Why Won't My Lettuce Seeds Sprout in Summer Heat?

You sow lettuce in summer for a continuous supply, water it faithfully, and... nothing comes up, even though the same seed sprouted easily in spring. This is one of lettuce's most counterintuitive quirks: its seed refuses to germinate when the soil is too warm. It is called thermo-dormancy, and once you understand it, you can outwit it and keep sowing lettuce through the heat. Let me explain.

What thermo-dormancy is

Lettuce is a cool-season crop, and its seed has a built-in safeguard: when temperatures are too high, the seed goes dormant rather than germinating. This stops it sprouting into conditions where the resulting plant would just bolt and fail. Above roughly 25°C, germination drops off, and above about 27–30°C many varieties shut down almost entirely. So in summer, warm soil — especially in the afternoon — keeps your lettuce seed asleep no matter how well you water it. This is the opposite of warm-season crops like tomatoes, which is exactly why it surprises people.

Start the seed cool

The trick is to germinate the seed somewhere cool, then move the seedlings out. Sow into trays or pots and keep them in the coolest spot you have — indoors in air conditioning, a cool room, a shaded porch, or even a cellar — until the seeds sprout, then move them into good light. Because lettuce germinates fast, it only needs a few cool days to get going. Some growers refrigerate the sown tray for a day or two, or chill the seed before sowing, to break the dormancy. Sowing in the evening, so the seed spends its first hours through the cooler night, also helps.

Cool the soil outdoors

If you are sowing directly outdoors in summer, cool and shade the soil. Water the bed well to cool it before sowing, sow in the shade of taller crops or under shade cloth, and keep the surface consistently moist and shaded until germination — a board or damp fleece over the row holds moisture and keeps the soil cooler (remove it the moment seedlings appear, since lettuce needs light to germinate and grow). Sowing in the cooler evening and choosing the shadiest part of the garden both reduce the soil temperature the seed experiences.

Variety and timing

Some lettuce varieties are far more heat-tolerant at germination than others, and choosing these makes summer sowing much more reliable. Pelleted, primed seed is also less prone to heat dormancy in some cases. And remember the simplest option: time your main sowings for the cooler parts of the year and use the cool-start tricks only for the height of summer. Put together — cool germination, shaded moist soil, evening sowing, heat-tolerant varieties — and you can keep lettuce coming even when the seed would otherwise refuse to sprout.