Building Hardscapes Around Trees

The photo of the deciduous tree (a black walnut, Juglans nigra) shows how much the tree has been cut back because it is dying back after the parking lot was added around it. The pavement is slowly killing the tree because the roots don’t get enough oxygen, “waste” gases from the soil can’t get out, roots were cut or damaged during construction and water can’t percolate into the root zone.

Here are some general guidelines to allow for the adequate space needed for root growth near hardscape areas (there are many exceptions):

• It’s best not to plant trees between paved areas with less than three feet of soil to grow. 
• Use trees that mature at less than 30 feet tall when you have a space with three- to four-feet between paved areas, such as between a sidewalk and the street next to a patio or foundation. 
• If you’re lucky enough to have a space of five to six feet between paved areas, you can plant trees that mature as high as 50 feet tall. 
• With trees higher than 50 feet you’ll need at least eight feet between paved areas. 

Also, consider installing expansion joints in sidewalks or the patio near trees to limit possible sidewalk replacements if the roots should heave a section of the hardscape to just a few sections rather than large areas. You might be able to reduce heaving by building curving and narrow sidewalk sections near trees.

Sometimes using a bed of coarse gravel is also effective at slowing, maybe even stopping, the heaving because the roots will not grow into the air pockets of the gravel, instead, they grow deeper. Use round gravel so there is lots of air pockets. Crushed rock packs in too tight.

Consider using lawn pavers, pictured right, with holes in the middle. These are what should have been used in a large area around the walnut tree, as seen in the other photograph.

BIO: Robert Kourik's books, blogs, and articles bring science to gardening. He has designed gardens for three decades and is a pioneer of edible landscaping, the art of growing food plants among ornamental plants.  He works throughout California and the country. Visit Robert's blog Garden Roots or go here to buy his books.