Harden(ing) off refers to the process of gradually acclimating tender seedlings which have been started in a controlled environment to their final outdoor growing conditions.
Gardening in most continental climates which experience cold winters require some method of starting crops in an artificially warm environment. Novice home gardeners start gardening by purchasing plants from a farmstand which may have invested in cost prohibitive equipment such as greenhouses, commercial grow lights, hydroponics, and other commercial seed starting equipment.
Many gardeners will attempt to “graduate” to starting some of their plants from seed indoors in their own homes. The benefits of seed starting indoors include trying cultivars which may not be available to them as plants and, depending on the quantity of plants grown, reduced cost. Many gardeners simply enjoy the challenge. The average home gardener can economically start seeds in a warm place inside the home with either access to a south facing window or a simple light fixture with special full spectrum filament(s).
Once the seedlings have at least two sets of true leaves and the outside average soil temperature has warmed sufficiently for transplanting into their final locations in the garden (which will vary depending on the type of plant), the seedlings must be hardened off.
By setting the seedlings outside for incrementally longer periods each day for at least a week, the seedlings will acclimate to the greater winds, temperature fluctuations and the direct sunlight of the outdoors.
The plant will undergo the following cellular changes: 1
- Plant growth slows.
- Natural waxes on leaf surfaces thicken as plants are exposed to more sun, reducing the rate of water loss.
- Cell walls develop more lignin to strengthen them.
- The amount of freeze-prone water in plant cells is reduced.
- The amount of carbohydrates in plant tissues increases.
- More rapid root development is stimulated.
It is best practice to begin to harden off seedlings for a few hours in moderate conditions. Excessively windy conditions at first can abrade and dry out the tender leaves. Too much sun can scald the leaves. Seedlings should be brought indoors during windy days and not exposed to the full noon sunlight until the end of the hardening off period. If it is not possible to bring the plants indoor at midday, set the plants outside in a relatively shady area until the last few days of hardening off. Care should be taken not to let the soil in the pots dry out during the process, which can occur within a few hours on a hot sunny day. The soil should remain moist but not saturated.
A properly hardened off plant should begin to thrive and grow within a week of transplanting.
Reference:
[1]https://lancaster.unl.edu/hort/articles/2007/hardentransplant.shtml