Whether you enjoy a fresh salad or crisp greens on top of a sandwich, there’s nothing quite like growing your own lettuce. While most people think of planting lettuce in an outdoor garden, you can also grow lettuce indoors!
Growing indoors allows you to enjoy homegrown salads, even if you don’t have access to an outdoor growing space.
Even if you do have access to an outdoor garden, I find that indoor growing provides numerous advantages. However, it also presents unique challenges!
I’m going to cover why you may want to consider growing lettuce indoors, how to provide the proper care, and tips for a successful harvest.
Why Grow Lettuce Indoors?
If you’re like me, there’s nothing quite like spending time outdoors with your hands in the soil.
Caring for plants during warm sunny days and cool rainy mornings connects me to the Earth and reminds me to appreciate simple pleasures.
However, growing outdoors isn’t always possible.
If you live in an urban area, apartment complex, or contaminated area, tending to an outdoor garden may not be possible or practical. In this case, indoor growing may be a good option.
And even if you do have access to an in-ground garden or sunny area for a container garden, growing outdoors presents a fair number of challenges. Bugs, cold temperatures, short days, heavy rains, hot temperatures, and high humidity can all present a challenge to lettuce plants.
Growing lettuce indoors allows you to better control these variables, which allows for a more consistent harvest.
How Is Growing Lettuce Indoors Different Than Growing Outdoors?
As mentioned above, the environment is much easier to control indoors than it is outdoors.
Yes, you can manipulate outdoor growing conditions with the help of shade cloth, fabric row cover, and high tunnels. But these alternations will only go so far.
It’s difficult to lower outdoor humidity on a sweltering August day or increase the amount of light present in December.
However, when you’re growing lettuce indoors, you can manipulate practically every environmental variable.
Too cold? Turn the heat up.
Humidity too high? Turn on a dehumidifier.
Only eight hours of light a day? Utilize a grow light to boost the daily hours of light.
With that said, you won’t be able to rely on nature for elements like water and airflow. Since no rain falls indoors and the wind doesn’t blow through closed windows, you are responsible for providing your lettuce plant with everything it needs to survive.
Best Ways to Grow Lettuce Indoors
While growing lettuce indoors is undoubtedly different than growing it outdoors, there are also differences between indoor growing methods.
Growing Lettuce Indoors In Pots
If you’re used to growing lettuce outdoors in a garden, growing lettuce indoors in pots can be an easy transition. And even if you’ve never grown anything in your life, you can still learn how to grow lettuce in indoor containers!
When you choose to grow lettuce in containers, two of the most important elements to consider are the container and the potting mix.
Choose the Right Container
Lettuce plants have shallow roots, so they don’t need much room to grow. However, if they are overly crowded or don’t have room to expand, they may become stunted or diseased.
When you’re choosing a container for your lettuce, pick one that is between four and twelve inches deep. This will provide the roots with room to expand while avoiding drainage problems that can result from extremely deep soil.
As far as above-ground space goes, think about what type of lettuce you’d like to grow.
Full heads of romaine lettuce, iceberg lettuce, and butter lettuce typically need about a foot between each plant. Smaller varieties like little gem can perform well with only eight inches of space between plants.
However, if you’re growing baby lettuce for lettuce mix, you can space individual plants two to three inches apart.
Select the Right Potting Mix
Once you’ve acquired a proper container, the next step is to choose a good potting mix.
Unlike when you’re growing lettuce outdoors, it’s best to use a manufactured potting mix rather than natural soil. A proper mix will provide your lettuce with all it needs to grow well in a container.
Since lettuce roots don’t like sitting in moist soil, you must select a well-draining mix. Elements such as perlite, pine bark fines, and sand will improve drainage.
While the potting mix should allow excess water to escape, it should also be able to hold a bit of moisture. Elements like peat moss, coco coir, compost, and vermiculite all help hold water.
Organic materials like peat moss and compost will also help hold nutrients until plants are ready to take them up. These materials may also provide a home for beneficial organisms.
Fortunately, numerous soil mixes work well for indoor lettuce plants.
Fox Farm Ocean Forest Potting Mix provides a good mix of drainage and moisture holding. It also contains a mix of natural fertilizers as well as beneficial microbes that help with nutrient absorption and disease prevention.
Another good option is the Bar Harbor Blend from Coast of Maine. This blend contains materials such as peat moss, aged compost, nutrient-rich seaweed, and perlite.
Growing Lettuce Hydroponically Indoors
Hydroponics is another way you can grow lettuce indoors.
This method involves growing lettuce in inert media like rock wool or perlite rather than a living soil mix. You then apply a nutrient-rich solution to the media to provide the plants with both water and nutrients.
Hydroponics can be a bit more expensive to get started with than container growing and often requires more advanced skills. However, it consumes less water than container growing and can be automated for ease.
There are hundreds of different hydroponic methods and setups that can work for lettuce, including pre-built kits and DIY options.
If you’d like to go with a pre-built kit, ensure the system will be able to handle the size of your lettuce plants. Remember that full lettuce heads can grow over a foot tall and a foot wide!
