Invasive Tree Root List: Trees That Have Invasive Root Systems
By: Jackie Carroll
Did you know that the average tree has as much mass below ground as it has above ground? Most of the mass of a tree’s root system is in the top 18-24 inches (46-61 cm.) of soil. The roots spread at least as far as the most distant tips of the branches, and invasive tree roots often spread much farther. Invasive tree roots can be very destructive. Let’s learn more about common trees that have invasive root systems and planting precautions for invasive trees.
Problems with Invasive Tree Roots
Trees that have invasive root systems invade pipes because they contain the three essential elements to sustain life: air, moisture and nutrients.
Several factors can cause a pipe to develop a crack or small leak. The most common is the natural shifting and movement of soil as it shrinks during droughts and swells when rehydrated. Once a pipe develops a leak, the roots seek out the source and grow into the pipe.
Roots that damage pavement are also seeking moisture. Water becomes trapped in areas beneath sidewalks, paved areas and foundations because it can’t evaporate. Trees with shallow root systems can create enough pressure to crack or raise the pavement.
Common Trees with Invasive Roots
This invasive tree root list includes some of the worst offenders:
- Hybrid Poplars (Populus sp.) – Hybrid poplar trees are bred for fast growth. They are valuable as a quick source of pulpwood, energy and lumber, but they don’t make good landscape trees. They have shallow, invasive roots and seldom live more than 15 years in the landscape.
- Willows (Salix sp.) – The worst members of the willow tree family include the weeping, corkscrew and Austree willows. These moisture-loving trees have very aggressive roots that invade sewer and septic lines and irrigation ditches. They also have shallow roots that lift sidewalks, foundations and other paved surfaces and make lawn maintenance difficult.
- American Elm (Ulmus americana) – The moisture-loving roots of American elms often invade sewer lines and drain pipes.
- Silver Maple (Acer saccharinum) – Silver maples have shallow roots that become exposed above the surface of the soil. Keep them well away from foundations, driveways and sidewalks. You should also be aware that it is very difficult to grow any plants, including grass, under a silver maple.
Planting Precautions for Invasive Trees
Before you plant a tree, find out about the nature of its root system. You should never plant a tree closer than 10 feet (3 m.) from the foundation of a home, and trees with invasive roots may need a distance 25 to 50 feet (7.5 to 15 m.) of space. Slow-growing trees generally have less destructive roots than those that grow quickly.
Keep trees with spreading, water-hungry roots 20 to 30 feet (6 to 9 m.) from water and sewer lines. Plant trees at least 10 feet (3 m.) from driveways, sidewalks and patios. If the tree is known to have spreading surface roots, allow at least 20 feet (6 m.).
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Preventing invasive roots in raised garden beds
Every year I remove and clean out the raised garden beds and add to the soil. This is the second year using the raised beds. I've noticed that when I mix in new soil and hit the soil, mostly clay, that the beds are sitting on there are tons off roots coming from the clay and spreading out within the raised bed soil. Is there a fabric of some kind that I can lay between the clay and the soil to prevent the invasive roots from getting into the bed? Also is there an alternative way such as placing stone, or some other type of organic base to prevent the roots?
Invasive Mulberry Tree - Gardening Advice
Q: My 15-year-old mulberry tree provides wonderful summer shade, but it’s growing up into the power lines, and my neighbor says its roots are damaging his underground sprinkler system. Can I prune the roots to keep them out of his yard? Should I replace the tree? - Sandra Todd, Los Gatos, Calif.
A: Garden diplomacy between neighbors is always delicate. And in this case, responsibility for solving the problem rests with both sides. Your mulberry’s roots didn’t cause the problem - at least, not at the very start. It’s physically impossible for roots to break into pipes or to drill through them to get at water, but they will follow leaking water to its source, and then squeeze through even the tiniest existing hole or fissure. Somewhere, somehow, your neighbor’s sprinkler system must have sprung a leak, luring mulberry roots next door for a drink. Don’t bother trying to cut back the roots or digging a trench for a concrete or plastic root barrier. Wherever the water is, those roots will find it, even diving under barriers as deep as 4 feet. First, the sprinkler pipes should be fixed - they are wasting water.
