We usually grow lots of lettuce for market all summer long without any problems. Last year we started all our transplants under shady maple trees, and I had about 100% germination every time I seeded my transplants. I direct sow into Speedling 128’s that have been bottom-soaked and I don’t cover the seed. This year I seeded the transplants the same way but they were under a shade-cloth covered canopy. Once summer really hit, I only had about 5% germination. When I started having trouble, I tried putting the seed in the refrigerator, but that didn’t help. I tried new seed. I have tried everything I can think of with no luck. Any ideas?
– Pete in Grapevine, NC
Lettuce is one of that 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. It 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 a 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 yields a poor stand and lets the weeds get a head start on it. In most years our average summer temperature is cool enough but sun on the soil surface even under shade cloth can 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 will help but I find that two inch sheets are less likely to blow away. That approach worked for many years but eventually the mice found those sheets very cozy 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 plan to reorder a few times through the season.
It probably goes without saying but seed with 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. For a while I switched to summer varieties in the summer but have since abandoned that approach. It seems to me summer varieties bolt slowly because everything about them is slow, including their growth rate. Slow growth gives fungi the upper hand in my view so I stay with spring/fall varieties and harvest them promptly. That 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 to keep the soil evenly moist and cool. We grow on landscape fabric but I suspect straw would work also. There could be a slug problem with straw. Our slugs are not a big problem with fabric in most years. Our cool field gets sun from about 10:00 to 5:00 which seems to be plenty for lettuce in the summer.
Disease Management– It probably varies from farm to farm but our two disease headaches with lettuce are aster yellows and lettuce leaf mold. 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 very common in our area. Over the years we learned that removing infected plants at the first sign of yellows, keeps damage at acceptable levels.
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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.