In an average garden probably half of the plants you come across are poisonous to one degree or another, some of them deadly. The good news is that a lot of it tastes horrible and chickens have been around for a long time and are not as daft as some of the ruminants when it comes to eating stuff that is harmful to them. For example, ivy, buttercups, laurel, rhubarb, azaleas, rhodedendrum, box and hydrangea are all poisonous when eaten, azaleas and rhodedendrum can be deadly in small doses others have a cumulative effect like say buttercup which burns and upsets the stomach. I have had all of these and apart from having a bit of a nibble at some box and deciding they didn't like it the chickens ignored everything I've just listed. If you really want to be thoroughly on the safe side do a search on poisonous plants for chickens, there are plenty of lists out there and then remove the deadly ones, I wouldn't worry about it too much, as long as they have food available they are unlikely to eat the harmful stuff.