Preparing Your Plant Babies for Leaving the Nest

Hardening off is not just for vegetables!
It will benefit herbs, flowers, any young
plants not used to the real world.
This is sweet woodruff.

       Crocuses are blooming, the temperature has soared into the sixties, and you can't wait to get those plants you started from seed and babied for a month out into the real world. You grab a trowel and a flat of  seedlings from your warm, protected, well-watered nursery and head straight for the garden to set them immediately into their new home.
       Don't do it! This is very much like telling your teenager that she's on her own right this minute without having given her any training in money management, doing laundry, cooking, or finding and keeping a job. The abrupt change will stress your plants, slow their development, or even kill them. Help your plants make the transition; harden off your seedlings, whether started at home or purchased from a nursery.
       Perhaps the most critical reason for hardening off plants is the change they will experience in quality of light. Plant leaves adjust to deal with the light they are used to. If they go suddenly from artificial light to full sun, the leaves will scorch. They need several days of gradually increased exposure to adjust to the change. Temperature and watering changes and an introduction to wind are other factors that hardening off addresses.
       Ideally, the hardening off process will take seven to ten days. Start by moving your plants to a sheltered area for only two or three hours. Perhaps the space between the garage and house? I put mine on my front porch to catch morning light rather than the harsher afternoon sun. Gradually increase exposure time. Protect the plants from strong winds, heavy rains and cold. You may want to bring them inside after their exposure period, or cover them with a floating row cover or blanket for the night, or set them in a cold frame. Decrease their watering, but not to the point where they wilt. Although they may appear to recover quickly from wilting, in fact recover takes days.
       Cold hardiness varies with plants, and has to do with a plant's ability to withstand frost. Hardy plants can tolerate temperatures in the 40's and should survive a light freeze after being well hardened off. Hardy plants include asparagus, broccoli, cabbage, chives, garlic, horseradish, kale, kohlrabi, leeks, mustard, onion, parsley, peas, radishes, rhubarb, spinach and turnips.
       Half-hardy plants like celery, lettuce, endive, cauliflower, chard, and potatoes can tolerate temps down to 45 degrees; squash, pumpkin and sweet corn down to 50 degrees; cucumbers and melons to 60; basil, tomatoes, and peppers to 65.
       The above guidelines are from a Washington State University Extension Office web page. Keep in mind that saying that plants can survive these lower temperatures is not the same as saying they will thrive in them. For instance, squash, pumpkins and cucumbers are all on the "very tender" hardiness list.
       After you've hardened off your babies, plant them during a cloudy day if possible, when rain is forecast. If you are blessed with clear skies, set them out during the late afternoon. Water them ahead of time so they will have time for a last drink before you disturb the roots. Set the plants at the same depth as they are in the pots, except for tomatoes. For tomatoes pinch off all but the topmost leaves and set it in the ground so that the stem is sideways, buried almost up to the leaves. If you are using peat pots, tear off the bottoms and any part of the pot that extends above the ground. If you leave it, it acts like a wick and draws water to the surface where it evaporates away. Water the seedlings in. Water the roots, not the leaves, and if you fertilize, use a weak solution. Mulching is always a good idea for weed and moisture control.
       If you absolutely cannot wait to plant, harden off for a minimum of three days. This is less than ideal, but will still benefit your seedlings.
        I'm still hardening off my Cold Set tomatoes, having barely resisted the urge to plant them as soon as the weather here went suddenly from winter to summer. But weather in Michigan is dicey. We can expect freezing and even a blizzard at this time of year, but this variety claims it will set fruit even at near freezing temperatures. I really, really want to find out if this is true. Imagine, fresh tomatoes in May! Well, I can dream. Good luck with starting your garden!