Lettuce seeds will not always germinate when soil temperatures are too high normally the height of Summer approx. above 26 degrees Celsius. This is normal and does not indicate poor germinating seed, sow them again in the Autumn or Spring.
There is also the probability that all lettuces, when grown in hot conditions will bolt to seed quickly, especially when a cool period precedes this.
Icebergs are hard to grow and very specialist plants for specific growing regions. In certain conditions they will not head properly no matter how good the variety or regional variation may be, these conditions include very warm weather, intense sunny weather following very cool periods.
This is because the plant gets set back when trying to form the head, classic signs are pale leaves, poor head, burnt tips of the leaves, very bitter leaves, brown spots on the leaves.
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Happy gardening - wherever it may be!