Far too late for most lettuces in most of the country. They are cool weather crops that bolt and turn bitter when exposed to summer heat. That may well be the problem you are seeing. So where are you located?

If you are in the far north where it is still cool then another possible explanation is that they are planted too closely to form heads. Proper spacing is required for head formation so you may need to heavily thin the patch out.

Dave

Thanks Dave! I am in the south west of the uk. It's been hot few weeks ago... I didn't see they bolt but as you suggested, I planted very closely... Will they develop into head after I thin them out then? What about those big flat green leaves? Should I remove them? Are those edible?

Thank you very much!

A photo would help.

Little gem is a head lettuce, not really meant to be harvested leaf by leaf. Head lettuces need space to head out. Little gem should have 6 inches between heads.

I find the best way is to thin the baby lettuces by pulling the entire plant, every other plant at least, and keep doing this until the mature plants are at their final spacing.

The OP is in the UK and lettuce can be grown pretty much year round here. I sow successionally throughout the summer.

loveVeg - some tips. 1. Sow the next lot thinner. 2. When the seedlings show start thinning them out i.e. removing some. You can eat the thinnings. 3. It does not take long to take a few leaves from each plant for a salad. That's what I do at the start of the season when the plants aren't fully grown. 4. The ones you buy have been trimmed for sale so the outer leaves have been removed. All of a lettuce is perfectly edible. You just have to experiment to find out whether you like the outer leaves or not. Fussy eaters might just go for the hearts. Personally I eat every bit that isn't actually yellowed or bug eaten. 5. If the plants are 8 inches tall they are nearing maturity and will start to bolt soon i.e. they will put up a stem and start making flowers. You don't want this to happen but you can't stop it so you need to eat up the ones which show signs of the centre pushing up taller. 6. Now you are growing your own lettuce you will discover that it actually has flavour and is not just a bland, crunchy substrate for a dressing. In fact it doesn't even need to be crunchy to be good. I always grow a mixture of red, green, cos, looseleaf and heading types so the salad bowl contains a wide variety of shapes, colours and flavours. You can buy packets of mixed seed so you don't have to get lots of different types in quantity.

Little Gem is my favourite lettuce but it isn't very big so imo you need to get eating.

I also suggest you do a bit of reading so the next lot is better organised. You can sow more now and keep putting in small batch of seeds until September.

Here is a link that might be useful: Growing lettuce

Thank you so much everyone! When I sow the seed, I kept 6 inches space in between. But once they grew, they were packed! I understand much more now. Definitely will thin out the lettuce before they mature next time... and I feel better now as I can still eat them although they are not perfect! Thank you!!!

floral_uk, thank you so much for all the information. It's really helpful. When I read point 4., I could imagine I was like those kids who never saw a real orange and thought orange only come in juice-form! lol and yes, I found the lettuce taste so much better. I don't really mind eating those big flat leaves which gave a different taste~ will definitely try mixed seed next time!

Thank you so much again xxx

Hi Hi~ Just want to let you know that I follow your advice to thin out my lettuce. Just now when I tried to picked some of them, I realised there were few leaves which were burnt or eaten by bugs, but when I removed those leaves, I found the lovely green crunchy heart inside and they were huge and bugs free! Thank you so much again!!!

Good stuff. Have you sowed more to keep up a succession?