Will Elevated Beds Prevent Tree Roots?

Root Growing Habits

• Roots naturally grow toward moist, rich soil. Because the top 24-inches of soil are typically tilled through natural microorganism activity, tree feeder roots grow horizontally within this layer for sustenance.These roots often develop much farther than the dripline. Your elevated bed is perfect for tree root sustenance; roots will eventually find their way into the bed if the tree is close enough. The best way to prevent roots in the garden is installing the garden far from the tree's dripline. In general, most feeder roots do not extend more than four times the dripline's radius, according to Colorado State University Extension.

Enclosing Your Bed

• If you only have a few tree feeder roots trying to encroach on your bed's soil, insert a black plastic liner at the garden's base. Although this strategy requires a complete excavation of your garden, you reap the benefits of a closed-system to tree root infiltration. You need to punch small drainage holes in the plastic to allow the bed to drain into the ground below. As you add the soil back into the bed, amend the soil with organic matter so that you improve the nutrient levels for your plants to absorb. It is possible for roots to eventually grow back into the garden as the plastic degrades so it is imperative that you keep the plastic covered with soil to prevent weathering from sunlight damage.

Root Barriers

• A more permanent tree root solution is using a vertical barrier. Because tree roots do not typically dive any deeper than 7-feet, a physical barrier placed between the garden and tree stops any elevated bed issues immediately. These barriers are often made of plastic layers approximately 5-feet in length. They need to be sunk vertically into the ground as an effective barrier -- tree roots encounter the barrier and slowly move downward or to the sides. It is critical that the barrier is not too short, such as one or two-feet long. Tree roots striking this barrier simply grow down and around it as they enter your elevated bed. (See Reference 1, 3, 4)

Avoid Damaging Critical Tree Roots

• When you install barriers around your elevated bed, you may need to cut tree roots away. Do not cut into large diameter tree roots, especially near the trunk. These roots provide anchorage for the tree. Cutting away small feeder roots is acceptable as long as you are not pruning too many away. The tree uses these roots for absorption -- you may stunt the tree's growth by removing too many feeder roots at one time.