When seedlings are started indoors or in a greenhouse, they become acclimated to an environment that is much less severe than the outdoors. It is sometimes hard for novice gardeners to comprehend that direct sunlight, strong winds, and cooler nighttime temperatures are simply too much for many seedlings or small plants to handle. The process of hardening off is meant to reduce the shock of transitioning from a sheltered environment to outdoor conditions. The three biggest factors affecting a plant’s transition from an indoor environment to an outdoor environment are direct sunlight, wind and temperature.
Many gardeners do not realize how much more intense the sun is than an artificial light source. Even plants grown under high intensity discharge lighting will have a hard time transitioning into the intensity of direct sunlight. Tender seedlings and young plants grown indoors on a window sill or under a grow light must be gradually transitioned into direct sunlight. Start by placing the tender seedlings in a shaded area. After a couple of days in the shade, the seedlings can be introduced to direct sunlight. Remember, gradual increases are the key to success in the hardening off process. Each day a grower can increase the amount of time the seedlings
receive direct sunlight. Depending on the particular plant variety, the grower can usually increase the duration of time in direct sunlight by 1-2 hours per day. Typically, seedlings should be acclimated to the direct sunlight in about a week’s time.
Many seedlings and young plants get accustomed to a high humidity environment. When placed outdoors, the wind and dry air can quickly dry out the medium and/or the seedlings themselves. If there is a strong wind present when a gardener is hardening off his or her plants, it is a good idea to create some sort of wind block for the seedlings. Cold frames or mini hoop houses are valuable tools for the hardening off process and can offer additional wind protection for transitioning plants.
Many gardeners start the hardening off process when the night temperatures are still too cold for the seedlings. It may be necessary to bring the seedlings back inside each evening until the temperature at night increases or the plants become acclimated to the colder temperatures. Cold frames or other temporary structures that offer cover can be used to protect the seedlings from the lower nighttime temperatures.
A cold frame is an unheated greenhouse, usually used to temporarily house plants during the spring or early summer. Cold frames are a valuable tool for hardening off seedlings and young plants before planting them in the garden. A cold frame works great for sheltering young plants while allowing them a “taste” of life outdoors. The most common type of cold frame used by home hobbyists is a box-style cold frame.
A box-style cold frame is nothing more than a bottomless box with an angled, transparent top. Box-style cold frames come in different styles and sizes and can be purchased or built from various materials. Some cold frames are as basic as a box made of cinderblocks with an old sheet of glass on top, while other cold frames are more advanced composite structures complete with a thermostatic auto-opener. Manufactured cold frames built from aluminum and polycarbonate offer the advantage of longevity and are backed by a manufacturer’s warranty.
Seedlings and young plants that are hardened off properly will transition more seamlessly into outdoor conditions. It only takes one bad experience for a horticulturist to realize how important the hardening off process really is. After all, the last thing a gardener wants is to have to replant all of his or her seeds and play catch-up for the rest of the growing season. Hardening off is most effective when done in incremental steps. When in doubt, take the slow route. In other words, rushing the hardening off process can be extremely counterproductive. A hobbyist who properly hardens off his or her plants will not only have healthier plants and a smoother transition to the outdoors, but will also have a more enjoyable experience in the garden.
Background information for this article was provided by Arcadia Glasshouse ArcadiaGlasshouse.com.