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Invasive Roots

Question from Carolyn:
I’m trying to figure out what plant in my back yard is causing the problem of invasive roots. The roots seem to be everywhere and any time I need to dig it makes the job extremely difficult. I have queen palms, pigmy palms and bougainvillas in the back yard. On the slope behind our house are Manzanita and Chaparral. Are any of those the cause? The roots are slightly thicker then a finger and usually flexible. Any information would be appreciated. Thank you.

Answer from Pat:
Invasive roots are a pervasive problem in the west, especially in old gardens with established large trees. Most of the problems I hear about are with trees and sometimes involving tree roots invading from an adjoining property, resulting in inharmony between neighbors. Many trees are obvious culprits, including eucalyptus, Monterey cypress, Melaleuca quinquinerva, camphor tree (Cinnamomum camphora), and certain pines. There are dozens more. Most of tree roots are hard and unbending, though some trees, including Monterey cypress and melaleuca, make a mat of surface roots like large-gauge wire wool. But the landscape you appear to be describing to me is a garden with no trees except for palms. Palms are not as much of a problem. Neither queen palms, nor pigmy palms have invasive roots.

Native plants also have roots that spread out to get available water, but in most cases they reach deeply into the ground and for the most part, the roots of native plants are wiry in nature. Of all the plants you list in your garden the most likely culprit is bougainvillea. Bougainvilleas have far reaching roots and they are flexible, as you describe. If the soil is friable these roots will extend far down into the ground, eventually hitting ground water thus rendering the plant totally self-sufficient, able to survive without irrigation. But when eucalyptus grows in an area in which there is a hard, largely impermeable subsoil, or hard pan, the roots will travel sideways out into the ground just under the surface of the soil, as the roots of invasive trees do. The plant is doing all it can to find water. Another plant whose roots travel far and wide in search of water is cut leaf tree philodendron also known as Silloum philodendron (Philodendron bipinnatifidum). When grown in the ground these plants send flexible roots out into the surrounding soil for fifty or one hundred feet. I mention this since it’s a common plant to grow with palms and maybe you have one.

Cutting off tree roots one finds in the ground will often kill branches all the way to the top of that side of the tree and if cut off close to the tree, those roots will never grow back on that side of the tree, but not with bougainvillea. With an established bougainvillea, you can cut off many of its roots without doing any appreciable harm, but those roots will grow back. When invasive roots are a problem, one way to go is to plant in large containers provided with a drip system and sunk into holes in the ground, with a stepping stone under each one to prevent the roots from entering on the bottom. Another way is to build a raised bed. Sometimes this is enough to allow plants to get a foothold and in many cases, as with bougainvillea, the roots don’t have a tendency to grow up into it but instead are satisfied with the water that drains down to them from above. With eucalyptus it’s different, the roots will climb up and fill the raised bed eventually and steal all the water they can get. In that case a barrier must be installed beneath the raised bed to foil the roots trying to sneak in from below.

Refer to books on raised beds in order to get ideas for how to design and build them. Fill them with good top soil and make a marriage of soils at the bottom so that you don’t create hardpan. You can also put in hardware cloth beneath them to keep out gophers. I am sending you photos of a bank in my garden that is also in a way a raised bed because it has a wall on one side to hold up the soil. An established bougainvillea grows at one end of the bank, above the rock wall. I have many raised beds in my garden and most of them need digging out and revamping from time to time. When revamping one can dig up the soil, remove invasive roots, amend, fertilize, and then replant as we did with this bank. I am not strong enough to do this any more but I have a gardener once a week who does this when required.

Comments

  1. excellent and thorough. I am interested because the house I just bought has three sissoo trees that are two years old. I researched the web and they present many problems with their root systems as well a being very messy.

    I am looking for good replacemet ideas that might shade the south west corner of the house without tearing up hardscape.Any ideas?? I live in SunCity Palm Desert

    • Indian rosewood or Sissoo trees (Dahlbergia sissoo) are handsome, fast-growing trees with very fragrant flowers in spring. They well adapted to desert climates though they look tropical and can take some shade. They need regular water, but yes they do have invasive roots and can be messy. Other fast-growing trees that you could substitute might use less water but might have a few of the same problems include Chitalpa, Desert Willow, Mesquite, Chinese elm and Brazilian pepper trees. Many trees that are known to be drought resistant will get into pipes and irrigation systems. If I were in your shoes I would plant palo verde (Cercidium x ‘Desert Museum.) Ask your nursery to order them from Monrovia Nursery. These trees are deciduous, fast growing and largely trouble-free. Once established they need little or no water. However, because this is a good tree for deserts, so many are planted that some people are tired of looking at them. Thus, before making a decision, if I were you I would drive around the area where you live and look at all the trees. If you see something you like, take a photo of the tree and its leaves, also its flowers if you should see it in flowering season. Take the photo to your closest botanic garden for identification. Then research the tree to make sure its characteristics fit your requirements, taking into consideration, size, shade, root growth and other considerations.

  2. Queen palms do have an invasive root system. It lifted up a side yard sidewalk about 8 inches. I finally removed three trees and the root system will grow under sidewalk and expand as far as it can across your yard. I had a whole rented bin full of roots. They also will grow toward water so I have roots I need to pull out of a backyard drain still. I have a picture if proof is needed.

    • Thank you so much for your excellent comment and helpful information. Almost any tree will lift up a sidewalk if planted too close and growing too large for the space allotted to the tree. Hard sub-soil, soil problems and drainage problems combined with shallow watering instead of deep watering can also affect root growth. I am not saying that any of these factors caused the problems you have encountered, only that existing conditions and cultural practices can do so in some cases so other readers can be aware.

  3. Elizabeth Hartshorn

    Hello Pat,

    I planted a small Bougainvillea as well as two Japanese Aralia next to a fence in the back yard about a year ago. I later learned that a sewage pipe passes beneath that area (about one foot away, actually) from those three plants. I am now a bit concerned about their roots affecting the pipe of about 12 inches in diameter. The Aralia are doing well and have grown a couple of feet in height since I planted it, whereas the Bougainvillea has grown just a little bit and flowered only meagerly, making me think it’s not getting enough direct sunlight. I can move any or all of these if they pose a potential problem to the pipe. I would greatly appreciate your thoughts on the matter.

    Thank you.

    Sincerely,

    Elizabeth

    • I would definitely move the bougainvillea or better yet, yank that one out and throw it away. Bougainvilleas do not transplant well, so why struggle? Read up on varieties, choose a great variety in a color you love and plant another in full sun with reflected heat. Japanese aralias like shade and do not have the reputation of being particularly invasive so I would leave then where they are.

  4. We have tons of eucalyptus near our garden beds, so removing trees cannot be done. Our garden beds are filled with small but invasive roots that are preventing our veggies from growing. I don’t want to spend a ton of money but I also want to prevent roots from coming back for at least 5 years or more if possible. Can I add a bottom to my garden beds and drill holes for drainage, and place my beds that now have bottoms on top of weed fabric with gravel on top of the weed fabric or pond liner or something else that’s proven to work? Will this be enough or will some how the roots still get into my beds? I’d like to add bottoms to my existing beds and then lay on whatever would be recommended to stop the roots if possible so I don’t have to build complete new beds. What kind of bottoms for the beds would be best? Will the roots get up through drain holes? PLEASE ADVISE.

    • The only solution is build bottoms on your beds. Make plenty of drainage holes in them and then place the beds on top of concrete blocks so there is a sizable air space under them. I have seen this done. In my own case, I cut down the trees.

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