What is the plural for lettuce?

The plural of lettuce can be lettuceor lettuces e.g.

The farmer grew lettuce for a living

I bought two heads of lettuce from the shop

This effect was not observed in glasshouse-grown lettuces where similar varieties are grown all year round.

The first red oak leaf lettuces from River Nene Organic Farm are now going into the boxes along with wet garlic.

Both variants are interchangeable