Could you add some photos of the tree?
Here in Savannah, Georgia we have lots of old trees that often require support. The Tree Department uses a system of metal cables (stranded or braided wire) and turnbuckles.
On this site there is information about something called "cobra cabling' that might work for you. The Zip ties you used may cut into the bark and harm your tree. I have seen homeowners use metal cables but they added a piece of old garden hose to protect the tree bark.
http://zimmermantreeservice.co...
Hope this helps.
