My wife was first to see the connection, digging deeper into the raised beds until she came upon an inch-thick woody root, obviously not from any vegetable crop. She pulled the root upwards to show how it was coming from the direction of the giant fir tree. I tried to convince her otherwise, knowing that if she were right then I had a big job on my hands.
For a few years we put off the inevitable – building a root barrier to keep invading tree roots from accessing our fertile garden soil. But as more beds became depleted and our planting options narrowed, the need for a root barrier became urgent, and so the digging began.
Our plan was to dig a trench along the 60’ western perimeter of the garden, blocking any roots migrating into our raised beds from that direction. The trench would be dug to ‘hardpan’ depth, about 3’ – 4’ deep, and lined with doubled-over sheets of galvanized metal roofing. The job went well, exposing many roots ‘mainlining’ into our beds, and within a few days the bulk of offending roots were severed, and blocked from entering the garden. Here’s how it is done:
1. Identify the source of invading roots
If you think tree roots are entering your garden, dig into the bed closest to the suspect tree. Pull any root upwards to get a better look and which direction its coming from. If the root is thin and breaks easily, it may be a viny root from an invasive weed like Horsetail. But if the root is thicker and woody, dig it out further to see its growth direction – it will lead to the source tree. The size of the tree will give you indication of the scope of the problem. If it’s a large tree, expect more than one or two roots. You can visualize the pattern the roots are likely following, since they radiate outwards from the tree. This will give you an idea of how long to make the trench.
2. Dig a trench wide enough to stand in and deeper than roots can go.
Begin the trench by digging down and about 18” wide, providing enough room for you to stand in while digging the deeper part of the trench. Leave a ‘step’ on one end so it’s easy for you to climb out. As you come across rocks, set them on the ground above, close to the trench since you’ll be throwing them back in when it’s time to backfill.
Before digging, set a tarp or ground cloth on the ground alongside the route you’re going to dig. As you shovel, pile the dirt on the tarp. This will make it easier to return all the soil to the trench as you fill it back in.
3. Use a pruning saw to cut the invading roots.
Before long, your shovel will hit into one of the roots. Dig the soil around to fully expose the root, then saw it from both sides of the trench. This will leave the trench clear of root stubs. Use a small hand pruning saw for this work. Set the cut root sections in a pile near to one side of the trench – you’ll want to show them to your wife so she sees you’re getting results. (This buys you future undisturbed hammock time.)
4. Set down folded sheets of metal roofing on edge, against the far side of the trench.
Try to find some old galvanized metal roofing sheets to use for the barrier. Another great option which we’ve used in another garden, is HDPE plastic sheets, doubled. HDPE will last indefinitely underground, and are tough enough to prevent root penetration. We found these HDPE sheets at a feed store where they are discarded after use on feed pallets (they prevent the feed sacks from being pierced by the rough pallet wood. You could ask at your nearest feed store if they are available. Otherwise, use the galvanized roofing.
It’s important to fold over the top edge of the metal sheets. This can be done by hand to form a loose fold, and when the dirt is filled back in, the fold will close more. Or you can use a heavy pry bar to force it closed. The reason you want this folded edge is that the top edge of the metal barrier may protrude just a bit above ground level when the job is finished. Any sharp edge would be a hazard.
Form a continuous wall with the sheeting, and overlap the ends at least 12”, so no roots can work their way through the barrier.
5. Throw the collected rocks into the bottom and fill in the trench.
Toss in any rocks you’ve collected, then fill the dirt back in. Overfill the trench, since the dirt will compress a few inches when walked on. The severed tail ends of the roots remaining in your garden beds don’t need to be unearthed and removed. Over time they will rot in place, providing nutrients for future crops.
Building a root barrier seems like a daunting task at first, but the job goes pretty quickly once begun. It’s satisfying to know you’ve shielded your garden beds from invading roots, and that all the special amendments, fertilizer and water you put into your garden will go to the plants they’re intended for.