Before your seedlings move from windowsill to planting bed, they need to be acclimated to outdoor growing conditions. The process is called hardening-off and transits seedlings from protected indoor growing conditions to outdoor realities of wind, cold and stronger sunlight.
Hardening-off seedlings is a process that should be done slowly over the course of a week or two. Done properly, you'll produce sturdy seedlings ready to withstand whatever nature throws at them. Rush the process, and you could burn, damage or even kill the young transplants.
Harden-off tender seedlings over a period of 6-14 days. The goal is to expose transplants gradually to outdoor conditions, each day increasing the time seedlings spend outside.
Gradually reduce watering over the course of hardening-off. You don’t want seedlings to wilt, just slowly stretch the period between waterings.
Stop feeding 3-4 days before you start hardening-off. Don't feed again until you transplant into the garden.
Start by placing seedlings outdoors in a sheltered spot – protected from wind and direct sun. Underneath a tree or overhang is perfect. Bring seedlings back inside for the first two nights. Each day, expose them to another 30-60 minutes of brighter sunlight. Work your way up to giving plants more and more direct morning sun, until they can handle mid-day sun without wilting. Fully hardened-off seedlings should be able to withstand the same amount of sunlight they'll receive when planted in the garden.
Watch the Weather
Check weather forecasts and keep tabs on predicted nighttime lows. If temperatures will dip below 35 degrees F., bring seedlings indoors or cover with a spun-polystyrene row cover or other protective material. Learn more information for Protecting Plants From Cold.
You'll get the best frost protection when you mount the row cover on hoops or stakes so it doesn't directly touch foliage.
Unless it looks like there will be a frost or freezing temperatures, plan to leave seedlings outside overnight by the third or fourth night. Place them under an overhang or beneath a table for protection. By the sixth night or so, seedlings shouldn’t need protection.
When to Plant
After a week or so of hardening-off, your transplants should look stockier and tougher. Plant on an overcast or drizzly day when winds are calm or in the evening. Water in with a liquid fertilizer solution diluted to half strength. If necessary, provide protection from pests, such as snails, slugs and cutworms.
Continue to protect seedlings from high winds, frost, hail or heavy downpours. A plastic gallon jug with the bottom removed makes a good cover for seedlings.