I sometimes feel like I am always telling the kids not to do something. Is this just a common feeling we all have or should I let them be more free?

For example, outside I am telling them to keep the sand in the sandbox, not to pick my flowers, stay off the new grass, heh heh now that I am typing it it sounds silly. My space is set up to be very child friendly so not to interfere with their play and there is lot's they can do. They just seem to be attracted to those couple of things I don't want them to do. I even made a mud table with worms and everything but they still go for my garden/flowers. Should I just let it go? I don't want to always say "no" but they don't seem to get it/remember/care?