Peas (Pisum sativum) and lettuce (Lactuca sativa) can be planted as early as 6 weeks before the last frost, as long as the soil is moist but not muddy. It's important to get peas in as early as possible. Yields drop dramatically on later plantings because peas tolerate heat poorly. Plant lettuce at 2-week intervals through early summer so you can extend the harvest into midsummer. Plant lettuce again in late summer for a fall crop. You can try a late-summer planting of peas for a fall crop also. They germinate poorly in warm soil and dry conditions, so try shading the seedbed to keep the soil cooler and retain moisture. This is good advice for summer-sown lettuce too.