Growing Lettuce Indoors

Growing lettuce in containers is very easy and can bring a very satisfying touch of green to the house in the middle of winter or in residences without gardening space. Growing indoors also allows lettuce harvests throughout the year, either in the form of cut-and-come-again harvests of young leaves or periodic harvests of mature leaves or lettuce heads. Always make sure you use containers that drain or the lettuce plants can rot.

Lettuce plants can be grown indoors in a few ways:

Start a dozen or more plants in a seedling tray. This will minimize the space needed to get plants started, but then transplant them into larger containers after a few weeks of growth.

Plant one seed into a pot at least 6 inches in diameter by 12 inches deep and let the plant go to maturity without transplanting it. This approach works well for head lettuce, which is harvested all at once.

Create a small lettuce patch by planting a few dozen seeds into a larger round or rectangular container. This approach works well for leaf lettuce, mesclun or mixed greens. This method also works well for harvesting the leaves while they are still small.

All of these approaches will work indoors, but choose the container size and shape according to your harvest goals.

One or more plastic pots or containers with drainage holes in the bottom

Tray to fit below each pot

Seed-starting mix

Lettuce seeds

Spray/mist bottle

Shop light, grow light or sunny window

Watering can or pitcher

Prepare each container and tray by rinsing or brushing them to remove any old materials. Fill each container with seed-starting mix, up to about 1 inch from the top.

To plant one seed per container, make a small depression in the soil and place the seed there. To plant a lettuce patch, sprinkle seeds over the soil's surface. If these lettuces will grow to maturity in that patch, sprinkle seeds lightly on the soil surface, with approximately 3 to 6 inches between each seed. If they will be harvested multiple times, such as cutting lettuces, seeds can be much closer together, with only 1 to 2 inches between plants. In either case, cover the seeds only very lightly with a very thin layer of more seed-starting mix (less than 1/8 inch deep). Mist the top of the pot to moisten.

Many lettuce varieties require light to germinate, so set up lighting right after planting. Recommendations vary for how much light to provide at this stage, ranging from four to 14 hours per day. That variation is due in part to the changing needs for light as the seed germinates and starts to grow. A middle-of the road approach of eight hours per day is sufficient for germination. Once the plants have germinated, the more light they can get the better. This is the stage at which 14 hours of daylight will be beneficial. If the sunny windowsill doesn't get that much light, the lettuce will need supplemental lighting. A simple grow bulb aimed at the plants can work for a few small pots. If you have more than a few or you are growing a number of greens in a larger container, use fluorescent shop lights instead.

If you purchased the seed-starting mix right before using it, it should be nearly ideally moist already. If you are using seed starting mix from an opened bag, it may have dried out. If that's the case, add enough water to fill the tray beneath the pot, then let the soil draw up moisture. This will help ensure seeds are not disturbed. Keep adding water as it is drawn up into the soil. When the soil won't draw up any more water, discard standing water.

Interior temperature and humidity levels will determine how often the seedlings will need water. Start with watering once per day. If the soil doesn't draw up water each day, back off the watering to once every other day. If the soil draws up water very quickly, increase watering to once every 12 to 18 hours.

Lettuce plants will germinate within a week or two, depending on the room's temperature. Anywhere between 60 and 80 degrees Fahrenheit works well for lettuce plants.

After several weeks, you should have a thriving lettuce plant (or collection of plants). If you used a seedling tray or a small pot for the lettuces, and they are starting to overflow that pot, transplant them into a larger pot. This is where you can really get creative with containers because the seeds have already germinated and simply need more room. The pot will need to be sized for the type of lettuce. Generally, a 3 gallon container is large enough to grow at least one or two lettuce plants, depending on the specific variety and its mature width. Heading types require a container that is approximately 5 gallons to grow two plants.

Romaine-type upright lettuce varieties need a pot wide enough not only for the eventual width of the plant but also so that it doesn't continually tip over because of the Romaine's height. The mature plants generally need at least eight inches of row space between plants, so a container should give each plant at least that much room.

Butterhead, buttercrunch and loose-leaf varieties such as Santoro, Buttercrunch and Red Oak Leaf, respectively, may need a bit more width than the upright Romaines. Giving them more elbow room will also make it easier to harvest a few leaves at a time. These varieties vary in mature size and need anywhere from 6 to 12 inches between plants. Check the seed packet for specific spacing information.  

Head lettuce varieties such as Iceberg, Ithaca and Summertime will need perhaps the most room of all. Not only does the container need size for the mature head but also for all the side leaves. Keep at least 12 inches of space between plants.

Mini-head lettuce varieties such as Winter Density, Bambi and Escale have the same growing habits as full-sized head varieties yet offer smaller dimensions and shorter maturity times. These can be attractive features if head lettuce is appealing, yet the better-known head types are too large for an indoor space. Check the seed packet for spacing information.

The seed-starting mix will provide a well-balanced start to the seedlings. The growing plants, however, will eventually need fertilizer either mixed into the potting soil or applied directly to the container on a roughly weekly basis. If the lettuces will live their entire lives in the same pot, simply apply a water-soluble, complete fertilizer every other week, per the label directions. If they are destined to be transplanted, mix a slow-release fertilizer directly into the potting soil. Some potting mixes already contain a slow-release fertilizer so check first. When choosing fertilizers, look for one that offers the three primary nutrients in equal ratios; it will be listed as 5-5-5 or 10-10-10 on the label.

For example, one water-soluble, complete fertilizer product suggests using 1 teaspoon of the product mixed in 1 gallon of water and pouring over the soil. Another slow-release fertilizer product suggests using 1 teaspoon of the granules and spreading it over the soil and watering in well.