Baby, it’s HOT outside! You wanna plant lettuce in your garden for fall harvests. But it’s August and it’s hot.
Lettuce seed has a hard time germinating when the soil and weather is too hot. They just won’t sprout when it gets over 80°F. Under hot conditions the seed experiences “thermo-inhibition." Kinda wild, right?
Wild lettuce originated in the Mediterranean in the Middle East, where summers are hot with very little moisture. If lettuce seeds sprouted under these conditions, they would likely die. Lettuce evolved here by refusing to germinate until growing conditions are right.
The best temperature for germinating most lettuce seeds is 68°F. Lettuce seed will sprout in temperatures ranging between 40 - 78°F but 68°F is best.
Both the temperature of the soil and the air temperature is important and they can vary by several degrees. And soil temperatures can get quite hot, especially if the sun is baking black ground with no cover.
That makes growing lettuce in late summer tricky! Lettuce likes moisture and cooler temperatures. So in the heat of the summer its best to start lettuce indoors.
Starting indoors allows you to control the temperature of both the air and soil in that ‘sweet spot’ for the best growth. It’s pretty easy to grow lettuce in a tray for at least a third of its life, and then plant it out. You can watch our latest video on starting lettuce in the heat of summer by clicking here.
One simple way to up the germination rate for summer plantings is to put the seed in the refrigerator in a sealed container for 24 hours before you plant. This will trick the seed into thinking its spring, bypassing its ‘thermo-inhibition.” Cool, right?
Tips to help lettuce 'keep its cool'
If you live in a really warm place and its the hottest time of the year, say July or August, just transplant your lettuce in a little shadier spot in the garden. Or transplant in the shade of taller crops.
Another option is to use a shade cloth to block some of the sunlight. Best to offer them shade during the hottest afternoon sun.
Heat stress can be reduced a little bit by moisture. Water helps keep the roots cool. Also, using mulch or landscape fabric will help keep the moisture in and the ground cooler, two things that lettuce loves!
Hope these tips and our companion video where we 'show you how' we do it will help keep your salad bowl full!
Our best to you,
Theresa & Dan