Why and when does lettuce turn bitter?

Potted lettuce "bolting"

Saturday, May 6, 2023

I love fresh salad and when the garden is giving, I try and eat them every day.  In our zone, it is hard to keep lettuce going that is not bitter once the hot temperatures arrive.  What causes this, when does this happen and what can you do about it?  This challenge is something I work on each season to have a way to keep sweet lettuce or other type of salad green producing all summer long.  Here is what I have learned so far.

When does lettuce leaves turn bitter?  In my garden, this starts as soon as we start having days that the high temperature hits the 80's for sure.  

What causes lettuce leaves to become bitter?  Lettuce starts turning bitter when it begins to make a flower stalk to make seeds.  There are 2 triggers for this-warmer temperatures and drier conditions.  

What can you do about it?  Keeping lettuce with consistent moisture and moving the plants to a cooler part of the garden can extend the harvest.  Look for lettuce varieties that are "bolt resistant".  You can also do succession sowing so you always have young plants producing.

I grow most of my lettuce in a self-watering pot that has a reservoir of water in the bottom.  This really helps keep the moisture consistent.  When the temperatures start getting into the low 70"s, it is good time to move your pot to a cooler part of the garden.  Another option if you are growing your lettuce in the ground is to sow seeds in full sun in early spring, then move to sowing seeds in the cooler part of your garden bed.  When it is warmer, lettuce does fine in even dappled shade.

I always harvest the leaves from the outside of lettuce plant to allow the inner part of the plant to continue to produce more leaves.  Since there are only 2 of us, this lets me harvest from a plant very early and for weeks without any going to waste.  

When the leaves start to get bitter, I just let it go to seed.  I save some of the seed and let some take their natural course.  I try different varieties every season along with the ones that have so far proven to be the best producers for my garden.  I look for bolt resistance, varieties that stay sweet even after bolting and the best self-seeders.  I let the seeds fly where they want and then keep an eye out for volunteer seedlings.  I'll transplant them to where I want them after they get sturdy.

I do practice "succession sowing" which means I sow new seed every few weeks.  I sow seeds about every 3-4 weeks unless I am getting volunteers naturally.

I also plant other varieties of greens that like the summer heat and stay sweet to supplement my summer salads.  The ones that have done the best for me so far are Hilton Chinese cabbage (use for salads and wraps), orach (comes in lime green, green, rose and purple), a variety of amaranths that are ornamental as well as tasty (Chinese Multicolor Spinach, Love Lies Bleeding, Pink Beauty), Red Malabar Spinach (a pretty tropical vine), and sprouting broccoli.  All do well in my summer garden.

I keep trying different lettuces to find the lettuce that stays sweet the longest, even after bolting, in my garden.  So far, the best performers are Red Sails, Butter King, Marvel of Four Seasons, Royal Oakleaf, and Bronze Beauty.

New ones that I am trying this year are Giant Blue Feather, Yedikule, and Solar Flare for their bolt resistance.