This is a grate place Ginjj! Lot's of great people and gardening knowledge in one place. Starting lettuce from seed is very easy. I use the small plug trays because they make transplanting very easy. I start one seed per plug and they sprout in under 3 days. Keep the sprouts about 1" under your shop lights for about 2 weeks watering as needed (plug trays are small so they dry out pretty fast). At about 2 weeks give or take a day, I start to harden them off to the outdoors. I transplant into the garden at 3 weeks. You can just gently grab the seedling leaves and it will pull right out of the plug. In the early spring I usually cover them with a frost blanket (Agribon 19)for a few weeks just in case we get an unexpected hard freeze. For fall lettuce planted out in August or September I use the same frost blanket as shade cloth to protect from August heat and sun.

So far the fastest growing lettuce I have grown has been from General Hydroponics Maxigro. I can take a pic of the label or have you check the msds to find the nutrient balance. Using generic, non hydroponic nutrients takes alot of special treatments and it really is not much of a payback to go cheap. A bag of dry mix Maxigro is $14 at My local hydro for 2.2lbs and does 400 gallons of lettuce mix. I use 3 tsp of it per 5 gal and achieve a 600ppm solution (170 ppm tap water included) to get my results. Looks like heat might have caused some of your issues as lettuce likes max of 80 degrees. Was the lettuce bitter? Any leaves pull off and milky substance come from from leaf break area? Right now i am in the dead of winter but my feeling is you are mid summer with shade cloth.

Yeah those heads look somewhat past picking time. You could have harvested outer leaves to use at any time and left the heart, the center, in place to develop more new leaves in the center but eventually the stem begins to stretch upwards and the leaves separate (called bolting). When that happen it can turn bitter and grow tough. In the pic the one on the right appears to be in the process of bolting now so likely neither will last much longer. And no leaving the roots will not result in new growth. Dave

Just noticed the couple of questions...No, no fillers needed. Last nights salad with a frittata. Easily 4 large salads. The rest put in a smoothy at work. this one was just a big blend. I've gotten so far ahead of myself I'm producing much more than we can consume. And a pic this morning of what is downstairs under lights. I have about 30 varieties I grow in rotation. Not always a blend but over-produced for the holiday long weekend. Sometimes just the daikon, red Rambo, china rose radish with fresh cucumber sliced. Simple lemon dressing. Or the crunchy sunflower shoots with the five blend pea shoots and a fresh sliced golden beet, different dressing. I will probably purchase once or twice a nice buttery Boston head for a avocado wrap at some point but the variety is so flavor punched I use mandolin paper thin veggies like a salad taco. No real need for any extra head lettuce. 10-12 days to harvest. And still fresh up to a couple weeks. Un-cut. Nothing fresher than a scissor snip an hour before brunch/dinner. Because these are young and not producing fruit, (like a tomato), a simple Ikea led light will grow macro greens. Last winter we had a month of gray snowy days. I set up a small shelf in my kitchen pantry and grew without issue.