How bitter? Bitter compared to the iceberg you buy in the store? Or tough, waxy, harshly bitter?
Fresh lettuce is more bitter than lettuce that has been stored cold, the compounds that give lettuce its flavor, and bitterness, are mostly metabolized withing a few days. So try putting it in the fridge for a while first and see if you like that better. Also, the type and variety make a difference. Iceberg is nearly all water, very mild, a little sweet. Leaf lettuce is the most healthy, and usually the most bitter. Romaine, bibb are somewhere in between and with all the various crosses available sometimes all of that goes right out the window. Some varieties are bred for low bitterness, but keep in mind they may have other problems. Some of the sweeter ones that come to mind are Sucrine, Falstaff, Mayan Jaguar.
If it is waxy, where it squeaks when you rub it between your fingers. That is from stress, most likely water stress. When you transplant, it will need water like every day it is above 75, not much water, just enough to wet the root plug. Then maybe every 2-3 days, depending on how fast your soil dries out, on hot days it likes a couple minutes of being sprayed down with a hose during the hottest part of the day to cool it off. When they are closer to harvest, having filled in the space, they might need water every day because their leaf mass uses it quickly, but are also more likely to develop rot issues from staying wet for too long.
Or, it might not have enough nitrogen, slow release organic sources that don’t leach out with the frequent waterings work best. Also, pH might be too low. Adding compost is a pretty good no think remedy, lettuce will happily grow well in straight compost for me.
The problem is genetics combined with climate. A few varieties have been bred for high temperatures. Jericho and Nevada are two that I’ve grown and liked. Fedco carries Jericho. Also, some of the varieties carried by https://wildgardenseed.com do fairly well. I like Outredgous and Really Red Deer Tongue though both get a bit bitter as the heat of summer sets in.
Better enough that the bitterness stayed in my mouth for about five minutes after I tried it. I’ll have to do the “wax“ test tomorrow. This was on a Sylvesta butterhead.
I have real good luck growing "Red Sails " lettuce.
It lasts a long time in edible condition , longer time before it gets bitter.
Also seams to overwinter well with protection ( row covers)
Have been saving my own seeds for years
Best to sow every 3-4 weeks ,so Allways some in good condition
Another vote for Red Sails. I also like Lolla Rossa, Black Seeded Simpson, Merveille des Quatre Saisons, Forellenschluss, and Rouge D’hiver. These are not bitter for me unless bolting.
Well sometimes just wait until I see finches on them, but need to pay attention .and be quick to bag once they find them.
Or just after the majority of the blooms have finished .
Some times they are still blooming as some seed is ripe enough for finches to eat.
Just cover when you think they are mostly done blooming .
Also a paper bag keeps the seed from blowing away, if you are late to pick seed heads.
As they are already in the bag , I will just label the bag and store in a dry place until needed.
I have a set of screens to separate the seed.
But often just take two seed head bags to the garden ,open, and beat the seed heads together over a new lettuce bed.
If you have enough seeds ( basically free) it’s easy.
A good lettuce plant full of seed is worth more than a big bowl full of lettuce greens. It’s a 50 ft. Rows worth of plants.
Just today I covered a 50 ft. Row of kale seed heads with remay to protect them from finches,a lot of seed in that row !
And glad to have them, as they seem sold out online .