The sad news for you is that the mulberry tree probably ought to go. Many people plant mulberries because they grow quickly they can also quickly grow too tall (30 to 50 feet in only 10 to 20 years). I’d replace your mulberry with a smaller tree, which will not spread its roots as far and won’t need a periodic buzz cut from the power lineman, either. Since the canopy of your replacement tree will cover less ground, plant it a little closer to the spot where you need summertime shade. Following are some appealing alternative trees. All drop their leaves in winter, like the ulberry. They will take many years to reach a height of 25 feet and can be maintained at that height or lower with moderate pruning.
For exotic bloom you can’t beat a chitalpa, a cross between the eastern catalpa and a desert cousin, chilopsis. This hybrid tree flowers over a long period in early summer, producing abundant orchidlike blossoms. Crab apples thrive in your region, and many varieties make a wonderful small but spreading tree to sit under. Ask a local nursery for a crab apple with fruit that persists into the winter. You probably think of crape myrtle as a large shrub, but it’s easy to train into a small, multistemmed tree. The best are modern varieties developed for disease-resistant foliage: ‘Sioux,’ ‘Comanche’, and others named for Native American tribes. Flowers range from purple through a wide range of reds to white. In the winter, crape myrtles display lovely peeling bark. Japanese persimmon trees are most glorious in the fall and early winter, when the leaves turn golden and the branches bear avocado-size fruit the color of pumpkins.
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Controlling unwanted trees and shrubs
Some commonly seen invasive woody plants in Minnesota include:
- Amur maple
- Norway maple
- Non-native bush honeysuckles
- Multi-flora rose
- Black locust
- Japanese barberry
- Russian olive
- Siberian elm
- Siberian pea shrub
Native woody plants can also be aggressive growers and spread into areas they aren’t wanted. An example of this is encroachment of box elder and cottonwoods into lands planted as grasslands.
Native plants that grow rapidly include poison ivy, blackberry, raspberry, sumac, hazel, prickly ash, dogwood, wild grape, Virginia creeper and poplar species like aspen and cottonwoods.
Controlling woody plants
Non-chemical (mechanical) control
Non-chemical treatment options may vary with species and age of the woody plant.
Mechanical control methods
- Pulling. Removing the root crown by pulling is effective. Success depends on the species. Hand pulling tools are available.
- Frequent mowing or burning. Repeated mowing may kill woody broadleaf plants. Burning can be effective on some woody plants.
- Burning every 3 years in a grass CRP or native grass/forb planting is recommended. Larger stems will not be controlled with fire.
- Cut and cover stumps. Covering stumps with plastic or material that excludes light for two years can reduce sprouting.
- This can be effective on species that stump sprout. Not effective on species that sucker or spread from lateral roots.
- Grazing. Grazing livestock, particularly goats, in a concentrated area of plants will reduce foliage and stems in the area. Herbicide treatments may be necessary.
- Note: Livestock will also eat desirable trees and shrubs.
Equipment for controlling undesirable woody plants varies from inexpensive hand tools to large power equipment.
Select tools according to your budget, size of plants, number of plants, size of the site being managed and who will be doing the work.
While mowing or cutting can be used as a management strategy, most deciduous trees and shrubs will re-sprout after the trunk is cut. Some woody plants like aspen can resprout from underground lateral roots several feet from the main plant. These species must be controlled chemically for complete control.
Chemical control methods
- Cut stump treatment. Cut the plant near the ground and treat the cut surface (cambium or bark ring) immediately with a labeled herbicide.
- Remove any berries or reproductive cut stems by chipping, burning, or hauling to a brush dump.
- Note: Transporting cut stems may spread seeds or reproductive parts. Non-reproductive stems can be left in the woodland to help prevent erosion.
- It is illegal to transport certain invasive species. Check the Minnesota Department of Agriculture for specific information on getting rid of them.
