The Importance of Hardening-Off Plants

Underneath the sheltering trunks of massive estate-grown beeches, the gardeners of Long Island's Historic Planting Fields Arboretum are gathering up small seedlings to replace their venerable parents. These mature trees and their offspring have shared this soil for a century, and are almost perfectly adapted to its conditions.

But what if you do not have your very own 400 acre garden, professional staff and extensive greenhouses? How will you assure the survival of your own carefully tended darlings? The process of adapting a plant to the location where it will eventually be grown is called "hardening-off" and a failure to harden-off plants is the leading cause of stunted growth or death.

The culprit is transplant shock. Symptoms include sudden loss of leaves, cessation of growth and in worst cases, death. Perhaps you had the rude experience of buying a magnificent plant from your local nursery or greenhouse only to find it on the verge of death. Commonly, plants from commercial greenhouses are grown under conditions of high humidity and optimum light and temperature. A sudden drop of several degrees or loss of humidity can induce dormancy.

Growth stops, leaves drop and the plant mimics its outdoor cousins in the Fall. For many tropical plants, there is no Spring, Summer, Winter or Fall, only a wet or dry season. Sudden drops in humidity mean that the dry season is beginning as far as the plant is concerned and it will wait for more moisture.

When a plant enters dormancy, watering should be reduced and feeding stopped. The plant needs time to become comfortable in its new home. Eventually growth will resume and normal watering can resume with it. To avoid inducing dormancy in a new introduction to the home, place a polyethylene plastic bag around it to maintain the same humidity levels that it experienced as a greenhouse grown specimen.

Each day, poke holes with a pin in the bag until eventually the bag is in shreds, then discard the bag. This will allow your plant to become accustomed to the lower humidity of your home. You may do the same thing with seed starts for your garden. Start your seedlings in a tray covered by a plastic bag. As the time for transplant draws near, poke holes until the plants are grown in air of normal humidity. Sometimes plants are sold that are mere cuttings with few or no roots. Should you be stuck with one of these duds, hardening-off using a plastic bag will give it extra time to develop needed roots.

Sometimes the problem is not humidity, but temperature. Suppose you need to transplant newly-grown seedlings from your greenhouse or home to the garden outside? In that case, you may want to slowly step down the heat. Move the greenhouse grown plant to a sunny widow at home. The plants on the sunny, warm window sill can spend some time in a cooler, closed porch and then finally, outside in a cold-frame before finally being planted in the garden.

To further reduce the shock, plant your seeds in peat pots or in cardboard milk containers or egg crates that can be cut away or will naturally disintegrate so that you do not disturb or kill delicate roots when you move them to their final homes.

Suppose you are forcing Spring-blooming bulbs for use indoors? If you have buried a few pots of tulips or hyacinths outside in the Fall for forcing, you will want to move the plants to a screened in porch a few degrees colder than your home, say the high 50's Fahrenheit during the late Winter. Same if you are blooming paper-whites or tender narcissi. Plants that naturally bloom in the Spring won't take kindly to heat, so keep them as cold as possible and move them towards warmer temperatures gradually.

Suppose you have the opposite problem? Your very costly amaryllis bulbs simply won't re-bloom if they do not get more light than they receive on your windowsill. In that case once the temperature is reliably in the sixties, take the plants, move them to the brightest window of your house for a few weeks, followed by a few weeks outside in deep shade, followed by a few weeks in semi-shade.

Don't place them in full sun until they have achieved a rich, green color in their leaves. Even plants sunburn. Chlorophyll, not sunblock, protects the delicate structures of the leaves and stems. If you see yellow or brown patches on the leaves, you have sped the process up too much and should not expose it to greater sun for a little while longer.

Hardening-off plants is equal parts patience and common sense. You may not be a "black-thumb" after all. Instead, the culprit may be a nursery that sells plants that have not been properly prepared for their eventual homes. Your lowly plastic bag and a couple of pins may be all that is necessary to turn your black thumb green.

Sources:
http://bbg.org
http://nybg.org
http://wavehill.org
http://www.plantingfields.org/