Will lettuce seeds germinate in summer?

Question

When the ambient temperature is between 30 to 35 °C in summer, will lettuce seeds germinate?

Answer 1

Lettuce seeds show "high-temperature inhibition", or, plainly put, won't germinate at temperatures over 85 °F / 30 °C. Considering the fact that they are native to the Mediterranean, that's a wise mechanism: summers tend to be dry there and the seeds would be at risk of drying quickly.

While they will germinate between 40 and 85 °F / 5 and 30 °C, you will get the best results in the 55-70 °F / 12-21 °C range.

If your nights are cool, sowing in the evening and shading / watering during the day might suffice; otherwise consider germinating them indoors, e.g. using the paper towel method or flats / trays, then transplanting them. Don't forget to harden them off, in this case not because of the temperature, but because of the sun, and choose heat-tolerant breeds or your lettuce will bolt and get bitter quickly.

Answer 2

You also have the option to germinate the seeds indoors assuming temperatures will be much cooler and conducive to lettuce seed germination. You can then transplant them outdoors. In my case I do that to avoid cold temperatures that may prevent germination.