We seem to be having trouble germinating the more heat tolerant lettuces. Jericho in particular is proving tricky, the others are offering ok results. With Jericho we are getting close to nothing. Are you treating the heat tolerant varieties any differently than others?
-Zac Jones,Twisted Ridge Farms
Dear Zac –
Lettuce is one of the top three vegetables that Americans consume (potatoes and tomatoes are the other two) and summer is salad season when no one wants to turn on the stove. Summer is also the most challenging time for lettuce production on our farm. This year has been particularly challenging with high temperatures and erratic rainfall.
To grow summer lettuce we need to provide the crop with three items – favorable conditions for germination, favorable microclimate, and an effective strategy for disease management.
Germination – The Johnny’s catalog germination guide indicates optimal germination for lettuce at 68 degrees. Lettuce will go dormant at temperatures in the 90s and may germinate eventually, but yields a poor stand and lets the weeds get a head start. In most years our average summer temperature is cool enough, though sun on the soil surface, even under shade cloth, may exceed optimum conditions. One way to solve that problem is placing a block of closed-cell foam insulation over the seed flat after sowing and watering. Any thickness of foam will help but I find that two-inch thick sheets are less likely to blow away. That approach worked for many years but eventually our mice found those sheets very cozy, especially with organic seed snacks close by. I now use a reach-in cooler and two old refrigerators (turned off) to keep the temperatures low until the seed coat cracks. Those metal boxes also keep the critters out. The refrigerator approach requires checking periodically but it works for us. Once the seed germinates, it will do fine in higher temperatures, even into the nineties. I do not direct sow lettuce but primed seed, sown in the late afternoon might work. A word of caution on primed seed – it is only good for a few months so consider reordering if you need more seed a few times throughout the season.
It probably goes without saying but seed with a poor germination percentage will perform poorly at any temperature. I suggest comparing your results with similar seeds from various seed houses. I have found big differences.
Early on, I switched to summer varieties in the summer but have since abandoned that approach. It seems to me that summer varieties bolt slowly because everything about them is slow, including their growth rate. In my view, slow growth gives fungi the upper hand so I stay with spring/fall varieties and harvest them promptly. Promptly means checking every few days in the summer.
Microclimate – Lettuce likes cool temperatures and moist soil. I find that shade to the west is a big help and mulch is also good for keeping the soil evenly moist and cool. We grow on landscape fabric but I suspect straw will work also. There may be a slug problem with straw. Generally, slugs are not a big problem with fabric. Our coolest field has western shade so it gets sun from about 10:00 to 5:00, which seems to be plenty for lettuce in the summer. Even a tall crop to the west of your lettuce crop may help cool your lettuce on hot, sunny afternoons.
Disease Management– Disease pressure probably varies from farm to farm but our two disease headaches with lettuce are aster yellows and lettuce bacterial spot. An Internet search will yield any number of great photos and management strategies. Much of the lettuce research is from California so recommendations to “eliminate insect vector populations within a half mile” are not very practical for us.
Aster yellows is very distinctive and is spread by leaf hoppers which are common in our area. Over the years we learned that removing infected plants at the first sign of yellows, keeps damage at acceptable levels. Photos are available on the web and through Extension but I look for yellow plants with distorted centers and tan blisters. With a little practice we can recognize it early in the field. By removing the infected plants quickly, the leafhopper vectors will have fewer infected plants to feed on.
Some varieties are more susceptible to lettuce bacterial spot than others. I find that dark red is a good color because the early stages of the disease are less obvious. Eliot Coleman once wrote or said in a talk that growing lettuce is “a race with rot” and I am inclined to agree. The best defense in my view for a wide range of fungi is great soil, good moisture, a vigorous variety, and happy transplants. Timing overhead irrigation to water when the plants are already wet from the dew is also helpful. Avoid extending the wetted period by watering for several hours just as the dew is about to dry out. Good circulation around the plants helps too so narrow beds may be helpful. The best disease defense is enthusiastic growth from a happy transplant to a harvestable head.
Happy growing – Tom
Tom Elmore is co-owner and operator of Thatchmore Farm in Leicester NC. He has grown certified organic fruits and vegetables for 25 years and serves on the Boards of the NC Greenhouse Vegetable Growers Association and the Organic Growers School.