This is good for seedlings, particularly plants like tomatoes and peppers that seem to be a little weaker when they are young. But when it is time for them to go out to their permanent spots, putting them out without gradually introducing them to the challenges of the outdoors could shock them (slowing their growth) or, without having had a chance to build-up defenses to these challenges, they might perish.
So, we must gradually introduce them to outdoor conditions. This process is called hardening off. The basic idea is ease the transition. Precisely how this is carried out depends on how much time and energy you are willing to put into it and the outside conditions when you are ready to begin the process.
Some of my resources suggested a simple process of merely putting the seedlings in a cold frame for a few days before planting them. This did not seem like a good solution for me, because by the time my plants were ready to go out, it was already pretty hot. The cold frame protects them from wind, rainstorms, and critters. And, they can stabilize temperatures so that plants don't have to deal with as much shocks in terms of temerature lows. In my situation the heat seemed to be the most difficult element, so putting them in a cold frame would have been detrimental.
The alternative is more time consuming. You gradually introduce them to the elements by putting them outside for short increments. How quickly you increase their challenges depends on how much time you are willing to spend moving flats of seedlings around all day. Pay special attention to water during the hardening off process. Here was my process:
day 1: put plants on covered front porch facing west for an hour in the late morning.
-they had to deal with outside temperature shifts and wind, but the wind was lessened by the walls of the porch. they did not have to deal with sunlight.
day 2: put plants on covered front porch facing west for a few hours in the late morning
-conditions same as day 1, but they have to deal with them longer
day 3: put plants on covered front porch facing west for several hours in the morning, then moving to the uncovered back porch facing east through the evening
-conditions same as day 1, but they have to deal with them longer (moving them to the east porch keeps them from being exposed to direct, unfiltered sunlight)
day 4: put plants on back porch facing east for one hour in the early morning, then move them to the front porch for the rest of the day (including the evening)
-they are getting the first taste of unfiltered sunlight and unfiltered wind. the early morning sun is gentler than the midday sun.
day 5: put plants on back porch facing east for a couple of hours in the early morning, then move them to the front porch for the rest of the day
-same as day 4, but increasing the time
day 6: put plants on back porch facing east for several hours during the day, closely watching them for wilt and sun scald.
-same as previous day, but adding the harsh midday sun.
day 7: put plants on back porch facing east all day and all night.
-adding the challenge of nighttime lows
Anytime the plants wilted or showed signs of sun scald, I immediately transferred them to gentler conditions and gave them extra water [and pulled back a little the next day - either repeating the step or even going back to the previous step. If anything, I wish I had been more careful. My tomatoes almost all survived the hardening off process, and my peppers handled it like champs. But three of my celery plants died, and two of them look like they will not last much longer. My actual process was usually much longer, because of days when the weather was too harsh or we were out of town.
Above, you can see my first hardened-off seedlings. Note the sun scald, particularly on the plant in the middle. I am happy to report that this damage did not deter the growth of this seedlings at all! These tomatoes (the romas) have been out for a couple of weeks and I can't even tell which ones had sun scald damage anymore - the new growth has completely overshadowed those leaves and all of the plants look equally healthy.
Ideally, you would not let the plants wilt or get sun scald. But it can be difficult to know how they will do. Had I started earlier in the season, this process on this exact timeline would probably be fine. By the time I started, even the first hour outside on the shaded porch made some of the plants wilt so it ended up much slower forme. It's easy to grow impatient (especially with the relatively time-consuming process of moving the trays of seedlings around) and difficult to guess what the plants are ready for. But although the peppers and celery only just recently went outside, I am proud to report that the tomatoes have really taken off since they were planted out!
And just as a note, it was fun to watch the plants get stronger - I could really tell that they were growing sturdier as the days went along!
Overall I was happy with the hardening off process. I will do the same thing next year, except I will be ready to start earlier and I will take it a little bit slower.
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