My 2 Marquis grapevines have been growing well (so far) this year. They have reached the 7' high top wire strand. Do I put another strand of wire up there so the vine can keep going? Is there a limiting factor here?
Larry just let them keep growing. They will tip over the other side. If they begin to shade the other foliage a lot, you can top them. Hold off on that as long as you can because it will force laterals to grow (at every leaf) and it will quickly become overgrown.
You don't want another wire, it gets too tall. My top wires are set at six feet because I am a short person of French decent.