Yesterday I decided to harvest my Bibb Lettuce in the morning for three reasons:
1. A high of 94 degrees was predicted.
2. It has reached the right size.
3. The center of the leaves had formed a "firm head".
I have two harvest options. I could have just take off the outer leaves and allow the plant to continue to grow. However with the hot weather continuing through the week, I decided to just end it here. I wanted to use the pot I was growing it in to propagate some strawberry plants. So I just gently used a small shovel and pulled the plants right out of the container. I twisted off the root end and noticed a white sappy substance. 
This can be a sign that the lettuce has turned bitter. And indeed some of the leaves turned out to be bitter, but some where still ok. The above average temperatures caused this to happen. In the average year I would have been fine timing wise. At least some of the leaves were still usable. We will probably plant some more bibb lettuce later in the year for a fall harvest.
One positive thing to note was that the lettuce grew well in the pot I had it in, making it an successful vegetable for a container garden.
Are you growing Bibb Lettuce this year? I would love to hear your experience. Leave a comment below.
To read all my posts on Bibb Lettuce, click here.
mike carolan
Hi - we are growing bibb lettuce this year for the first time. We grow all of our veggies in a large, table box garden (we had purchased a bean plant and put it on the patio as we hadn't finished finalizing the box yet - only to realize that my black lab thought she was vegan and ate the leaves off the plant). Well the other day, my wife had taken one of the leaves off of the bibb plant and tried it and she said it was extremely bitter. Well - I've looked online and apparently the best chance to bringing this guy back is to soak ONLY THE LEAVES in very cold water and baking soda and then rinse them in very cold water and store them in the fridge overnight. Going to try this out and I'll let you know how it works out.