Disambiguation of Some Common Plant Names 3: Marigold, Mountain Ash, Bugleweed, and Loosestrife

Common names are locally-used names. Nobody standardizes them. So it is easy for people to use the same common name for different plants. 

marigolds, genus Tagetes

Common Names--Too Many Shared Names

Calthus palustris  Do you call it cowslip or (marsh) marigold?

Nobody regulates common names. That's one of the reasons for scientific names. The rule on scientific names is: each organism has one and only one name, not shared with any other organism. 

Common names don't obey either of those rules. I wrote previously about multiple common names for the same plant. (link) That is annoying, because sometimes you don't recognize that someone is talking about a plant you know only because they're using a different common name. 

I think the same common name for different plant is even more unfortunate. In this day of using words on the internet to learn about things, shared common names lead to at best, time wasted working out which plant you want, and at worst, possible poisoning because one plant with that name is toxic and one isn't. 

another cowslip Primula vulgaris

another marigold, Tagetes sp.