Q: Four years ago, I transplanted seven fifteen-foot oaks from the woods into my yard. Three died and four are doing well. How long does it take for the roots on these transplanted trees to gain the same resistance to overturning as they would have had growing naturally?
A: I think four years is plenty of time. Vigorous new growth on the branches this year would be evidence that the roots have spread and started making new anchor roots. If the trees look healthy but have not grown much, keep a guying system in place another year.
Remember that any time you guy a tree it should be a loose system, not tight. The gentle swaying of the tree trunk stimulates root growth. Also never use wire (even padded wire) or chain around the trunk. Nylon straps an inch wide are best to avoid trunk damage.