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.

Direct Sunlight
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.

Wind
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.

Temperature
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.

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