Lettuce is one of the easiest plants to grow indoors and can be harvested after ~30 days depending on the variety so you can have a fresh supply of greens all winter long.  Most lettuce varieties are considered cold weather crops so the only real constraint on growing lettuce is temperature.  However, as long as growing conditions don’t exceed 26℃ (~80°F) you should be fine.

Things you will need:

1. Container – any small container that is at least a few inches deep should be fine.  Remember to drill drainage holes in the bottom if there aren’t any already.
2. Indoor potting mix – most peat-based mixes will work fine for this purpose as long as it has relatively good drainage.
3. Lettuce seeds – There are many different varieties of lettuce out there.  There are loose leaf and head type lettuces and others that fall somewhere in between.  There are also some more heat tolerant varieties that are suitable for higher temperatures so pick the variety that fits your preferences and growing environment.  
4. ***Optional: A spray bottle to moisten the soil – This helps because the lettuce seeds are very small and watering by pouring water on the soil may cause the seeds to float and could affect germination. 

Sowing and germination

Lettuce seeds can germinate at temperatures as low as 4℃ but typically won’t germinate above 26℃.  They are also one of the few seeds that require light to germinate properly. 

To germinate lettuce seeds, pre-moisten an indoor potting mix with good drainage and put it in your container.  Make sure the container has drainage holes in the bottom so the roots don’t become water logged.

Next, sprinkle the seeds over the top and gently press them into the soil so they make good contact.  You can either leave them uncovered so they germinate quickly or cover them in a very thin layer of dirt.  Remember that they need to receive some light to germinate.

Once the seeds are sowed, it’s a good idea to spray them with water to ensure they have enough moisture to germinate.

Place the seeds under a grow light or in a sunny window sill and after 3-5 days, you should see the seedlings emerge.

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Thinning out your lettuce seedlings

If you planted more than one seed per cell or at a high density in a larger container, you will most likely have to thin out your lettuce seedlings so they don’t compete with one another. Start by picking the best looking seedling in each cell and remove all the others. This can be done by gently pulling on the seedlings until they come out. It is best to do this while the plants are still young before their roots develop and get all mixed up with their neighbors.

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If you started you lettuce seeds in a container, you will want to thin them out so there are at least 6 inches between each plant. They will need this room to grow to their full size.

Transplanting your seedlings

Once your seedlings have grown their first true leaves (not the cotyledons that emerge from the seed like those in the pictures above), it won’t be long before they need to be transplanted to a larger container. If you started your seeds in a larger container and have thinned them out, you don’t need to worry about this part since your plants should have enough room to grow to full size already. If you started your seeds in starter cells, the roots will be starting to run out of room after a week or so and need to be moved to a larger container.

Start by filling a container with potting soil up to about an inch from the top. The container should be large enough to allow the plant to grow to full size. I would recommend something at least 5″x5″x5″ for a single plant but you could transplant several seedlings into a larger container. Once you have prepared the new container, gently massage and squeeze the bottom of the seed starter cell to try and get the soil and roots to separate from the walls of the container. Once the roots seem separated from the cell, gently pull on the stem of the plant near the base and lift the seedlings out of their cells. Remember these seedlings are still very fragile. In your new container, use your fingers to move some soil to the side to make a small space for the seedlings roots and place the seedling in the hole. Move and excess soil around the seedling and gently push down on the soil around the stem to make sure the roots have good contact with the new soil.

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Once all your seedlings are transplanted, give them some water. In their new home, they should flourish and provide you with fresh greens in no time!

Growing requirements

Temperature

Like I mentioned earlier, lettuce is typically considered a cold weather crop that is normally grown in the spring and fall so it prefers cooler temperatures.  High temperatures in the summer can cause the lettuce to bolt and go to seed resulting in bitter tasting leaves.  Since we are growing inside however, just make sure the lettuce plants aren’t placed in a window that gets really warm on sunny days or over a heat register.

Light

Lettuce does not require very much light but should get between 4-8 hours of direct sunlight per day.  Another option is to grow them under grow lights.  The intensity of the grow light will determine how close your light should be to the plants.  Leave the light on for 12 hours per day. 

Water

Lettuce roots like to be kept moist.  Depending on your growing environment, they may need to be watered twice or three times a week.  The best way to know when to water them is to stick your finger in the soil up to the first knuckle.  If it feels dry, they need water.  If the soil still feels cool/moist, wait a day or two before watering again.    

Nutrients

Lettuce is not a heavy feeder so it only requires a little bit of fertility.  Most indoor potting mixes with added nutrients will be enough for the plant to grow to maturity.