How to Grow Salad in Containers. Homegrown salad simply tastes better than the stuff you buy in a box at the grocery store, and you don't need a garden to grow your own greens. These easy-to-grow vegetables thrive in containers because they have very shallow root systems and don't need rich soil. I've grown greens on my front porch, back deck, and even my roof, and you can too. All you need is a spot that gets some sun, a pot filled with potting soil, and seeds or seedlings.
Place your container in a spot that gets at least 4 hours of sun a day and is convenient for you to water. Greens grow well in any container that has at least 6 inches of space for soil and a drainage hole. Good drainage is imperative for growing greens because their roots rot in water logged soil and they are more susceptible to fungal diseases like bottom rot and damping off.
Fill your container with a high-quality organic potting mix to within 1 inch of the rim and then water it in.
If you're using seeds, sprinkle the seeds as thinly and evenly as possible across the soil. Gently press the seeds into the damp soil to ensure good soil to seed contact and then cover the seeds with a very light layer of potting soil (about 1/8 inch). Water them in with a gentle stream of water. If you are planting seedlings, space them 6 inches apart. Consider mixing leaf lettuces into your ornamental containers.
Keep the soil evenly moist. Lettuce needs a constant supply of water to keep the plants growing fast and to prevent them from turning bitter or going to seed prematurely. Remember that pots dry out much more quickly then garden soil, so you may need to water a couple of times a day if the weather is warm or your container gets a lot of sun.
Harvest greens planted by seed at the baby greens stage. When the greens are about 4 inches tall, simply mow them down with scissors, leaving about a 1 inch stub. After cutting, water with one cup of diluted fish emulsion to give the plants a little kick start. The greens will re-grow in just a few weeks and you can get two to three harvests from one sowing. Allow seedlings to grow into mature heads. Pinch the leaves as needed or harvest the whole head by slicing it off at the soil line with a sharp knife.