Although I haven’t used it myself, one hydroponic grow kit that many recommend is this growing system from iDOO. It comes with an automatic timer to turn lights on and off, a fan to promote airflow, and 12 growing pods.
The kit also includes sponge pods to support plant roots as well as liquid plant food to fertilize your lettuce. The height is just shy of a foot tall, so it’s best for smaller lettuce plants or baby lettuce mix.
You can also design your own hydroponic system. This method allows you to create the precise setup you’d like, but it can also require some tinkering to get things just right.
If you opt to DIY your hydroponic lettuce setup, consider using either the nutrient film technique or the floating raft system, since both of these methods are well suited for growing lettuce.
The nutrient film technique involves placing plants in small plastic cups and nestling these cups inside holes in a small channel or PVC pipe. The plants’ roots are left to dangle down from the cups and into the bottom of the channel.
A dilute nutrient solution continuously flows down the channel, allowing the plant roots to absorb both water and nutrients.
The floating raft system involves placing plants in rafts that float on top of a nutrient solution. Oxygen is pumped into the solution to provide plants with the air they need.
When to Start Lettuce Plant Indoors?
Since you can control environmental factors indoors, you can start lettuce plants whenever you like! However, you should consider a few factors when determining when to plant lettuce seeds indoors.
Use Your Desired Harvest Date as a Guide
First, think about when you’d like to harvest your lettuce. You can then use this information and the lettuce’s days to maturity (DTM) to determine when to plant.
Let’s look at an example of Coastal Star romaine lettuce. According to its seed packet, this variety has a DTM of 57 days.
However, this refers to the number of days from transplant to harvest rather than from seed to harvest. When you factor in the roughly 30 days it takes for lettuce to grow from seed to transplant, you learn that a head of romaine will be ready in about 87 days after you plant a seed.
Of course, the exact number of days lettuce will take to mature will vary depending on factors like light and temperature, but you can use this number as an estimate.
So, let’s say you want to harvest a mature head of Coastal Star romaine at the end of May. Using the DTM as a guide, you can determine that you’ll need to start seeds 87 days prior, or around the end of February.
Think About Lighting Sources
While you can grow lettuce indoors year-round, the way you grow will have an impact on the best time of year to plant.
If you’re relying on natural light, there will be more light in the summer than there will be in the winter. Therefore, lettuce will take longer to mature in the winter than it will in the summer.
However, if you’re using supplemental lighting to grow indoors, the time of year will have less of an impact on the lettuce’s growth rate.
Starting Lettuce Seedlings Indoors
If you want to start lettuce seedlings indoors with the intention of later planting them outside, you should use the outdoor weather as an indicator of when you should plant.
Generally, lettuce will take about 30 days to grow from a seed into a three-inch tall transplant. That means if you want to plant lettuce in your garden on a certain date, you should start your seeds 30 days before this date.
While lettuce plants can survive cold and even a light frost, seedlings are more tender than full-grown lettuce. Additionally, plants that are grown indoors and then moved outdoors will be more susceptible to cold damage.
Therefore, you should not plant lettuce transplants outdoors in the spring until the danger of frost has passed. If you’re unsure of when this occurs, look up the average last spring frost date in your area.
Generally speaking, March is a good time to plant lettuce seeds indoors if you wish to later transplant them outdoors.
You can also start lettuce seedlings indoors in the summer to plant outdoors for a fall harvest. Germinating lettuce seeds indoors is often easier in the summer since lettuce requires cool temperatures (60-70ºF) to germinate.
No matter when you are starting lettuce seedlings, you should use a nutrient-rich and well-draining potting mix. This will provide plants with all the nutrients they need until they are planted outdoors.
As far as seed-starting trays go, it’s best to opt for small plug trays that allow you to easily remove the plant’s root balls when it’s time to transplant. These 72-cell trays are one good option.
How to Care for Lettuce Plants Indoors?
No matter where you’re growing lettuce plants, you’ll need to provide the same elements: water, nutrients, light, proper temperature and humidity, and a place to anchor.
Here are some elements to keep in mind when you’re growing lettuce indoors.
One of the trickiest parts about growing lettuce indoors is providing enough light. Lettuce plants require at least eight hours of bright light each day to grow well, and they will thrive with 12 hours of daily light.
Without enough light, they won’t be able to complete photosynthesis—the process in which plants use light energy to convert water and carbon dioxide into sugars, oxygen, and water. This means they won’t have the sugars they need to fuel essential plant processes.
A lettuce plant’s response to a lack of light will depend on what stage of growth they’re in.
Small seedlings may become ‘leggy’ as they reach for existing light. This will make them elongated and weak.
As plants grow older, a lack of light can slow their growth and also lead them to become stressed and more susceptible to disease.
So, where should you place your indoor lettuce plants to ensure they receive enough light?
If you have a bright area, you may be able to get away with using just natural light. The best place for indoor lettuce plants will be next to a bright, south-facing window.
An east-facing or west-facing window can also work well in the summer.
However, if you don’t have a bright window or are trying to grow robust lettuce year-round, you may benefit from using grow lights. These lights make up for dim conditions and allow you to provide your lettuce plants with as much light as they’d like.
