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okiedawn1

Hardening Off Seedlings Raised Indoors

Okiedawn OK Zone 7
15 years ago

When you are raising seedlings inside under lights, they do not naturally develop the ability to deal with direct sunlight, heat, cold or wind because they have been raised in a very protected environment. If you move those seedlings directly outside into full sun, heat, cold or wind, they are going to sunburn and/or windburn and their growth may slow down in response to the drastic change in air temperatures. If the sunburn and windburn are bad enough, the plants can die. So, you have to provide your seedlings with a hardening-off period.

To harden off seedlings, if it is possible, place them in full to dappled shade their first day or two and let them just get used to the brighter light, different air temperatures and wind. If it is a very windy day, place them in a location sheltered from the wind to avoid windburn.

I like to put my seedlings on the screened-in back porch the first couple of days because they receive only indirect sunlight and very little wind exposure there. It also stays pretty warm (almost too warm on hot days) because the screened windows are covered in greenhouse plastic for the winter months. Or, sometimes I put them on the east-facing front porch, the south-facing side porch or the covered patio.

After they have had a couple of days of exposure to outdoor conditions, I move them out into full sun for an hour the first day, two hours the second, three the third, etc. I prefer to move them back inside before sunset for the first few days so they don't have to deal with the colder nighttime temperatures. Eventually, though, I am leaving them in the sun most of the day and on a protected porch or patio at night. After a week or 10 days, they are ready to go into the garden.

There are times, though, when plants are completely or almost completely hardened off and a really cold spell arrives and the seedlings have to go back inside. If they are back inside for more than 2 days, I harden them off again for 3 or 4 days before leaving them out all day long.

When you start seedlings too early and have hardened them off, they can get pretty big (too big to go back under the lights) and you may have weeks yet before you can plant. In that instance, I'd try to leave them outside as much as possible but you'll have to take great pains to keep them moist and growing. You don't want for them to get too dry and for their growth to stall. You also may need to pot them up to a larger container so they do not become rootbound.

When you pot up, be sure you water in the soil in well around the plants so it will settle down around the plants. If you potted up the plants into larger containers, they may need to be fed. Feed them with a water-soluable fertilizer at half-strength to full-strength per package directions.

If you have a cold frame, you can keep older, hardened off seedlings in it. Or, if your soil is warm enough for root growth, you can plant the rapidly growing seedings in the garden if you cover them with frost blanket type row covers (or the scrounged up equivalent of such) when cold nights threaten.

Sometimes it seems all I do in spring is carry flats of plants in and out, but it pays to harden them off gradually.

Dawn

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