Arguably the best time to hire an arborist is spring, before leaves grow and of course before the tree undergoes a growth spurt. Why? Because once the tree is pruned or cut down there is much less mess to tidy up afterwards. Also, there is less of the tree to cut down in the first place, so the job is quicker and the arborist will charge you less.
Given that trees are constantly growing, the tree will never be smaller than it is now; So you could argue that in fact, the best time to hire an arborist is now!
Of course, that’s purely from a cost perspective. From an aesthetic perspective, you could say that it’s best to hire an arborist sooner rather than later. This is because triming tree’s back sooner means that the tree can be shaped as it grows, rather than ending up with a small tree with long, lanky branches.