It is a little bit disappointing to put care and hard work into your garden, only to get bitter lettuce in exchange for all of your effort. However, there are specific reasons that lettuce becomes bitter, and luckily, there are also ways you can stop it from happening.
So, why is your lettuce leaves bitter? Lettuce leaves become bitter when the plant bolts and starts to produce flowers and seeds. This is more likely to happen in hot weather, when temperatures are consistently over 75 degrees Fahrenheit (23.9 degrees Celsius), since lettuce is a cool-weather crop.
Let’s take a closer look at why lettuce becomes bitter when it bolts, and what bolting means. Then, we’ll look at some tricks you can use to prevent lettuce from bolting by keeping the plant a little bit cooler.
Why Are My Lettuce Bitter?
Lettuce is what is known as a cool-weather crop. According to Clemson University, the optimal temperature for growing lettuce is 55 to 65 degrees Fahrenheit (12.8 to 18.3 degrees Celsius).
When temperatures are over 75 degrees Fahrenheit (23.9 degrees Celsius) for multiple days, lettuce (in particular the Bibb variety) tends to bolt.
What Does Lettuce Bolting Mean?
When lettuce bolts, it is switching from vegetative growth (producing leaves) to reproductive growth (producing flowers and seeds). This will occur naturally in lettuce with age, but it will speed up in hot temperatures, as mentioned above.
When a lettuce plant bolts, it sends up a flower shoot, which eventually produces seeds. This is the method by which the plant reproduces.
Unfortunately, when a lettuce plant sends up the flower shoot and produces seeds, this takes energy away from the growth of leaves. It also causes the leaves to become bitter, since nutrients and energy are put into the flowers and seeds instead of the leaves.
Note that lettuce may also bolt in response to a change in the length of the day (natural passing of time). When lettuce knows that the days are getting shorter and cold is approaching, it may bolt to produce seeds before it dies.
Likewise, bolting may also occur in lettuce due to a lack of water or nutrients. For example, if the plant does not think it can survive a drought, then it may bolt quickly in an attempt to produce flowers and seeds before it dies.
For more information, check out this article on bolting from Wikipedia.
How to Stop Lettuce from Being Bitter (How to Prevent Lettuce from Bolting)
You cannot control the weather to stop the heat that is often the cause of bolting. However, there are still plenty of ways to prevent lettuce from bolting so that you can harvest plenty of good lettuce for salads. It all starts with choosing the right varieties of lettuce to plant.
Choose Heat-Resistant or Fast-Maturing Lettuce Varieties
Certain lettuce varieties are heat-resistant, meaning that they resist bolting in high temperatures. Being able to resist bolting in temperatures even a few degrees warmer than other lettuce makes a big difference.
Other lettuce varieties are fast-maturing, meaning that there is less time from planting seeds to harvesting lettuce leaves. Some lettuce varieties can mature in as little as 48 days (less than 7 weeks!). The table below has more information on times to maturity for different lettuce varieties.
In fact, the University of Maryland recommends such varieties to extend the lettuce-growing season.
I have included a table below that shows various heat-tolerant and fast-maturing lettuce varieties, along with their time to maturity, color, and type.
| Lettuce Variety | Days to Maturity | Color | Type |
| Coastal Star | 57 | green | head |
| Muir | 50 | green | head |
| Tropicana | 52 | green | head |
| Nevada | 48 | green | head |
| Adriana | 48 | green | head |
| Sparx | 58 | green | romaine |
| Cherokee | 48 | red | head |
| Red Cross | 48 | red | head |
All of these lettuce types (and more) are available from Johnny’s Selected Seeds.
Plant Lettuce Twice: in Early Spring and Late Summer
Planting lettuce seeds twice, in early spring and late summer, has two major benefits.
First, you get two seasons’ worth of lettuce in one year. Second, you avoid planting lettuce at times when it will be exposed to the hottest summer temperatures, which helps to prevent bolting.
Lettuce is a cool-weather crop, so it can tolerate cooler soil temperatures down to 40 degrees Fahrenheit (4.4 degrees Celsius). Lettuce seedlings can also tolerate light frost.
