Here’s an excerpt from a photo-documented post I did called Greenhouse Tips: Starting Seeds Inside in “Gardening Tips & Tricks” Group which thoroughly covers every aspect of seed-starting.
Hardening-Off your Plants: Whenever you start plants inside, you will need to harden-off the plants before planting out. Hardening is 10% about acclimating to temperature and 90% about building a resistance to the sun’s UV Light. Direct exposure to the sun’s UV will cause the leaves to sunburn and turn white. Proper hardening is not an easy task, especially for people who work outside the home. Most people start out by placing plants in the shade, then moving them to partially sunny areas for an hour or two, then back into the shade, etc. All it takes is one distraction and you could lose all your plants from sunburn. The small portable greenhouses are all UV-coated to block UV light, so you can’t harden in these. Any greenhouse plastic is UV coated to make it last several years in the sun. It has absolutely nothing to do with protecting the plants from UV.
Here’s the trick to fool-proof hardening. The first step is to purchase a 10′×10′ 4-mil clear plastic dropcloth from Home Depot or any paint store. Make sure it is at least 3-mil thick material, and it can be clear or translucent. Greenhouse film blocks 98%-99% of UV light, while normal poly film blocks significantly less. A 4-mil thick clear plastic material will block about 25%-30%, and 6-mil plastic blocks 35%-40% of UV light. Ultraviolet (UV) light is the harmful rays that give you a sunburn at the beach, so the plastic sheet acts like a good sunblock, preventing plants from sunburning. I picked up a roll of 4-mil 84″ × 72″ clear plastic bags on Craigslist. Depending how many plants you have to harden, I would suggest that you make a plastic covered lean-to. I took a plastic patio table and turned it upside down, then covered it with clear plastic with duct tape. Keep the North side open so you have access to water you plants. Here’s an even simpler hardening tent using 2 lawn chairs and some duct tape. After 5 days under the tent the plants are completely hardened and ready for direct sunlight.
As I started more and more plants, I covered an old 10’x10’ screen-house frame with plastic. Now I use a 6-mil translucent plastic covered 10′×20′ portable carport frame for my hardening hoop-house.
Make sure that one side is open to get ventilation or you can fry your plants in a sauna.
Once again, I accidentally do something smart. I put my plants into those translucent storage bins, mostly so I could cover them easily, without having to fuss with plastic sheeting, both when it was too cold, AND when it was too hot.
Great idea—-wish i would have read this 1 week ago, as I’ve scarred my little buddies pretty awful over the last week trying to harden them off fully…egads. Will be trying this trick next year for sure.
I am with lex! I will keep this in mind for next year! I think I have the plastic laying around my house too!
I should have ready this before I lost my cucumbers. I was one of those that got lost in a moment at work and forgot to go out and pull them back. Lesson learned. I like all these ideas. I did a much smaller scale version by cutting out the bottoms of large mayo containers (from school districts food cafeteria) and used them as mini greenhouses for the rosemary. Worked brilliantly.
A word of Warning. Don’t close up the tents tight or you’ll turn it into a sauna. I always leave the North side open (South for Aussies) to get air circulation.
@Cristina…. Sorry I wasn’t more clear about that.
@hotwired – no problem (and maybe I have misunderstood Xan’s message too..)
After hardening off for a few years, this was my best year yet! I learned that the key is patience. There is no rushing or short cuts (like most things in gardening/life). I would have never taken this long, but it has been very sunny and windy here, so I was waiting for a better day to plant out. Here was my recipe:
4 days under plastic sheeting
4 days under window screen
2 days of 3-4 hours of direct sun
The plants never moved, I used a “lean-to” approach for both the sheeting and the window screen (I used the sliding glass door screen.) I planted out with most being planted in direct sun and gusty winds. After two days, everything still looks great!
so do I leave the plants out all the time—day and night right from the start and how high at night would you recommend it be before doing that?
Make sure the overnight temps are over 45F / 7C. I’d definitely bring them in at night until there’s no chance of frost.
thanks. I am anxious to start the process, but I guess it will be a bit more of a wait. Still having some frosty nights, but the promise of some double digit over night temps is in the forecast.
My hardening off process would also work for people with jobs I believe. I have a rolling wire rack that can fit many trays of seedlings on it. It can be rolled in and out of the garage, and left in either a sunny or shady location as desired. I just rolled it out for a bit longer each day. A person with a full time job could start out with evening outings, then add additional time in the mornings as needed until the total is at least 8-9 hours. Then it should be OK to leave it all day while at work I’d think. Especially if you have a neighbor willing to throw protection over it in case of a storm.