HGTV's Gardening by the Yard explains why, when placing plants, you should avoid putting them too close to an exisiting structure.
Many plants have a tendency to get a lot wider as they get older. Among the most obvious are blue Atlas cedars, Southern magnolias and Bradford pears. Sometimes these plants are placed in spots where they shouldn't be, then grow too big for their site.
"Just about every week I see a blue Atlas cedar (figure A) planted too close to a house or even grouped together as a trio of trees spaced just a few feet apart," says master gardener Paul James. "The problem is that this tree, in time, can grow to 100 feet tall and at least 40 feet wide, much bigger than most homeowners realize. The same is true of the Southern magnolia. And don't forget the Bradford pear (figure B), which in the trade is sometimes called "Fatford" pear because of its tendency to outgrow its place in the landscape."
So what's the solution? Take into account how tall and wide your plants will ultimately grow. Avoid planting tall trees under power lines or large shrubs too close together. If a plant is likely to grow too large for a particular spot, place it elsewhere in the landscape.