The average date for the last frost in this area is May 15, though each year’s weather conditions can vary widely. Early to mid-may is generally the time to start the hardening off process for tender vegetable seedlings started indoors, e.g. tomatoes, peppers, squash, cucumbers, basil etc. Hardening off in preparation for planting out in the garden involves slowly exposing young seedlings to the elements to strengthen them.
About 2 weeks prior to transplanting plants outside, it is recommended to start the hardening off process. Start by placing plants outside during the warmer part of the day (typically between 12-5 PM) for about 2-3 hours; gradually increasing the amount of time each day. After working the plants up to being outside for 10-12 hours a day for a few days, leave the plants outside for 24 hours for a couple days. Once you complete this, your plants are ready for transplanting. During this period of hardening off, gradually reduce how often you water; however, don’t allow the plants to wilt. It is also not recommended to fertilize the plants before or during the hardening off period.
Illinois Extension: Good Growing
Now that the days are warming up, it’s safe to bring seedlings outside to acclimate. It’s best to place them in a shady and not too windy spot the first time they are exposed to the elements. Strong sunlight can quickly burn young leaves and strong wind can be equally as damaging to tender stems and leaves. A warm and shady place is the best bet for the first time outdoors.
Hardening off seedlings requires consistent attention: seedlings need to be transferred outside and back inside for several days, placed in and out of shade when outside (too much mid-day direct sun too soon should be avoided) and sheltered from strong winds. Weather conditions and nighttime temperatures need to be monitored regularly. Temps may still dip in to the lower to mid 40’s, in which case the seedlings will need to be brought back in so they are not damaged by the cold, even later in the process. A good rule of thumb is to begin to leave seedlings out all night only when they are ready and the nighttime temperatures are above 50 degrees. After a few days of being out all day and night, the seedlings are ready to be set out in the garden.
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