How to Grow Lettuce Indoors
Growing lettuce indoors is not only easy but economical too! Although not technically a microgreen, we're going to categorize it as such because we harvest baby leaf lettuce before it's a mature plant.
We'll show you how to grow lettuce indoors from seed to harvest.
The methods, steps, and equipment we use are a bit different than how we grow most microgreens, but it's easy, inexpensive, and you can get more than one cutting from a tray.
Below, we'll explain and show you a video on how to grow lettuce indoors step-by-step.
Click here to skip down to the video.
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What's Needed to Grow Lettuce Indoors
As we mentioned, the equipment and procedure are a bit different than our usual microgreen methods.
First, let's go over the trays and required light for growing lettuce indoors.
Trays for Growing Lettuce
Microgreens can be grown in smaller trays (although there's nothing wrong with growing them in large trays either) because they're usually added to salads for flavor, color, and nutrition. They're also added in small quantities to dishes and food as ingredients.
We find the Home Microgreen tray at 38 square inches an excellent size for most microgreens.
For Lettuce, Not So Much
While lettuce is generally the bulk of a salad, more product is needed per person. Therefore, it makes sense to sow lettuce seeds in larger trays than microgreens.
We use shallow 1020 trays (shown below), the same trays we use to grow microgreens that we sell at grocery stores, customer subscriptions, and farmers' markets. You can purchase these at Bootstrap Farmer.
You can also use 1010 shallow trays, we often use these to grow several different lettuce types at the same time.
You can purchase the 1010 trays in the Home Microgreen Store.
We also recommend using a humidity dome.
Lettuce seed needs humidity and, to a lesser extent, light to germinate.
The humidity dome will keep moisture around the seeds and seedlings, even out temperature fluctuations, and keep the upper surface of the soil moist, so the tender roots don't dry out.
Before we discuss how much light you need, I'll introduce the video that shows you the step-by-step process on how to grow lettuce indoors.
How to Grow Lettuce Indoors Video
Lettuce, unlike microgreens, does need quite a bit of supplemental light when grown indoors.
The type of light needed depends on the variety of lettuce you're growing as well as your expectations.
Green Leaf Lettuce
Green leaf lettuce grows to the baby leaf stage (1.5- to 3.5-inches long) requires the least amount of light. A south-facing window can work, although we do recommend using an LED light in the color spectrum of at least 5,000k. We have used these Barrina lights with success.
Red Leaf Lettuce
Red leaf lettuce, however, requires more light. A full-spectrum LED grow light is recommended. The red leaf will grow fine under the less powerful LED lights, but the color will be dull. In that case, you might as well grow green leaf lettuce as it will grow more vigorously than red leaf under poor light.
We've had great success with the Mars Hydro SP 150 LED grow light. You can see a review of this light (growing red leaf lettuce) in the video below.
Hopefully, we'll get a Mars Hydro SP 250 to test as it will cover a larger area and be more powerful bring out even more intense red colors in the leaf lettuce.
The general rule is that lettuce grows better in locations that receive more natural sunlight, or the artificial light has a broader spectrum and more wattage (power).
Pin the photo below to your Pinterest Microgreen Board!
Steps on How to Grow Lettuce Indoors
Fill a 1010 or 1020 tray with a professional potting mix. We use coconut coir based potting soil as it is easier to wet in the beginning, wicks water better from below, and doesn't oversaturate as peat-based mixes do.
Level the soil in the tray, tamping it firmly but not compacting it.
Wet the surface of the soil with a spray bottle. The soil doesn't need to be saturated all the way through. Only the upper half needs moisture.
Add lettuce seed to the shaker bottle.
Here's the part most people can't believe.
Only add between 0.7 and 0.8 grams for a 1020 tray. I know it doesn't seem like much.
As the lettuce grows, the leaves will take up the room. Add more seed, and you'll have die-back inside the tray. The leaves get so dense that the inside leaves with brown and whither.
Trust me. Don't even use all of the 0.8 grams.
Usually, we only sow 0.6-grams! The plants will be healthier, and the second cutting will be better.
Sow the seeds evening across the tray. This is the hard part.
There are so few seeds, and the seeds are so small and light, it's hard to space them evenly.
