Mature willow trees are often 13.7 to 21.3 m (45 to 70 feet) high and at least as wide. The roots below can stretch several times as wide as the tree branches. While they generally won't interfere with the growth of other plants, willows' roots can wreak havoc on man-made structures. Plant them at least 30.5 m (100 feet) away from buildings, pipes and septic systems.
Willow trees, like cottonwoods and poplars, produce many shallow roots that may extend two to four times the length of the tree's crown, or outer branches. Most of the roots lie within the top 30 to 45 cm (12 to 18 inches) of the soil. In moist, well-aerated soils, they remain closer to the tree; in poor, sandy, dry soils, they extend farther in search of nutrients and moisture.
Willows are notorious for burrowing into septic systems and water pipes, although they won't damage a foundation unless moisture collects there. The roots may also damage paths and gardens. Because the roots lie so close to the surface, lawnmowers or excavation projects can easily damage them.
Plant the willow in full sun and water it regularly. Take care not to nick the tree or roots with the lawnmower. If the roots protrude into the garden, consult an arborist about options. An arborist may be able to cut them back without harming the tree. For a less invasive option, replace the grass with perennials that don't require mowing and will hide the roots.
Willows are rarely a good choice for residential areas due to their invasive roots, large size and propensity for disease and branch breakage. Replace them with slow-growing, long-lived trees to reduce problems with clogged pipes from roots. If you wish to keep the tree, hire a plumber to clean the pipes annually and use a product in your toilet designed to reduce root growth.