With grow lights, the right soil, and comfortable temperatures, you can grow lettuce inside all year long. Growing lettuce in a bright sunroom or in a large windowsill may give the plant enough light to grow during the spring and summer, but in the winter you’ll likely need to add a supplemental light source. Also, leaving your lettuce plants—especially seedlings—on a smaller windowsill may cause them to grow long and weak. This is called getting leggy. The plants stretch toward the sunlight, and in doing so, they grow long and spindly, instead of stout. Here is where the grow lights come in handy. With the lights directly overhead, the plants grow straight up and won’t get leggy. But you can certainly grow salad green indoors all year long with this method.