Answer: Hot weather will turn lettuce and spinach bitter almost immediately, so it is really important to get them in the ground as early as possible and to begin harvesting as early as possible. For the tastiest of greens, begin harvesting as soon as the leaves are large enough to snip with a scissors; the greens will continue to grow for weeks and weeks and you'll maintain a steady supply of the most tender new growth throughout the season.
In addition to regular watering, and picking early, I would suggest mulching to keep the soil cool. You should also ensure an ample supply of nitrogen to keep them growing quickly. You might also want to check that your pH is between 6 and 7. Finally, you might experiment with several varieites to find the ones you like the taste of the best.
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