You can get more information from the California Turtle and Tortoise Club , but be aware that their information is on toxicity to humans, and may not apply to turtles in all cases. Whatever the vegetation, be prepared for the turtles to trample it down or dig it up to some extent. Needless to say, no pesticides should be used in the enclosure. If rain is lacking, a quick sprinkle would be appreciated in the mornings. Dump the water every few days to remove mosquito larvae.
Turtles also tend to relieve themselves in the water, so monitor it and clean it when necessary.
The fouled water makes fine fertilizer. Box turtles are solitary creatures. They do not need companions of any sort. They must also be protected from family pets, and the enclosure must not attract nocturnal visitors such as raccoons or skunks. If you must keep box turtles inside during the winter, make a large enclosure.