By Contributing Editor
Nancy Hulse

You’ve chosen the right planting time, planted your seeds indoors and nurtured your tiny seedlings into sturdy healthy plants. The final step is hardening off: successfully introducing your plants to the outdoors.


Plants have very different requirements in the conditions they need to thrive, and this will help you to determine when to begin to harden them off. If they are hardy cold-season plants, like asparagus, broccoli, lettuce, and spinach, you can begin to harden them off a couple of weeks before the date of last frost (just remember to bring them in if frost actually threatens). Other plants such as tomatoes, peppers, eggplants, melons and annual herbs like basil require a warmer soil temperature to thrive. Setting these out before conditions are optimal will not gain you a jump start on your garden, rather it will stress the plant and result in slower growth and decreased resistance to disease.


To prepare your seedlings for the real world, you can begin while the seedlings are still indoors. You’ve provided a stable environment where your seedlings are protected from the three main stresses they’ll encounter after they are transplanted: wind, drought, and direct sun. Now you can begin to gently introduce some of these elements.
As the plants grow and produce leaves, find a way to move the air around them. A small electric fan stirring the air will do nicely, or even fanning the seedlings with a piece of stiff paper a couple of times a day. In the interest of providing information that you no doubt will be able to work into your gardening conversation at some point, the name for this process is thigmomorphogenesis (from a Latin word meaning touch, morph to shape or form, and genesis to bring something about).
When you begin to move the air around the plants, you add the benefit of increased evaporation from the leaves of the plant. As a reaction to increased water loss the plant begins to grow a thicker cuticle. So now you’ve begun to condition your plant for the second stressor, drought.
A couple of weeks out from your planned transplant date you should reduce the amount of water the seedlings get. Let the soil become a bit dry-looking between watering. You’ll know you’ve gone too far if the plants wilt a bit. If so, water them right away, and try not to let things go so far in the future. The reduction in water has an effect similar to that of the moving air; the plant responds to drying by toughening the cuticle to retard water loss.


At least a week out from transplant time, start exposing the plants to the great outdoors. The first exposure should be numbered in hours. Put them out in a shady, protected place for a few hours (say, mid-morning to early afternoon). If the day is breezy, avoid putting them in the direct path of the wind.
If you have just a few plants, you might not mind schlepping them in and out each day. However, if you’ve started several trays of plants, or you are in a cold area, you may want to have a shaded cold frame available. Unless you are really pushing the season, or have very cold-tender plants, a cold frame will give your plants enough protection to allow you to leave your plants in place after the first couple of days.
There are many ways to construct a do-it-yourself cold frame. Readers of my article on building a straw bale garden might not be surprised to find that the versatile straw bale can also make a great cold frame. Four straw bales placed in a rectangle will leave a space in the center for several trays of plants. Place a recycled window or plastic over the top. As the straw decomposes, you’ll even get additional heat for your plants. But be sure you’ve put your bales in a place were they can stay for the season, you’ll not want to try to move them after they soak up a couple of days worth of rain.
You might also want to consider a manufactured cold frame. I’ve found the FlowerHouse SeedHouse to be a reasonably priced and convenient way to protect my seedlings. It’s useful to have screened vents and doors so that you can control the temperature and humidity. Remember that whatever method you use, you must be sure to keep the seedlings protected from too much sun.
After a couple of days of short exposure, unless there’s a danger of frost, you should be able to leave the seedlings out for the day, still in the shade. Each day, nudge them closer to a spot that gets full sun, or uncover more of the cold frame. Assuming the weather remains stable, by the end of the week you should be able to leave those seedlings fully exposed to the elements, day and night. Water as needed, of course.
If the plants aren’t showing any signs of distress, and the weather is looking good, the seedlings are pretty much ready to go after a week and a half to two weeks of hardening-off. These times are, of course, generalizations. In regions with really cold climates, a longer and more gradual buildup to total exposure is a good idea. Be prepared to bring the plants back indoors any night the weather is likely to be too cool for the plants, and you should have hot caps and such ready to use at a moment’s notice after the transplants are in the garden.
Follow through on the hardening-off by transplanting on a calm, cloudy day, or by shading the transplants if sunny weather prevails. Be prepared for sudden changes in the weather; have shade ready for unusual heat, and row covers for that late spring cold snap that always seems to be waiting for you to get the transplants in the ground. Watch the transplants for signs of stress, and don’t be too quick to douse them with fertilizer; let them settle in a few days before giving them anything stronger than compost tea.