I’m going to say, right up front, that what follows will never answer every question about cutting tree roots. And what’s provided here should be not be considered the ultimate authority on the subject. But they are rules that most of the tree experts I know follow, and they’re the guidelines I’ve lived by over the years.
There’s no one-size-fits-all formula, and many questions need to be answered before jumping out there with an axe or machete or chainsaw. And by the way, a chainsaw should never be used around dirt, unless you wanna ruin it.
The most important rule: Cut out only one big root per year on a mature tree.
Be warned, however … that will be a giant effort in futility if you don’t first get the tree on a deep-root watering and feeding program. Exposed roots above ground are the tree’s way of screaming that more water needs to get down deeper into the soil. So, unless you immediately get on the deep-root protocol I’ve pushed for the past 20 years, you’ll just be spinning your wheels by cutting an above-ground root; more will be on their way.
The tree flair – the large roots slanted upwards at the base of the tree’s trunk - and other big roots actually form the tree’s anchor. If they are pushing themselves closer to the soil surface to find moisture, you can see the problem. To be a strong and effective anchor, those big roots need to borrow deep into the earth.
That’s why you can only prune out one big anchor root per year! And, theoretically, you shouldn’t do this kind of work yourself. Unless you have genuine experience, you should call on a real tree expert. It’s actually much harder work than it may appear, and if you get overzealous you could actually do more harm than good
There are also some folks who want to prune roots because they’re worried they’re headed toward a sidewalk, a driveway or the house foundation. In those cases, you actually need a root barricade. That’s because to effectively head off sidewalk or driveway damage, or to prevent roots from sipping moisture from under a foundation, you would need to cut more than one big root.
I don’t normally recommend root barricading to protect sidewalks, but for driveways and foundations, a professional should be called in. If you’re thinking, “Hey, I can just get a Ditch Witch and create my own,” please don’t! The best barricading is done by hand to prevent traumatizing the roots. In Houston, I only recommend Root Wall Pro.