Lettuce can be grown in the summer in Florida– the only question is what types are best adapted, and how much of a nudge is needed?
Heat can contribute to bitterness. However, there are a lot of other factors at play: age, availability of water, and of course genetics. I found some lettuce variety trials from 1999 that tested lettuces grown in the summer in Florida for bitterness. Butterheads all seemed to avoid bitterness fairly well, whereas Lollo Rossa bombed. There was at least one non-bitter cultivar in each of the Romaine, Looseleaf, and Oakleaf categories as well. Good information to have as we head into this project!
A more recent trial tested Florida summer-grown lettuce cultivars for tip burn. I think this may still be working its way through the bulletin process, so I’m missing a couple important pieces of information (such as whether the lettuce was grown open or under shade, and if this was early or full-on summer). Nonetheless we have some good data: cultivars Ridgeline, Coastal Star, and Livigna all had low tipburn. Skyphos, Rex, Livigna, and Antago all had extremely low or ZERO tipburn.
You may note by comparing the two trials that few or none of the same cultivars were trialed in each one, so we have no way of knowing if there are any cultivars that score well on both bitterness and tipburn. I chalk it up to the studies being done 13 years apart and the availability of lettuce cultivars having changed a bit in that time. I’m still just kind of stoked knowing that lettuce will in fact grow down here in the summer….
If anybody wants more details and it still hasn’t made it to publication yet, make a note in the comments and I can email you the link.