Even early in the year, sun shining directly through glass or plastic can have a burning effect on delicate seedlings. Basil1In an unshaded and unventilated greenhouse, propagator, coldframe or cloche, this can result in temperatures well above safe levels for your seedlings.

This problem becomes more acute as the season progresses and the sun gets stronger. Seedlings can the easily be scorched, particularly if they are touching the glass or have droplets of water on the leaves. For this reason, young seedlings under glass should not be placed where they will be in full sun even for part of the day.