Early spring and fall are fine times to plant many trees and shrubs, though there are differences among species.
Poplars, cottonwoods, aspens, and other Populus species are best planted in spring as soon as you can get into the ground. But some species are better planted in early fall, such as Amur maple, Norway maple, Barberry, Birch, Hackberry, Hawthorn, Russian olive, Honeylocust, Apple, Cherry, plum, apricot, Pear, Sumac, Willow, European Mt. Ash, Greenspire Little-Leaf Linden.
Whether you plant in spring or fall, plan ahead for the eventual size and care requirements of the plants you choose, and be sure to select varieties that will grow in your part of the state. If you shop from catalogs, look for information, not pretty pictures. Most importantly, is the hardiness zone right?
In general, western Montana lies in USDA Hardiness Zones 4 & 5, eastern Montana in 3 & 4, and a large part of central Montana in 4. But this system is accurate to only plus or minus one zone and much depends upon elevation and site aspect.
The "Tree and Shrub Selection Guide," published by MSU Extension, is a resource for Montana landscaping projects and home reference. It describes the size, shape and ideal growing climate for 226 specific tree and shrub choices that are adapted to Montana growing conditions. It also includes basic planting instructions and lists possible pitfalls of each species, including details like sprawling or brittle branches, soil limitations, need for winter protection and even odor.
For additional help in picking out the right plants for your landscape, contact your local MSU Extension agent. They can provide information regarding which plants are right for your particular soil, as well as other free and low cost yard and garden publications.
For free and low-cost gardening information, visit MSU Extension's online catalog at http://www.montana.edu/publications
or the Montana Tree and Shrub Guide at https://www.montana.edu/extension/gallatin/documents/naturalresourcesdocuments/2B0323.pdf
Specific to Montana cities.....
Many municipalities in Montana have adopted a specific street tree recommendation list. These selections meet or exceed requirements set forth by city planners, city foresters and tree boards. It is recommended that you contact your local city tree source, local nursery or arboretum before purchasing any plants. They can assist you in making an informed decision. Below you will find links to the latest tree lists from some of the cities in Montana ....