Get some cheapo plastic laundry baskets from the dollar store....you may even have some on hand already. Use the baskets that have a wide open-weave pattern in their sidewalls. Invert them over the flats of seeds you are hardening off outside. Put a brick or an old cooking pot filled with water on the basket to weight it down and keep it from blowing away. The baskets allow for some air flow but will protect the seedlings from direct overhead sunshine which will sunscald them, they will get filtered indirect light through the sidewalls and that is sufficient for the the first week.
If you dip below 45 degrees at night you should bring the seedlings inside for the evening, but if it's been a very sunny day and the deck or patio where your flats are has absorbed a lot of solar-heat then you can simply toss a heavy blanket over the laundry baskets and at night the deck/patio will radiate it's retained warmth back into the air trapped under the basket by the blanket.
After about a week outdoors you can remove the basket for the early morning or afternoon, but still cover the flats with the baskets for midday. Obviously, if it's overcast, you won't have to worry too much about sunscald, but on sunny days it's a danger to the seedlings. It takes about two weeks to get indoor-started seedlings hardy enough to stay outdoors without protection once frosty nights have passed.
During the hardening-off period it is vital that you check the soil moisture of the flats on a daily basis. Most flats will need a thorough watering if it has been a sunny and/or windy day.
Good luck with your seedlings!
Entered by Trudi_d
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