Nope. Well, probably nope. Lettuce bolts because of heat. If your raft system has a large enough thermal mass (acre feet!) it will slow bolting down. Commercial hydroponic lettuce farms must resort to cooling the air to prevent what is happening to you.

Your EC should be from .3 to .8 and the pH should be at or near 6.3 for best results, but 99% of the time, heat is the enemy when it comes to bolting.

I agree about the heat - because the lettuce is "afraid" of drying out or not surviving the heat and hence starts to bold in a rush to assure reproduction.

I know this is "disputed territory", but apparently the bolting process can be controlled with planting in respect of the moon calendar. I do not believe in this philosophy that much in general - but apparently, the lettuce and other leafy vegetables that tend to bolt to early can be controlled a way better. This is what I have been told by the gardener of a non profit organisation in Luxembourg that has experimented with moon calendar for years. And she says that they have got the most clear and positive results of all tests with avoiding the premature bolting of lettuce. In fact they only plant letuce according to the moon calendar, while for other vegetables they do not consider it anymore.

Freeman mentioned some stuff along those lines from the farmer's almanac once. I chose to go ahead and do some research and find that the moon phase planting schedules are, indeed, much debated. Essentially, from what I've gathered, the only reason it's really debated is because there are so many reputable growers that have supposedly proven it works and there aren't any scientists willing to say, "it works but we don't have a clue why". Until it was mentioned by Freeman and I started researching, I thought it was a load of crap. I'm actually seriously reconsidering and plan to try planting according to the almanac next year. At least in my ground garden.

As far as the bolting, summer is a good time to eat baby greens. Grow plants to flower in spring (or winter) and collect the seeds to grow baby greens and then start the full heads again in late summer to grow in cooler fall weather.

Now, all this is common knowledge about the air temps, right? I don't think that is the only factor, though. What is one way to extend harvest? Shade. Shade doesn't reduce air temperatures that much, especially if you use other plants to do it as has been done for many many years. I think light intensity also has a lot to do with it. Possibly even photoperiod. Both of these will affect the heat inside the plant by leaps and bounds. It's like the difference of a black car in a garage at 90 degrees or a black car in the sun in 80 degrees. Which one gets hotter to the touch? The one in the cooler temps with direct sun.

Look, I have done my own tests as well - but with pepper and tomatoes seedlings instead. I was that much disappointed by the results that I gave up for good. I was planting one group during a ideal "fruit period", and another at a random (actually unfavorable) day. Those were even seeded later but grew that much better from start. Even after repotting both at vavourable- versus unfavourable days, my "randoms", still lead the race. At that point I was not even interested in some future development of both groups. Actually I finished eliminating most of the "moon seedlings", just because they were the weakest! And don't anybody tell me now that they would have more tomatoes at harvest times, because my "random" Oaxacan Jewels were doing perfectly fine...

But still, I trust my friend's results with bolting salad. Though, unfortunately I do not plant much salad, and haven't tested this yet. Actually I have a suspicion here; people do not like partial results and "unfinished business". So they tend to make up- and spin some full story around a partial knowledge and truth, - instead of accepting it as what it actually is. This is so common.

I'd rather pick up- or even buy -a few good advices and facts instead of getting brain washed with a whole doctrine comes along with it. Unfortunately most people are not interested in single facts that haven't got the necessary references and expanded background story that goes with it. Same as most people will not go to a restaurant that has only one perfect dish on it's menu. They always want the whole enchilada, even if half of it would be pure c.r.a.p. :-D

Actually you only need to understand how things evolve and why they sometimes become contradictory, - most of the time debating is useless. If I find a perfect dish in some restaurant and also discover that most of the others are not of my taste, I will not argue about the menu with the Chef either. I will rather order that fine dish only and enjoy it!

Interesting comments.

This is the first summer I've attempted to grow lettuce in my greenhouse. I'm located in Tucson and even with cooling the air temperature gets up to mid-80's.

It appears that going from seed to start-to-harvest takes about 30 days. So if I want to have relatively fresh lettuce all summer, I'm going to have to start new plants every three or four weeks.

I hadn't considered additional shading suggested by Joe.jr, as the greenhouse is already shaded with 50% white. It would be easy to add the shade, but the problem with running an experiment in a small greenhouse is that the conditions are less than constant. But then, what's the harm. If I get some interesting results, I'll post them.

No the ec and ph will not effect bolting. I have learned recently that although heat can play a roll in bolting it's mainly controlled by day length. If you can manage to block the light in the evening and remove in the morning you would be in good shape. I would shoot for 12 hours. http://journal.ashspublications.org/content/120/3/460.full.pdf