Harvest head lettuce when the heads are firm. Hard heads may be overmature. Pick leaf lettuce before the stem elongates by cutting or pulling off the outer leaves. The leaves are bitter and tough after the stem elongates. Tipburn is the drying of the leaf edges and usually only head lettuce is affected.
High temperatures are a factor.
Use tipburn resistant varieties. The ribs of some leaves turn brown possibly due to high temperatures. Plants will form seedstalks in response to hot weather. This information is for educational purposes only. References to commercial products or trade names does not imply endorsement by MSU Extension or bias against those not mentioned.