Answer: Lettuce generally tastes sweetest when grown in cool weather. Hot weather causes plants to bolt--that is, to form seed heads. Once this process begins, the leaves can become bitter. In regions where spring is relatively short, it's sometimes tough to get a good crop of lettuce, because the weather warms up before even an early crop is mature. I like to plant a succession of leaf lettuces every two weeks or so, so that I always have some young leaves to harvest.
Some lettuces are also more bitter than others, so the variety of Romaine you are growing could also be a factor in the taste. Though lettuce does benefit from fertile soil, if the plants are growing well the addition of fertilizer probably won't affect taste. As far as watering goes, you need to keep the soil evenly moist. The amount depends on weather, soil type, and temperatures. Young seedlings need to be watered more often than established plants. Check the soil every day at first, then every few days once the seedlings are growing well. Water when the soil is dry to a depth of a half inch or so.
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