- Low volume basal spray. Wet the bark area from ground level up to 12 to 15 inches. The bark should be wet but not dripping.
- Leaf application. May be used on seedlings, small plants and resprouting plants. Spray leaves to wet around the entire tree. Avoid spraying non-target plants.
Always read and follow label directions, wear recommended protective clothing and avoid contact with non-target plants. The label directions will list plants controlled, areas where the herbicide can be used and application methods.
Two common active ingredients found in brush killer herbicides are glyphosate and triclopyr.
- Glyphosate is a non-selective herbicide which can injure any green plant.
- Triclopyr is a selective herbicide active on broadleaf plants. Triclopyr has two formulations: amine (water base) and ester (oil base).
Before you purchase a brush herbicide, read the label to verify that the product is labeled for your site and will control the plants you want to eliminate.
Even after treatment with an herbicide, re-sprouting and seedling emergence may continue for years. Monitor sites for re-growth annually and retreat accordingly.
How Invasive Are Fig Tree Roots?
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Fig tree roots generally are very invasive, although much depends on the tree’s variety, its planting location, and the overall soil quality. Most fig trees, if they are planted in optimal conditions, spread their roots far and wide, which can make then troubling if they are planted in the center of a landscape design. These trees usually do best on the outskirts of a yard or surrounded by plenty of open space. Gardeners can often take steps to minimize root invasion by planting trees in pots or building underground retaining walls to keep the roots structured.
Much of a fig tree’s invasiveness depends on its variety. All fig trees are members of the ficus family, which is marked by shallow, fast-spreading roots. Still, smaller trees such as the celeste or Malta fig trees typically keep their root systems more or less contained, and larger trees such as the brown turkey, magnolia, or Florida strangler fig trees have more of a tendency to dominate a space. These roots often trample or choke out other plants, and they can damage sidewalks, driveways and other objects in their paths.
The roots of a fig tree typically are woody and dense, and they generally grow near the surface, if not above it. They do best in warm, consistently dry climates such as their native Mediterranean and Middle Eastern habitats. Colder weather and thinner soil often impede the growth of roots. A fig tree will produce fruit in almost any condition, but it might not grow to its maximum size — or spread the roots that it needs to support its size — if its environment is less than ideal.
Gardeners who hope to plant fig trees in residential spaces often look for ways of naturally curbing root growth without depriving the plant of nutrients. In most cases, this involves confining the fig tree roots to a small space before moving the plant outdoors. Fig trees, like most ficus plants, do well as indoor potted plants. Beginning growth indoors can help the roots grow in a contained way, and it prohibits premature spreading. The roots will, of course, expand after the plant is moved outdoors, but usually not as dramatically.
Another option is planting young fig trees in prepared plots with built-in root restraints such as underground brick walls or planting liners. These restraints allow the roots to begin growing naturally but set up impediments to slow them down and keep them contained. Trees are usually able to grow all of the roots they need to support production with this kind of restraint, but the roots are kept close to the trunk. This sometimes also forces the plant to burrow down instead of out, which is far less invasive.
Regularly pruning fig trees, particularly when they are young, is another favorite way of containing root invasiveness. Pruning generally works only for gardeners who want small trees, however. Routine pruning is a part of caring for fig trees, because it helps the plants focus energy on fruit production and keeps the branches strong. To limit root growth, however, the trees must be aggressively pruned, which essentially stunts their growth.
There is nothing particularly wrong with invasive fig tree roots, except that they can disrupt other garden and landscape elements. People who are interested in planting fig trees on their property would be wise to carefully research the options before making a purchase. A bit of planning can save a lot of trouble.