In light of this, the Old Farmer’s Almanac recommends sowing lettuce seeds directly into the ground two weeks before the last spring frost, and again 8 weeks before the first fall frost.
It is advisable to sow lettuce seeds directly into the soil outside. However, if you want to start seeds indoors, you should do so 4 to 6 weeks before the last spring frost date. You can find the frost dates in your area on the Old Farmer’s Almanac website.
Stagger Your Lettuce Planting
The Old Farmer’s Almanac also recommends staggering your lettuce planting during the growing season. By planting lettuce seeds at different times, you get a constant supply of lettuce plants that are maturing at different dates.
This is a good way of “hedging your bets”. Even if an earlier planting of lettuce starts to bolt due to the heat, later plantings may still avoid bolting and bitter leaves.
Provide Shade for Your Lettuce
Providing shade for your lettuce is very important to avoid bolting, especially in the hottest weeks of the summer. The temperature in the shade can be several degrees cooler than the temperature in the sun. This can make all the difference in whether a lettuce plant bolts or not.
One way to provide shade for your lettuce is to plant them near other taller plants. Since most plants are taller than lettuce, you have lots of options!
Vining plants that climb stakes or trellises, such as tomatoes and cucumbers, are great shade-providing neighbors for lettuce. You can plant lettuce close to them to provide shade during part of the day, while still giving the lettuce enough light to grow.
If you use an A-frame trellis, arbor, or pergola to support climbing plants (such as tomatoes, cucumbers, squash, or grapes), then you can plant lettuce underneath the structure. That way, the structure itself and the leaves of the taller vining plants will provide shade to the lettuce.
For more information, check out my article on trellises, arbors, and pergolas.
Another option is to use shade cloth or floating row covers to keep some of the sunlight (and thus the heat) from getting to your lettuce. Row covers are basically a fabric material that filters out sunlight (you can choose from different materials that allow more or less light through).
Finally, when planning for next year’s garden, scout out an area for planting lettuce that has a bit of natural shade during the hottest part of the day.
Use Mulch to Insulate Your Soil Against Heat
Another way to keep your lettuce a bit cooler is to insulate the soil they are growing in. To do this, use some type of mulch to cover the surface of the soil.
In addition to preventing weeds, mulch will help the soil to retain water and prevent the soil from getting so hot.
Wood chips are often used as mulch, but don’t forget that you can use many different materials as mulch to cover your soil, including:
- Hay
- Straw
- Grass clippings
- Leaves (whole or chopped up by a lawnmower)
- Cardboard boxes (flattened)
- Compost
Be careful about using manure as mulch – it should be fully decomposed before applying it to your garden. Otherwise, you might burn your plants due to high salt content.
Keep Your Lettuce Watered
It takes energy to evaporate water. This is the way that your body cools off on a hot day: you sweat, and when the water in the sweat evaporates, it cools you off.
The same principle applies to gardening. If you keep the soil around your lettuce watered, it will stay slightly cooler than the surrounding soil due to evaporation.
Also, keeping your soil moist will prevent water stress on your plants. Water stress is another reason that lettuce sometimes bolts and becomes bitter, so make sure to keep an eye on the soil moisture and on the lettuce itself.
If the lettuce leaves look wilted, check the soil with your fingers. If it is dry down to a few inches, make sure to give the plant a thorough watering to help the lettuce perk back up and stay cool.
Keep Your Lettuce Picked
When harvesting lettuce for salad, pick younger leaves, which tend to be less bitter than older ones. To prevent the lettuce from bolting, you can also pick the older bitter leaves off the plant.
You can either boil these bitter leaves and season them to taste, or give them to chickens or other livestock (they will love it!)
Conclusion
By now, you have a much better idea of why your lettuce is bitter, and also why it happens (it all comes down to heat!) You also know what you can do to keep lettuce a bit cooler and prevent bolting and bitter leaves, at least for a while.
I hope you found this article helpful – if so, please share it with someone who can use the information.
If you want to read some of my most popular posts, check out the “Best of GreenUpSide” page here. Enjoy!
~Jonathon