Meemur...is your previous late lettuce looking bad because of poor germination, or did it germinate ok, but the heat is beating it up?
For germination issues due to the soil being too hot, there are a couple of tricks. "Too hot" for leaf lettuce is 80° or above...above 85-90°, it won't germinate at all.
I've used ice cubes for cool weather seeds that hate (or simply die) transplanting. I sprinkled the seed on the soil, patted it in (palm of my hand, just pressing tiny seeds down so they have good contact with a firm soil surface), and then spread 10# (for a 4 x 8' bed) on top. I also hung shade cloth over the bed. If the weather is very hot and dry, a second application of ice cubes 24 hours later may be needed.
Or, try planting in seed trays, and put them in the fridge for 24 hours, then bring out and let them sprout indoors. Transplant into moist soil (thorough watering reduces soil temps a few degrees if the water is from a cool source), and use shade cloth and frequent misting to give them a good start.
You can also use the "paper towel seed tape" trick.... moisten a double layer of paper towels. Sprinkle the seeds evenly across the surface. Lay another single layer of damp towel on top, press gently. Then *loosely* roll the towels up, and slide them into a plastic bag...DO NOT seal! Put it somewhere the cat can't get it, and check it after 3-4 days. Once healthy sprouts have appeared, unroll the towel onto well watered, prepared soil. *Lightly* sprinkle soil on top, until the white towel isn't visible. Water frequently for the first few days.
You dont want to see a corner or edge of towel sticking above the soil, because it will "wick" moisture away from your seeds or baby plants.
For heat issues, shade cloth makes a huge difference. Placing organic mulch (fine compost works well) over the soil around the plants can help keep the temperatures at the root zones a bit cooler.
Last year, we had extreme heat...its rare to see it over 100° here in north central NY. The broccoli and cauliflower got hit pretty hard... it either bolted early, or turned dark and bitter. I did get quite abit of good tasting cauliflower from a few late plants, and the side shoots of broccoli I harvested after our first frost were delicious. But this year, I decided to try a couple options.
One bed of 8 broccoli plants was covered with a small "screen house" (Temu), which is meant to exclude insects. It sure did that! But it also must have kept the plants cooler...the plants I left uncovered in a raised bed (granted, that soil was probably significantly warmer than the covered plants which were in the regular garden rows) had similar issues to last year. Uneven growth in the heads, loose flowerets rather than the nice, firm buds...the flavor was *much* better, though. But the plants in the covered bed...holy cow! Huge (10" diameter) heads with bright green buds, and they tasted *sweet*!
And ZERO bugs! I put up 2 bushels in the freezer, and didn't find a single cabbage worm!
The cauliflower in a screen covered bed looks great...it's just getting ready to produce heads, there is no insect damage, and I'm hoping as it grows in our cooling late summer weather, the flavor and texture should be excellent.
I ended up skipping using actual shade cloth, because we haven't had the extreme heat this year.
It all takes a bit of fussing, and I need to plan out my plant spacing better, so the "screen covers" will fit over the plants without crowding or not fitting inside well. But its been well worth the trouble and time!
Summerthyme