Are leafy vegetables OK for pet rats?

Excessive consumption will cause diarrhea. Iceberg lettuce has no nutritional value, though mustard greens, dandelion greens and such are good in small amounts. Edible flowers such as Hibiscus, Marigolds, etc. are good, but make sure you know what flowers or parts of flowers are edible before feeding them to rats. I am trying to work on a list of safe edible flowers.
English thyme sprigs seem to relieve respiratory symptoms in mice..There are many herbs that rodents can safely eat in small amounts, but research them before feeding and feed only a tiny amount when first offering it to avoid stomach upset.