Invasive Tree Roots - Knowledgebase Question

Name: Garden Friend
Thousand Oaks, CA
Question by scfamily
January 26, 2000
We just bought a house with nice railroad tie vegetable beds already in place. Unfortunately, the neighbors have planted two elm trees near our fence. The surface roots have invaded the vegetable beds and made them unsuitable for growing. I am thinking of putting down hardware cloth and building raised boxes on top of the beds about 2 feet deep so that I can do my vegetable and flower gardening. Do you think this will work? If I totally redo my yard because of their trees, how far away from an elm should I plan on building vegetable beds? How far away from an elm can I plant, say an avocado tree? Is there a way to sink a barrier into the ground to stop the roots from invading our yard?

Answer from NGA
January 26, 2000


My raised bed veggie garden sits right next to a forested area, so I know exactly what you're experiencing. Tree roots just can't resist soft soil, lots of nutrients, and plenty of water, so they naturally grow into the beds. Sadly, tree root competition for these elements is generally successful. The smaller root systems of annuals and perennials, hard as they try, can't outcompete the tree roots, so they generally produce stunted plants with poor yields.

I cope by lining the bottoms of the beds with hardware cloth topped with weed barrier. It's not a permanent solution, though. Every 3-4 years I empty the raised beds, cut back and remove the invading tree roots, replace the weed barrier, and refill with soil.

Tree roots can extend one and one-half times the diameter of the canopy (farthest ends of the branches). So, place your beds as far away from the Elms as possible. You can discourage root invasion if you place a barrier along the property line, deep enough to keep most of the small feeder roots on their own side of the fence. The same principle is used for containing bamboo roots: dig a trench 24" deep and 6"-8" wide, then place galvanized or fiberglass panels in the trench, setting them horizontally, about 3" below the soil surface. Fill in around the panel and water to settle the earth.

An Avocado tree will require lots of sunshine so place yours where the elms won't shade it. A mature avocado can reach 40' in height and width. The roots of the tree can extend 60' in all directions (but that's just a potential; actual growth will depend upon soil conditions). Give your avocado tree as much sunlight and elbow room as possible so it doesn't become overcrowded as it matures.

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