Trees serve as the backbone of the garden. Trees add structure and height, clean the air, filter water, prevent soil erosion, provide shade and can be a habitat for wildlife. If you have ever sat beneath a maple tree in the heat of summer and enjoyed the cool shade it provided you understand the value of that tree. To me planting a tree is a no brainer, the more the better!
It takes time for trees to grow into a well structured specimen with a great canopy so they should be the first plants put into a landscape. You could splurge and buy trees that are already mature but these are expensive and the root ball can be difficult to plant by yourself. Smaller trees can catch up significantly to the larger ones in a couple seasons since those large root balls have to recover for the missing roots that were cut when they were removed from the nursery.
When buying a tree check the root systems to make sure it isn't pot bound. Look at the foliage for damage caused by disease and check the trunk for bark damage.
Our landscape had two trees in the front yard, both of which are Bradford pears (one of the most over-planted trees in Tennessee).
Here is what we added so far:
1 Red Sunset Maple (Acer rubrum 'Franksred')
4 Canadian Hemlock (Tsuga canadensis)
1 Green Weeping Willow (Salix alba)
2 Eastern redbud (Cercis canadensis)
We planted the maple for the fall color and the shade and the hemlocks to form a privacy hedge on one side of the house. I've always like the form of the weeping willows and this one will help to make a summer screen for our deck. The redbuds are native to Tennessee and have great purple spring blossoms.
There are quite a few more trees coming from the Arbor Day Foundation so once I get them planted I'll add to the list. We also added several shrubs but I'll save them for later.