If buying lettuce in the store that was Aquaponically grown and has the roots you can set the plant in a bowl of water in the frig and harvest the exterior leaves. Setting the bowl/glass on a sunny window and the plant will continue to grow from the center. Getting roots to form from a cut bottom can happen but it would need sun, warmth and moisture with no chemical treatments on its way to market. It can be fun to experiment with lettuce and celery hearts but it is more of a way of extending “shelf life” and having a fun curiosity than “growing food”.
Sorry, I live in a very temperate climate and can’t grow many warm climate plants. Also need heat mats to start some seeds, etc. You are right, you need to find the coolest areas of your home to get propagation of lettuce.
Here’s what I found on lettuce seed....so I’m guessing around 20C. Pay attention to night temperatures also.
”Germination temperature: 40 F to 85 F (5-30C)— Seed can go dormant at high temperatures. Best germination is below 70 F. Days to emergence: 7 to 14 - Germinates in about a week at 50 F, 2 days at 70 F.”
You are quicker and easier to buy some cheap lettuce seeds, (I recommend Paris Island Cos) & start again.
Since you get a lot of heat, sow them in the shade,
We grow ours that way all the time. We just sow rows and don't thin them, then cut the nice tender leaves as we need them and they come back just fine.
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