Gardening instructions can be full of peculiar phrases from British gardening traditions. “Potting on,” “pricking out,” and “hardening off” are all things we do with seedlings to help them prepare for eventual transplanting out into the garden. Hardening off seedlings is probably the most important concept that new gardeners can grasp to improve successful transplants. It is the process of gradually introducing seedlings started indoors to the much harsher conditions of the garden outside.
As bright as any indoor grow lights might be, none compare to direct sunlight. Plants that have not been introduced gently to direct sunshine may show rapid and dramatic signs of shock. The leaves may become bleached out, curled under, or simply fall off. Sudden transplant shock can even kill many types of seedlings.
In addition to bright sunshine, seedlings started indoors will be exposed to (sometimes high) wind. This can cause stress to the stems of the plants, and it combines with the sunlight to strip moisture from the soil and leaves. Also, fluctuations in temperature caused by cool nights and hot days can cause further stress to the delicate young plants. Heat loving plants like tomatoes, peppers, and basil, need particular care before transplanting, but hardening off is a sensible step to take with ALL seedlings.
We recommend hardening off seedlings over the course of at least several days to one week.
Day One: Choose a warm, sunny day, but place the pots, flats, or trays out in full shade, bringing them back inside at night.
Day Two: Place the seedlings in dappled shade, with some exposure to direct sunshine, but bring them back inside for the night.
Day Three: Still in their pots or trays, allow the seedlings increased exposure to direct sunlight for several hours, and leave them out over night.
Day Four: Transplant the seedlings to their permanent growing spots, and provide lots of water. it can be helpful to provide partial shade to the plants as they recover from transplanting.