A Chinese proverb says, "The best time to plant a tree is 20 years ago. The second-best time is now." This is so true, and fall is a particularly good time to plant a tree!
Although there are some exceptions, (aren't there always?) September to mid- October is considered to be the perfect time to plant trees and deciduous shrubs in Central Illinois. Water loss through leaf transpiration is lower in the fall, as the temperatures are more moderate and the sun's rays are lower in the sky. October is typically a rainy month, and this helps the roots get their all-important moisture consistently with even distribution. The soil temperatures are still warm, yet the air is cooler- which encourages root growth and not top growth. Besides this, many trees and shrubs are on sale at the local nurseries. I say this with caution, because the flip side to this last point is that selection can be limited. On this, I highly recommend you call around or visit a tree farm to get what you really want, and what will really fit nicely into your landscape set up. A tree's life span is too long, and their landscape impact too large to let a bargain be your deciding factor!
You will thank yourself over and over in years to come for the homework you do now to select what tree you plant, and where you will plant it. Landscape trees are like visible neighbors that you can't hide or run from. Your choice can be the difference between added expenses and frustrations later on, or lower expenses and lasting pleasure. A wonderful guide to tree characteristics for our Zone 5 is found at http://www.mortonarb.org/trees-plants/tree-and-plant-advice/tree-selector. A great tree planting brochure can also be obtained at our Extension office, you may call the DeWitt, Macon or Piatt Extension offices to ask for one.
After making this momentous and exciting decision, where are you going to plant it (actually this is also how you help choose what kind of tree to plant). Believe the facts, because that 8 foot cutie can grow into an 80 ft. beauty. All good, unless there are power lines nearby or it towers over your one story ranch.
So, you've completed the compatibility process and selected the best tree ever, one you will love for its extended life. Congratulations!
But wait, taking such care to select the best tree and site, I know you'll want to give it the best chance of success for a long and healthy life. Simply, there are just a few very crucial steps here. First, dig the hole at least two to three times larger than the container, especially if the soil is at all compacted or has clay structure. If the tree has been grown in a container, pull and even cut the roots to disrupt any circling root growth. Failure to do this will result in root girdling, an eventually fatal condition. If the tree was grown in burlap or wire, cut away as much as the material as you can and loosen the roots, as much as possible. Plant as deep as the root ball, but the root flare should be even or slightly above ground level. It's a sad fact, trees that are planted too deep start to weaken and die after 12-18 years, and rarely if ever live past 20 years. Remember, tree trunks should not look like telephone poles or lollipop sticks rising from the ground. To plant, hold the trunk straight and refill the hole with the same soil it was dug with. Do not add fertilizer, but DO add 2-3 inches of mulch from an inch or two away from trunk to the drip line of the branches. There is a good visual guide on planting http://web.extension.illinois.edu/jsw/downloads/706.pdf
I haven't mentioned shrubs yet, but decision criteria and planting guides are the same. Of course, shrubs don't get as large or live as long, but they add great value and beauty to home landscapes as well. The Morton Arboretum link above is also a good place to start research on types of shrubs to consider. There are perhaps even a larger choice of shrubs to select from than there are trees!
Lastly, I'd like to add something not always thought of. Besides the physical attributes of the tree or shrub you need for your landscape, is there a specific function you would like the tree or shrub to provide? These lovely plants can and do provide shade, wind block, fall color, winter or three season interest, spring flowers, wildlife home or food, privacy, noise reduction and more; what do you want? Choose the one that will give you that, and it will enhance your world.