That's why I provide 0.8-grams. Just in case you sow all the seeds and don't cover all the area. I've done it. But resist using all 0.8-grams.
Mist and spray the seeds with water.
Gently at first, the seeds are light, and too much water pressure will throw the seeds out of the tray. Mist them first, then increase the spray pressure.
All you need to do is wet the soil surface. Don't saturate the soil. The mist will not only wet the seeds but help make seed/soil contact.
Put the seeded tray under the grow lights or near a sunny window (or both).
Place the humid dome over the top of the tray. If your dome has once of those twisting vents. Make sure it's closed. We want to keep the moisture in the tray.
We use the Mars Hydro SP 150 and it works great!
You can read my review of the Mars Hydro SP 150 LED Full-spectrum grow light by clicking this link.
Wait. How long depends on the temperature, the lettuce variety, and probably some other factors that aren't coming to mind this late at night...
You should see some seed germinating in 3- to 4-days. Lettuce seeds don't appear to germinate all at the same time, so it might take up to 6-days for all of them to start growing.
As long as your humidity dome is secure, you shouldn't have to water the seeds (again, coconut coir base mixes work well at holding water). If, however, you see the soil is turning a lighter brown, by all means, use a spray bottle and mist the little guys.
Once the majority of the lettuce seedlings are 3/4-inches tall, take off the humidity dome. Using the mist sprayer, soak the surface of the soil until it's dark brown again. Don't worry if some of the lettuce seedlings tip over. They'll stand back up.
The tray may look sparse. It's ok, it will fill in.
Also, by add 1-cup of water, it will allow the soil in the bottom of the tray to uptake water.
Yes, I know that is a tray of microgreens and not a tray of lettuce. But you get the idea.
Check the tray each day for dryness, but don't water from the top again. Only bottom water. After an hour or so, check the watering tray. If there's water, dump it out. As much as lettuce loves water, they can still be overdone.
The foliage is going to start closing in as the days go on, you'll see that the sparse looking tray is now full. So overwatering can lead to fungus issues as little air gets to the soil once the lettuce gets bigger.
After 25 to 35 days (again, many factors involved), you can take your first cutting of baby leaf lettuce.
Cut the lettuce leaves 1/2- to 3/4-inches above the soil level. This way, the lettuce will regrow.
Either cut what you want to use that day or harvest the whole tray. Either way, remove any debris such as cut leaves or brown leaves and put the tray back under the light.
The lettuce will start to regrow.
You can often get as many as 3 cuttings off of one tray. However, each cutting gets a little more bitter. I find that the 3rd cutting is ok, but the 4th cutting is often too bitter to enjoy.
After you've grown your first tray of lettuce, you know a few things.
How long it took you to grow the lettuce from seed to harvest under your conditions. Everyone will have a bit different results depending on the amount of light, warmth, and attention you gave the lettuce plants.
You also know how many servings the tray of lettuce provided for you and your family.
With these two knowns, you can calculate how often and how many trays you need to grow to keep your family supplied with fresh-cut lettuce.
Even in the winter.
Things to keep in mind is that the temperature and light available with change with the seasons. More warmth and light will increase production.
Want to Grow Lettuce Indoors?
You can purchase all the supplies you need from the sources below. Let's start with the seed.
The Home Microgreen Store has packages of lettuce pre-measured for 1020 trays. Click the button below to see the currently available lettuce seeds.
Heavy-Duty 1020 Trays
These are indescribable. You will never have to buy more trays again if you use these. Worth the investment.
Looking for fewer and smaller trays? The Home Microgreen Store also sells heavy-duty 1010 shallow microgreen trays.
Full-Spectrum LED Grow Light
The Mars Hydro SP 150 is worth every penny. It has the power to grow green and red leaf lettuce quickly and with great color. See the photo below.
Use the code "microgreens" to save money!
The Home Microgreen Store has bags of soil that will fill your 1020 shallow microgreen tray. Click the button below to visit the Home Microgreen Store.
We hope that this article was helpful to you.
It's so easy to grow lettuce indoors.
You'll never want to eat the lettuce from the store again. Your lettuce will be fresh, sweet, and silky tender. Say goodbye to grocery store lettuce.
Have a Question?
If you have any questions about the information in this post or microgreens in general please leave a comment below or reach out to me using the Ask a Question page.
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