This is a very important question and one many do not even think about. Plants are beautiful how can they be harmful :). But sadly there are many that are extremely dangerous to mildly dangerous. When I first started my cattery and building my catio .. I wanted to place plants out in the catio for my babies to enjoy. So I went on a search all about plants :). I came across one site that they list every plant that can be poisonous or dangerous for your cats. As well as every plant that is not harmful. Here is the link to Animal Poison Control. This site is great because it also has what human foods are bad for you pets as well as household products. Everyone that has pets should check it out and see if they have any of the harmful items in their homes. It lists for Dogs .. Cats and Horses.