Answer: Lettuce does best in bright light, rich soil with a pH of about 6.0 to 6.5, and even moisture. It also needs an ample supply of nitrogen to support those leaves. You might want to run some basic soil tests and see what if anything needs to be added to your soil. Lack of heading is most often caused by high temperatures, (lettuce grows best below about 75 degreees) but can also be caused by dry soil or oddly enough an oversupply of nitrogen. Since your leaf lettuce is so slow I woudl suspect a lack of nitrogen and/or moisture as well as possibly a lack of light. I hope this helps.
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