Impossible to tell with info provided. Likely inadequate water or too much heat or both. Maybe small possiblity inadequate N.

Dan

The one thing lettuce can't stand is stress: water stress and heat stress are the two big no-nos.

Don't plant against a wall where they will pick up even more heat. Try planting in a semi-shaded area, esp where they can get shade in the hot part of the day.

If you can get hold of a packet of Jericho (romaine type) or Nevada (Batavia looseleaf) varieties, they are more heat tolerant (there may be others, too).

Lettuce also seem to go bitter once they start to bolt (getting taller and starting to form a flower stalk).

It's a common problem, so don't think it's just you! Lettuce can be tricky in warmer climates.

Sue

Thanks! They are in full sun, so maybe that's it. I did have one head that bolted in the fall, but the rest all look very good. I have raised beds and just planted one bed with corn - I think I will try my lettuce inbetween the corn, maybe that will give it more shade and not get so hot. Thanks for taking the time to respond - and I do feel better knowing that this is common.

We still don't know where you live. I find overwintered lettuce are pretty tough and bitter until they start to grow again in the spring. But that's just in my climate.

Summertime iceberg-type lettuce is another that stands the heat well.

Lettuce is cool weather crop. Likes cool overcast skies better than full son all day. Plus, lettuce love water.
Therfore, a combination of less watter, too much sun, too high temperatures can contribute to bitterness. My experience shows that in my climate, when it starts getting hot and lettuces start sooting high and bolting, then they get bitter. Some people dont mind it. If one can eat bitter melon, can also eat slightly bitter lettuce. Actually some bitternes in lettuce is benefitial (according to THE BITTER PRINCIPLE)

I've got 4 vars of overwintered lettuce, and making them cold makes them sweeter (purposely chosen cold-tolerant vars). Here, from mid-July to mid-Sept I don't grow it, as it bolts, requires too much water, and generally is unhappy.

Dan

I had a bit of trouble with this too. I just put up a trellis this year for peas, and later for vining beans. I my try to plant my loose leaf lettuce near or underneath the trellis to shade them a little.

I also may plant them to the north of taller plants to avoid that intense afternoon sun. It seems to go from snow to 80 degrees pretty quick. Frustrating, to say the least.

Thanks everyone. I live west of Fort Worth (Texas Heat), so I bet the comments of too much heat and not enough water is the main factor. When I plant more, I will make sure they get in the shade of other plants (still may try some with my corn - but already getting into the 90's here, so may wait until the fall). Thanks again for everyone taking the time to share your suggestions!!

I'm in Phoenix but lately, your temps have been similar to mine, just cooler at night.
I grow winter lettuce, it's all over by April but yo can't do that due to harder freezes. You can grow spring and fall.
When I want a little slightly out of season, like now, I plant a leaf lettuce mix in part-shade, water the heck out of it and eat as baby greens. I don't expect to get more than 3-4 weeks of eating before it gets bitter.
I have a little patch now and it's still very sweet. Planted late Feb or early March.
Make sure you give plenty of nitrogen too. These need to be fast growing little salad gardens. Don't plan on keeping them around too long. Once bitter, it's all over. Plant a warm season crop in it's place.

I live in San Antonio and have been eating lettuce for the past 6 weeks. We planted a loose leaf variety in early February and then we had a very deep freeze. The seeds came up fine (albeit a little late). So far, so good. We do water often. It is definitely starting to warm up, so I'm not sure how long it will last. I think the variety we have is Black seeded Simpson. I don't really care for it, but the rest of my family doesn't mind. I mix it with some Romaine from the store. I am looking for some more crisp varieties for the fall.

I'm using a very pretty red and green mix from Baker creek here but can't find the seed packet.

I'm in northern Ohio and usually grow lettuce in the spring and then later in the fall. Our summers get too hot for lettuce, too. The last couple of years I've used small hoops over the lettuce along with a shade cloth and this works quite well. I've become a fan of the hoops over crops with various covers for heating and cooling plus insect control. Significently extends growing season here.

luke