re lettuce bolting
when I had a garden I grew many varieties of bibb and other thiings
never bolted w shade clolth
get 50 %
I got black as that was all they had in the previous century
white might be better
easy to ' hang ' from a few posts about 3 - 4ft high
http://www.gemplers.com/shade-cloth
best stuff ever.
never bolted all summer long
also consider Remay woven cloth for covering beds in very early spring and fall
you will get 2 more months of lettuce and etc.