This week's article from Linda Gilkeson's email list is about hardening off seedlings. You can get on this list by emailing Linda at info [at] lindagilkeson [dot] ca. She lives on Salt Spring, so her tips are directly useful for us. You can read all her past articles at the link she cites below.
Quoting Linda:
How you harden off tender seedlings can have a long term effect on the crop. Hardening off is the process of reducing the growth rate of seedlings by exposing them to cooler conditions and less water and fertilizer. As their growth is checked, plants accumulate food reserves, which they can use to produce new roots faster when they are transplanted. Hardening off also thickens the cuticle and wax layers on leaves, which helps plants to withstand wind and weather and protects leaves from sunburn.
Some plants can be hardened off to withstand frost, including the cabbage family, lettuce, most greens and onions. If transplants plants are too large, however, exposure to temperatures under 5-10oC for more than a couple of weeks can make some of them send up seedstalks.
The tricky thing is that there is a disadvantage to over-hardening plants. Such plant are slow to begin growth and may never really recover, resulting in lower yields and later maturing crops. Transplants suffering from uneven watering, from being rootbound or chilled become over-hardened. Plants for sale have usually been hardened off by the time they reach the market so they become over-hardened if they hang around too long before they are set out. [Find out when your local nursery brings in new stock, buy your transplants the day they arrive and plant them out immediately or else pot them on]. Home grown seedlings that were started too early suffer the same fate if they end up being held too long in small pots.
Hardening off is least helpful for tender plants, such as tomatoes, peppers, cucumber, melons, squash, and celery. Ideally, you want to time the seeding date for these so that transplants reach the right size to go into the garden just as the weather becomes warm and stable (good luck with that!). Such tender plants are better off being started later rather than earlier so they don’t experience a check in their growth. In any case, if seedlings are becoming rootbound and the weather still isn’t warm enough to put them out, they should be potted in larger containers and kept in.
Hardening off for tender crops mainly means getting them used to direct sunshine. Tender plants that have been started under glass can be seriously damaged or even die from sunburn if suddenly moved outdoors (really!). Sunburn damage on cucumbers and squash (the most susceptible group) appears as light tan spots and blotches on leaves and stems.
What to do: Gently harden off transplants you have grown by gradually exposing them to direct sun and outdoor conditions. Starting with an hour or so the first day, set them out for a longer period each day, taking about a week to get them used to a whole day outdoors. Move them indoors if nights are cooler than normal. Once they are set out, keep sheets of plastic, floating row cover, cloches or other covers handy to protect transplants from an late cool spell.