Often, people posting for the first time in the Plant ID Forum mark a thread closed after the first wild guess. Or, they ask a question in this forum and the first person who responds doesn't answer correctly but the OP closes it to future comments. If OPs could close a post, they may not come away with the right info.
Its a problem for crowd sourced websites but, bigger problems could be caused by allowing the closing of threads by OPs. It would be best if we just remembered to stay on topic unless the OP joins the weird direction of the conversation.
The Admins can close a thread to future comments at the request of the OP.
