The potential for the plants harming the newts depends on the plant, whether it is aquatic, and whether the newt will be crawling through it or on it. Some plants have irritating sap that can cause skin damage, and some plants may release toxins into the water. In general, plants that are safe for freshwater/tropical aquarium fish should be safe for the newts.
You can also learn more about which plants are safe for herps and which may cause problems as well as the nature of those problems here:
Edible and Harmful Plants
Good luck!