When it comes to choosing grow lights, you have lots of options—and all these options can easily become overwhelming! The truth is that a simple and inexpensive grow light will work just fine for lettuce.
Look for a full-spectrum light—a light that contains all the different wavelengths of light. This light is similar to sunlight and appears to the human eye as white or light yellow. Fluorescent and LED lights are both suitable options.
If you’re growing many trays of seedlings or a large number of potted lettuce plants, a hanging light may work well. Another option is an easily adjustable light.
No matter which type of light you choose, it’s best to position your plants only a few inches away from the light source. This is especially true when plants are small.
And remember there is a thing as too much light! Ensure you provide your lettuce plants with at least ten hours of darkness during each 24-hour period.
Temperature and Humidity
Fortunately, lettuce plants are generally happy with temperatures that humans fin comfortable. If you keep your home between 50-80ºF, your lettuce plants will be just fine.
Along with providing the right temperature, you should also avoid sudden swings in temperature. That means keeping plants away from drafty heating and air conditioning vents as well as frequently-opened exterior doors.
As far as humidity goes, lettuce plants prefer lower humidity. Average household humidity is typically fine.
If you’re growing your lettuce plants in potting soil, watering is an important part of keeping them healthy and happy.
If you don’t water enough, the plants may wilt or lack the water they need to complete processes like transpiration and photosynthesis. And if you water too much, the plants may become stressed and/or develop fungal diseases.
In general, you want to keep the potting soil moist but not wet. Choosing the proper potting soil is an essential part of keeping soil moisture at the proper level.
The frequency which you’ll need to water your lettuce plants will depend on the following factors:
- Size of container: soil in small containers will dry out more quickly than soil in large containers
- Potting mix: materials like peat moss, coco coir, and compost will hold moisture while perlite, pine bark, and sand will improve drainage
- Temperature: higher temperatures lead to higher rates of evaporation and transpiration, both of which cause soil to dry out faster
- Humidity: higher humidity decreases the amount you need to water
- Light: more light leads to higher rates of transpiration which requires you to water more often
With those factors in mind, you can expect to need to water your lettuce plants anywhere from a few times a day to once a week. Seedlings in small cell trays may need to be watered daily while large plants can generally go a few days without water.
A good rule of thumb is to use the soil moisture as your guide. If the top inch of potting soil is dry, water well. If it’s still moist, wait to water.
If you are using a nutrient-rich potting rich, you may be able to grow baby lettuce and lettuce seedlings without additional fertilizer.
However, if you want to grow larger heads of romaine or iceberg, you’ll need to apply fertilizer. The amount you fertilize will depend on factors including soil mix and plant size, but you can start by applying a diluted fertilizer solution once every two weeks.
Look for fertilizer that will encourage vegetative growth. This liquid fertilizer from Fox Farm is one good option.
Indoor Lettuce Pests and Diseases
While indoor lettuce is better protected from many of the pests and diseases that impact outdoor plants, that doesn’t mean it’s not susceptible to threats. Keep an eye out for the following, and act accordingly.
These tiny sap-sucking pests can impact lettuce plants both indoors and outdoors. They often come into your home on infected plants and/or potting soil.
Many different types of aphids can attack lettuce plants, and they can be brown, white, black, red, yellow, and green.
All aphids use their mouthparts to pierce plant leaves and then suck out the sap. These pests can also spread diseases as they feed.
Aphids generally attack the tender interior lettuce leaves, so you may not see them immediately. These pests can multiply rapidly, so when you notice them it’s best to remove them ASAP.
You can wipe off small numbers of aphids with a soapy rag. Larger infestations can be sprayed with neem oil or insecticidal soap.
As the name suggests, this fungal disease causes the bottom of a lettuce plant to rot. It is caused by the fungus Rhizoctonia solani.
Bottom rot will only occur if the fungus exists in the soil. Preventive practices involve choosing healthy potting mixes and sanitizing containers between uses.
Pythium Root Rot
This is another fungal disease, but it mainly attacks lettuce roots. It can occur in both soil-grown and hydroponically-grown lettuce.
Plants infected with this fungus will develop stunted roots, which can lead to stunted above-ground growth.
Prevention is the best strategy for root rot, as there is not an easy and effective treatment after plants are infected. Use potting mix sources you trust and sanitize containers between uses.
Harvesting Indoor Lettuce
Just like with outdoor lettuce, you can harvest indoor-grown lettuce at any time. Harvesting lettuce when it is small and immature will simply result in a smaller harvest.
If you want to harvest a mature lettuce head, simply cut the plant near its base. While this will leave you with a nice, full head, it’s important to note lettuce will not regrow if you use this harvest method.
You can also harvest individual outer leaves and allow inner leaves to regrow. This will allow for a continuous harvest over multiple weeks.
What Types of Lettuce Can You Grow Indoors?
The good news is that you can grow any lettuce indoors that you can grow outdoors!
That means you can plant buttercrunch or romaine in a hydroponic system or sow a row of baby green leaf lettuce in a soil-filled pot.
You should determine whether you want to grow mature or baby lettuce ahead of time since this will impact the amount of space your plants will need.