Lettuce is a cool-season vegetable, it grows best in temperatures around 60 – 65°F. Once temperatures rise above 80°F, lettuce will normally start to “bolt” or stop leaf production and send up a stalk to flower and produce seed. The leaves become bitter when this happens.
Romaine types such as Mayan Jaguar and Pomegranate Crunch, Butterheads, such as Tom Thumb and Speckled Amish, and loose-leaf lettuces, such as Buckley and Slo-Bolt tolerate heat better than tighter heading lettuces like Iceberg.
Several things you can do to grow lettuce in summer, at least a little longer.
Mulch Lettuce tolerates a higher air temperature if the soil around its roots is cool and moist. Keeping your soil cool and damp encourages your lettuce to grow longer without bolting. Since lettuce has wide and shallow roots, a thick mulch keeps it happier in warm weather.
Partial shade is one way to keep lettuce growing later into warm weather. Deep shade isn’t good, but a system allowing sun during the morning while sheltering the plants in the afternoon keeps them living longer.
In the long run, when it gets as hot as it is going to get in the upcoming weeks (100’s), there is very little you can do to prevent your lettuce from bolting and turning bitter. To salvage it, harvest it and it will keep n the refrigerator for weeks. Much longer than anything you can buy at a grocery store.
If you are starting seeds during these times, just know that most lettuce seeds won’t germinate as temperatures rise above 80°F, a condition called “thermo-inhibition”. This trait is a carryover from wild lettuce in the Mediterranean Middle East, where summers are hot with little moisture. If the lettuce seeds sprouted under these conditions, they would soon die out and the species would go extinct.
The hot weather looks to be here. This morning I saw that it’s going to over 100 degrees for the next six days. (Unusual for our area) That means I won’t be going outside except to water. (I may stick a picture of myself in my horse’s paddock for her to remember me!) Although most people assume that tomatoes love hot weather. They don’t.
Fruit development slows as the plant focuses on moving water through its system. A heat wave can also keep tomatoes from developing into a deep red, resulting in orange fruit.
Most varieties of tomato plants take a break, even those who are bred for warmer climates. They don’t care for excessive heat any more than we do. When daytime temperatures are up in the 90s and nights are in the 70s or warmer, tomato plants may keep on blooming, but the flowers often fall off and fruit does not set. Pollination doesn’t occur when it it is too hot. Once the flower opens, it has a short 50 hour window in which to pollinate or they abort, dry up and drop off. Don’t worry, they will soon return to normal as it gets cooler.
Here are few things you can do to help them through this time:
Mulch Adding mulch around the base of your tomato plants can help keep the ground a few degrees cooler and and aids in keeping moisture fro excessive evaporation. Use two to three inches of mulch, things like leaves or grass clippings.
Avoid Overwatering You might think the plants need more water than normal. They don’t, they need the same amount but more often. In pots, I water throughly every day in hot weather. I soak them until water come out the holes in the bottom of the container. If it’s a smaller pot with very little volume you might need to do this twice a day. (See why I recommend LARGE containers?) Tomato plants need an inch or two of water a week. A deep soaking is better than a little water every day. The best way to tell if your plants need water is to poke your finger into the soil. If it’s dry more than an inch down, it’s time to get out the hose.