How to Get Lettuce to Persist into Summer
"So I started using shade cloth over my lettuces and similar cool-season crops. I purchased 40% Sunblock Shade Cloth and clipped the fabric to the rabbit/groundhog fence which is about two feet above the raised bed. Note: Be careful not to let any fabric dangle onto the ground because groundhogs can climb the fabric. This has extended my lettuce season (except for the groundhog issue). Perhaps this year I will extend the shade cloth to cover the tomatoes if things get too hot.
A. Eric, if summer gets too hot for tomatoes in central PA, I'm moving to the tropics of New England!
Anyway, you point out the challenge in trying to enjoy your first ripe tomatoes in a summer salad; by that point in the season, heat sensitive lettuce will send up a tall central stalk, and the leaves will become bitter, as evidenced by a white latex-like liquid on the cut leaves.
Shade cloth is an excellent way to slow this process. Available over the Internet and at hipper local independent garden centers, shade cloth comes in a variety of 'percentages', indicating the amount of direct sun it blocks. It is typically suspended on hoops to hold it above the plants. The taller the hoops the better, as a too-close-to-the-plants set up might trap excess heat and defeat the purpose.
Professional shade cloth is draped over very tall hoops and left open at the ends to protect against trapped heat. Used correctly, this will lower the temperature at the soil level, especially with adequate watering, which is important. If the water is applied with sprinklers, it should be done only in the early morning. If you use drip irrigation, do it when the sun goes down to chill the soil.
Remember that lettuce plants thrive in cool soil, but lettuce seeds do not. Start your first run or two indoors and transplant them into the soil as soon as it's "workable", which means not frozen. If you're in an area where unfrozen soil quickly morphs into hot days, install the plants at night. If you're in a cold clime, remove all mulch from the planting bed two weeks in advance to allow the sun to directly warm the soil. If necessary, use row covers to protect the plants from extreme cold, then switch to shade cloth when the heat gets hot.
An easy alternative is to situate the plants where nearby trees will provide shade as they leaf out, but allow full sun to hit the plants before that.
Note: Lettuce sown in the 'Fall' (late August in most regions) will always grow better and easier than Spring lettuce. In late summer/early fall the soil is warm and ideal for direct seeding. Then the shorter days and cooler nights prevent bolting naturally. Do you really care if you enjoy most of your tomato and lettuce sandwiches and/or salads in early September? It's a heck of a lot easier to achieve. (Pick the tomatoes and greens early in the morning for the sweetest flavor.)
Variety choice is critical. In the Spring, you should only plant varieties touted as heat resistant, heat tolerant or 'slow to bolt'. Back in 1994 garden writer Lois Trigg Chaplin compiled a list of heat resistant varieties for the July/August issue of ORGANIC GARDENING magazine. Her first choice was pretty obvious: SLOBOLT, which was indeed the slowest variety to bolt in her Alabama garden, while the popular variety GREEN ICE lasted the longest at a nearby neighbor's place. Both are leaf lettuces.
A Florida gardener found SIERRA, a 'loose head' lettuce to be the best at beating his extreme heat; it was also the winner in Londonderry, VT, a polar opposite in climate. Oregon State University researchers developed a head-forming variety called SUMMERTIME, which seems self-explanatory. In Dallas, MISSION, another head lettuce, won the war.
The romaine variety PLATO excelled in both California and Clemson University test gardens. (Dr. David Bradshaw of Clemson asserts that romaine lettuces in general always outlast other types.) The classic Amish variety DEER TONGUE (which I grow every year) also gets a shout out. And finally, the vibrant red lettuces RED SAILS (another variety that graces my garden every season) and ROSALITA (Bruce Springsteen's favorite) resist heat well.
When the weather gets warm, make sure you have two inches of mulch around the plants to keep the soil cool. Excellent choices would be compost, shredded fall leaves or pine straw. Terrible choices would be any kind of dyed, bagged wood chips, although arborist wood chips can do in a pinch. If you do use them, keep the layer fairly thin.
Groundhogs: They are a daunting pest. They can easily burrow under your beds, and they are excellent climbers. Any fencing must go down deep and have an unsupported 'baffle' at the top to drop them back down on their furry little butts. My first line of defense would be a motion activated sprinkler; when it detects something moving near your beds it throws a coupla cups of cold water at them.
Sorry Gus (Pennsylvania's second-most-famous groundhog and spokesmammal for the PA lottery); hope we didn't ruin your scratch-off tickets!