The perfect tree
I am looking for a deciduous tree that is perfect for Colorado to plant in a small-medium yard with no grass -- pavers, gravel, ground covers. Need something tough enough for our climate, resistant to diseases, tolerant of alkaline soil, xeric, relatively quick growing and of course, gorgeous. I am hoping for a broad shape that will provide shade, with room to sit underneath its boughs. I'm thinking a Golden Rain Tree or a Catalpa might fit the bill. Can a Japanese tree lilac get tall enough/broad enough? THANK YOU!
Arapahoe County Colorado trees and shrubs
Let's start with your site, no lawn, pavers, gravel and ground covers...You do not state if the area has an irrigation system or if you rely on snow/rain precipitation or if you will water the tree by hand as needed. Newly transplanted trees need to be watered frequently and deeply in order to survive the transplanting process and to re-establish a good root system that not only transports nutrients but also anchors the tree.
The Golden Rain Tree, mature size of 25'H x 20'W, has a moderate growth rate, low water requirements once established and is best grown in dry sites. It does bloom in July when few other trees bloom which is a plus. There are a few downsides to this tree: if grown in a gravel or bare soil area the seeds can germinate easily and there will be unwanted seedlings to deal with and the red-shouldered bugs (similar to box elder bugs) tend to congregate to feed on the immature seeds. While they do not harm the tree they can be a nuisance. It is also a somewhat weak branching tree that can break under heavy snows.
Western or Northern Catalpa, is another flowering tree, flowering in the spring with big, heart-shaped leaves and white flowers. Mature at 50'H x 25'W. Has a slow growth rate and a more upright growth habit. Bean pods form in the fall, can measure 20" in length, may stay on the tree all winter and can be somewhat difficult to gather up on the ground.
Tree lilacs mature at 20'H x 20' W with white flowers in late spring, have a nice fragrance. Once established the water requirements are moderate and they have a moderate growth habit. The red-brown bark has a shredding habit which is a nice visual interest.
These are the three tree species you specifically mentioned. There are many, many other choices that are appropriate for Colorado and the Front Range area. I would avoid planting any Ash trees because of the Emerald ash borer that is now in Boulder County.
The following CSU Fact Sheets will provide you with some other tree species to consider.
Selecting a tree is a very individual thing, what I like may not be what you like. The fact sheets can help you make an informed decision. If you have any additional questions, please let us know.