The evergreen is in a class of it's own, but the options range from ground-hugging shrubs to hundred-foot giants, from haphazard arrangements
with branches sticking out every which way, to plants compact enough to make topiaries out of--or at least with the ability to be shaped. The obvious advantage to the evergreen is the fact that it is . . . always green, but don't just run out and buy the first thing you see. I visited several places that sold trees before I found one with one that would be the shape I wanted, with the color and type of needles, and that wouldn't grow so large that I would eventually have to cut it down because of space. As I mentioned, many evergreens grow to be real giants, and though that little tree may look lovely for the next few years, choosing wrong can mean a major hassle later to have it removed.
Side note: I've heard rumors that the reason nothing grows under evergreens is because the needles are so acidic: it ruins the Ph of the ground. Some even suggested mixing pine needles into garden beds that were too alkali, but I read about a study a while back that disproved the theory that doing so made the slightest difference.
As for deciduous trees, the options are incredibly diverse. Because of my incoming water line, the sidewalks and my irrigation line all running through my front yard (and nowhere near each other), my husband and I selected a spring-flowering tree that will never get very large. Smaller trees tend not to have as big of a root spread, so I figured it less likely to have water-line issues. The roots also don't get as large, so they cause less damage to foundations and sidewalks. We picked a tree that will grow kind of round on top, liking the shape, as well as the reddish leaves all summer long, and the spring explosion of flowers.
In my back yard I'll use more shade trees, and further back yet are my fruit trees. Aside from options like spring flowers, fall foliage, and general size, you need to consider what kind of seed pods the tree may put out. Does anyone in your family have allergies to specific trees (such as cottonwood?), does your climate or water conditions foster the type of tree you want--a desert tree is not going to be happy along the Oregon or Washington coastline, while trees that thrive in Seattle's regular rains may not be the least bit happy in Utah, Nevada, Arizona or New Mexico. Thankfully, your local garden centers are unlikely to stock trees that won't survive your weather conditions.
Don't plant trees too close to your foundation as larger ones, especially, can cause a great deal of damage, and some smaller ones may not survive the hot, dry space on the sunny side of the house without a great deal of supplemental watering.
Trees are plants, so yes, they do need watering when rain is scarce, and they need deeper watering than grass and flowers. My local nursery owner reminded me to water my trees when we have a dry winter, or, as happened a couple of years back, a winter with lots of snow fall, but no melting for months on end. The trees are alive and need moisture even if the leaves have fallen off for the season. Deep watering from time to time also encourages the trees to put their roots down into the soil so they are less likely to be uprooted by strong storms, or affected by drought.
How fast do you want your trees to grow? Willows grow pretty fast, adding interest and shade in only a few years, but they don't live as long as the oak or maple that takes many more years to get established.
Do you have a beetle outbreak in your area? Some trees are susceptible to certain kinds of bugs. If you are interested in a specific tree, it wouldn't hurt to contact your local extension center and see if there is a problem with growing that variety in your area. The pine bark beetle is causing real havoc in many of our forests right now, and some deciduous trees have similar problems. Be especially aware if you are bringing trees in from out of the area, or are buying from stores that don't specialize in plants as they many not be aware of infestations in your general area.
One more thing--Be aware of where the shade is going to be cast. Planting a leafy tree on the south or west sides of the house (in the northern hemisphere) can save you a bundle on your summer cooling bill, but planting one to the south of your flower beds or vegetable garden can make growing what you want nearly impossible.
These pictures all came from the National Arbor Day Foundation Website.
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