a few hours of real sunshine -- it looks like bleaching or white patches.
• Wind, which both desiccates soft leaves and snaps unflexed stems. A sheltered spot for the first few days is as important as shade.
• Drying out. A small cell dries fast outdoors in sun and wind, and a seedling that wilts hard during hardening off loses days of progress.
• A late frost on a night you did not check. This is the one that takes the whole tray at once.
• Cold soil, which stalls heat-loving crops even without frost. Tomatoes and peppers sulk for weeks if planted into cold ground.

1. 1

   Days 1 to 2 — one hour, full shade

   Out in a sheltered, shaded spot for an hour in the middle of the day, then back inside. Do not start on a windy or very bright day.

2. 2

   Days 3 to 4 — two to three hours, shade with a little dappled sun

   Still sheltered. Check for wilting -- water before you put them out, not after they flag.

3. 3

   Days 5 to 7 — half a day, some morning sun

   Morning sun only. Afternoon sun is the stronger and the one that scorches. Still indoors overnight.

4. 4

   Days 8 to 10 — a full day outdoors

   Out in the morning, in at dusk, with a few hours of direct sun. By now they should look visibly sturdier and darker green.

5. 5

   Days 11 to 13 — overnight, if nights are mild

   Leave them out overnight somewhere sheltered, only once night temperatures are reliably above the crop's tolerance. Check the forecast every evening.

6. 6

   Day 14 — plant out

   Plant in the evening or on an overcast day rather than at midday, water in well, and keep row cover handy for the first week.

Treat that as a template rather than a rule. If a cold snap or a storm arrives, pause at the stage you are on -- you lose nothing by taking three weeks, and you can lose everything by pushing through bad weather.

Moving trays in and out twice a day for two weeks is the reason people skip hardening off. A cold frame turns it into propping a lid: the frame gives shelter from wind and heavy rain, a few degrees of overnight protection, and a gradual increase in exposure that you control by how far you open it.

The discipline it demands is venting. A closed frame on a sunny spring day can cook its contents by lunchtime, which is the opposite of the problem you were solving. Prop it in the morning, close it in the late afternoon. The best cold frames covers the options, and greenhouse vs cold frame explains why a frame is often the better buy for exactly this job.

An unheated greenhouse does the same thing with more room and more risk of overheating. If you have one, use it with the vent and the door open rather than sealed.

The practical consequence is that one tray going out together is usually wrong. Brassicas and lettuce can be hardening while the peppers are still indoors under lights.

1. Plant in the evening or on a cloudy day. A transplant put in at midday in full sun spends its first hours wilting.
2. Water in properly, enough to settle the soil around the root ball rather than just dampening the surface.
3. Cover for the first week. Floating row cover softens sun and wind, buys a few degrees against a surprise cold night, and keeps early pests off.
4. Expect a pause. Transplants sit still for several days while roots establish. That is not a problem.
5. Keep watering until they are away. A newly planted root ball dries out faster than the soil around it.

If you are working out when all of this should be happening, when to start seeds indoors counts back from your own frost date -- and hardening off is the two weeks you need to have left room for.

One practical note on trays: hardening off is hard on flimsy ones. Carrying a flexing tray of wet cells in and out twice a day is how plants get tipped, so heavy-gauge reusable trays earn their cost during exactly these two weeks.