Heat may cause seedlings to wither and fall over after emergence. Plants may also fail to develop. Heat injury can sometimes be confused with damping off: Stems of seedlings affected by damping off usually have a dark, shriveled portion at the soil line. Prolonged hot weather may cause older spinach and lettuce plants to form seed stalks. Plants begin to grow tall and send up flower stalks.
Solutions
Heat damage can be avoided by shading plants. If planting during warm weather, use floating row covers to protect plants in the seedling stage. Once there are enough leaves to shade the stem, covers can be removed. Plant lettuce and spinach during cool weather to avoid seed stalk formation.
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