Is it legal to tell a neghbor they can not come onto you propery to trim their bushes?

My neighbor's bushes run along side our property line. I have had issues in the past with this neighbor about where the property line is. He has a string running from the front of the yard to the back of the yard claiming it as his property line. This does not give him enough room to trim his bushes. There is only about a foot from the bottom of the bushes to the string he put up. I can not afford a survey at this time to determine where the property line. The bushes are getting out of control on my side of the yard because he has not timmed them. Do I have to allow him on my property to cut them? CanI trim them myself?