Top 12 invasive trees found in gardens
Did you know that there are 379 NEMBA-listed invasive plants in South Africa? Invasive trees are a serious problem as they very often consume much more water than indigenous trees. They are also land transformers, transforming fynbos and grassland into dense stands of invasives, ultimately effecting and impacting negatively on biodiversity. In savanna and forests, they compete directly with indigenous trees and shrubs. All category 1a and 1b listed trees must be removed. A total of 559 invasive species are governed by the National Environmental Management: Biodiversity Act, 2004 (Act no. 10 of 2004). The Alien and Invasive Species Regulations (AIS) for this legislation became law on 1 October, 2014. A further 560 species are listed as prohibited from entering the country. Here are some Category 1a and 1b listed invasive trees and shrubs commonly found in gardens and highlighting their negative impact to the environment. All plants that are listed under these two categories must be removed.
Top 12 invasive trees which must be removed:
These are invasive species which must be combatted and where possible, eradicated. Any form of trade or planting is strictly prohibited.
These are established invasive species which must be controlled and wherever possible, removed and destroyed. Any form of trade or planting is strictly prohibited and landowners are obligated to control Category 1b plants and animals on their properties. A species management plan should be drafted for large properties.
1. Bugweed (Solanum mauritianum) – Category 1b
This tree replaces indigenous riverine and forest margin species. It also competes with young trees in plantations, particularly pines and black wattle, inhibiting growth and causing stem deformation. It is a host of the KwaZulu-Natal fruit fly which is an economic pest. The unripe fruits are poisonous and the hairy leaves and stems can cause allergic dermatitis and asthma.
2. Port Jackson’s willow (Acacia saligna) Category 1b
This invasive tree is potentially the most damaging invasive species in the coastal lowlands of the south-western Cape. It competes with and replaces indigenous species.
3. Tree of hell (Ailanthus altissima) – Category 1b
Its “rapid growth and aggressive root system” posed a threat to urban infrastructure. It also produced a chemical known as ailanthone that prevented the growth of other plant species.
It is highly invasive and leaches chemicals into the soil which hinder indigenous plant growth. It threatens the fynbos, savanna and grasslands biomes and spreads rapidly through seed dispersal. It also suckers profusely.
4. Australian albizia (Paraserianthes lophantha) – Category 1b
Australian albizia is a huge invader of the Western and Eastern Cape Provinces. It competes with and replaces indigenous species. Dense stands along watercourses are likely to reduce stream flow.
5. Strawberry guava (Psidium cattleianum) – Category 1b
Strawberry guava can form dense thickets that exclude indigenous vegetation and reduce species regeneration. It is considered to be the worst invasive plant species in several islands in the Indian Ocean.
6. Black locust (Robinia pseudoacacia) – Category 1b
This invasive tree is widespread. Dense stands are formed mainly by suckering from the roots and can cover vast areas stands along watercourses restrict access to water by domestic and wild animals. It is also poisonous to humans and domestic livestock.
7. Yellow bells (Tecoma stans) – Category 1b
This plant competes with and has the potential to replace indigenous species. It can invade hot and dry savanna where it may reduce grazing for domestic and wild animals.
8. Chinese tamarisk (Tamarix chinensis) – Category 1b
This tree competes with and replaces indigenous species. Dense stands could significantly reduce stream flow and groundwater reserves.
9. Ant tree (Triplaris americana) – Category 1a
The ant tree is a problem in subtropical KwaZulu-Natal. This is an unwanted tree and it must be removed from the garden. It can outcompete and replace indigenous trees. The leaves are also poisonous.
10. Pearl acacia (Acacia podalyriifolia) – Category 1b
In South Africa, where it is relatively widespread, pearl acacia competes with and has the potential to replace indigenous species and is therefore regarded as a ‘potential transformer’ of natural vegetation.
11. Brazilian pepper tree (Schinus terebinthifolius) – Category 1b KwaZulu-Natal, Mpumalanga, Limpopo and Eastern Cape, 3 in rest of South Africa
This problematic invader has the potential to replace indigenous species. It is poisonous and irritant. Indigenous birds could neglect the dispersal of indigenous plants as a consequence of their preference for the fruits of this alien species.
12 Jerusalem thorn (Parkinsonia aculeata) – Category 1b
This is a spiny shrub or small tree native to tropical America and a popular ornamental. It has however, become a problematic invader and Jerusalem thorn is noted as being a potential transformer species.