Our grandchild is a toddler. We want to plant a tree that would be large enough to be used for a treehouse two years from now. We were thinking of planting a sycamore, but are there other trees that would grow fast and be usable for this purpose?
Most trees that grow extremely quickly have very weak branches (poplar, willow, elm and others), and some of these trees are considered invasive or noxious (like Empress tree [Paulownia tomentosa] or tree-of-heaven [Ailanthus altissima]). Sycamore (Platanus species) typically grows 3 feet a year, which is moderately fast, and has medium branch strength. If you were to find a tree that was already a couple of years old, even after planting it and waiting 2 years, the wood would not be strong enough to support a treehouse. Also, a fast-growing tree will continue to grow larger and taller, its branches reaching higher and its trunk expanding in girth. This could cause trouble with a structure built too early on in its lifespan. Such a tree might also need a significant amount of space in the garden: its roots might reach out quite far, and its branches would also need sufficient room to spread.
You could construct a free-standing treehouse that abuts a tree, so that the branches would not need to support its weight, and visitors would still have the experience of being in the midst of foliage and branches.
The book Treehouses by Paula Henderson and Adam Mornement (Frances Lincoln, 2005) states that “most of the world’s most durable treehouses sit in healthy mature hardwoods. Good host trees include oak, fir, maple, beech, ash, and willow. Regardless of species, experts recommend that load-bearing branches have a minimum diameter of 17.5 cm. [that is about 7 inches].”
Here is an article entitled “Tree-Literate Treehouses,” about treehouse construction, from University of Georgia Forestry.