ArborGard+ 9″ Trunk Guards. Arbor-Gard is a tree trunk guard that protects the young and maturing tree trunks from damage caused by string mowers, reel mowers and effectively eliminates the need for unsightly chemical control. ArborGard+ is manufactured from linear low density polyethylene with ultraviolet inhibitors. The guard is 9″ tall and expands to a 4 inch diameter. Guards can be locked together to accommodate any size tree.