I planted a batch of "zesty blend" lettuce seeds. They seem to be doing fine, but suddenly they started flowering:
Have they bolted? They look healthy otherwise. Still edible?
Yes, bolting means "flowering and ready to set seed". When it sends up that long flower stalk, it's all done.
As for "is it edible": it will probably be bitter enough that you won't want to eat it.
Lettuce in summer is tricky -- long days and heat makes it want to bolt. Now that you've seen it happen once, you know what it looks like and you can keep an eye on it next time around and harvest it once it starts to show signs. Giving it some shade can also help it last a little longer.
If you still have seed, I'd go ahead and start another batch -- starting a few new plants every couple of weeks will keep you in lettuce from now until the ground freezes solid.
Lettuce often runs to seed prematurely when it is stressed. Apart from the reasons already mentioned, drought and overcrowding can cause it to bolt. As pointed out, you can eat it after it has bolted, but it will have a bitter flavor.
Mine do better when under a thick-mesh netting grow tunnel (see photo) which provides them with some much-needed shade and also protects them against bird and slug damage.
Simple answer is yes, flower shoots on lettuce is a sure sign of bolting.
Lettuce is a cool season crop, meaning it prefers to be grown in mid to late Spring and (early) Autumn (Fall).
If your lettuce was planted in full sun and temperatures have constantly been over 75°F (24°C) during the day time, that is the cause of the problem.
Nothing about going to seed means you can't eat them, but as others have pointed out, you may not want to at that point due to a tenancy for the greens to get tougher and a bit more bitter the further into the seeding stage the plant has gone. That said, I'd still consider giving the greens a taste even after they've flowered. I've found different varieties hold up for a bit longer than others. Greens that are tougher or a bit bitter to begin with (rocket/arugula and mustard greens) still taste pretty good even though there might be a flower or two, whereas softer lettuces (buttercrunch) might as well go straight to the compost bin.
I did meet someone who said that bolted lettuce was good for making lettuce soup, which would certainly deal with the fact that the leaves get tough after bolting, though I suspect that a bitter lettuce will make a bitter soup. I can't say that I've ever tried this, though I've always been curious.
If you're going to keep re-sowing your greens as the year gets warmer and lettuce is more likely to bolt sooner, you should start harvesting the plants earlier. As long as they have a few well developed leaves, a young lettuce salad is very nice. Pull the whole small plant, cut off the root, clean and eat. (As bstpierre pointed out you can snip the plant rather than yank the whole thing, see his comment below this post for more information).
And, of course, once you've reached the end of the season, it's never a bad idea to let a plant go all the way to seed, so you can harvest the seed for sowing in subsequent seasons (see here and here for some Q&A on saving seeds).
It appears as though it has likely started to bolt. You can attempt to eat it but be warned as it may be slightly tougher and bitter than you are accustomed to. The longer you wait the worse of it will potentially taste but it is still edible. If it's not to your liking, simply pull it up and put it in your compost pile (if the seeds are not yet mature) and attempt to grow some more as the season allows you to.
Yes, if your lettuces are putting up a long tall central stalk and/or flowering then they have bolted.
You might want to check out the question "Culinary uses for bolted lettuce?" on the cooking Stack Exchange (asked by, ahem, your correspondent some months ago).
technically bolted means run to seed prematurely. as opposed to flowering and seeding by usual time for species. factors for bolting include unusally long hot spells, drastic fluctuations in weather over few days, poor soil structure and/or soil moisture problems, unbalanced chemistry due to fertilizer use, crowding, improper handling of seedlings when planting out and planting seedlings of species that hate handling altogether and should be planted as seeds. almost forgot, untimely planting of crop, planting crop outside normal climate zone. thankfully species most prone to bolting are also quickest growers so you can progressively sow.
When my red leaf lettuce bolted this year it tasted very bitter, to the point of making me gag; and I like Arugula. When the Romaine bolted, the higher the leaf the more bitter it tasted. I have pulled all our lettuce plants now, and will try to replant some soon. This is my first year with a vegetable garden in about 5 years; and at a new house and in new planting beds.