Can People Put Up Their Own Signs in a Public Park?

A reader writes:

We noticed that someone put up 'please be respectful' sign in the park (on the same wall where someone keeps trying to plant a vegetable garden...) 99% sure this is not any official city sign.

Can people just do that? Put up their own signs in a public park? How does one find out if that's even a real sign?

Looking online it does show that park supposedly has a 'community garden' but that seems impossible considering the layout.

I feel like that park is getting more and more unwelcoming, with people turning it into their living room with cook outs and a TV when the weather is nice, the home made sign on the grass hill about not disturbing the grass, and now the sign stating that dogs are not allowed to to their business on the mulch.

We would assume you can't just put a sign on public property, but then again we doubt the city is going to do much stop inconspicuous things like this.  All that being said...just as equally as they probably shouldn't be doing that...you probably can just take it down if you would like?

(Hat tip:  SL!)