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Plant Answer Line Question

Keywords: Tree roots, Conifers

Can you suggest any larger growing conifers (ex. Lawson's cypress) whose root systems are not invasive? The area I'm interested in planting is near water lines.


There are a number of conifers listed on the locally developed web pages of Great Plant Picks.

I would suggest looking at some of these, and then checking the web page of SelecTree, where you can select trees for low root damage potential.

For instance, if you are interested in planting a fir tree such as Abies grandis or Abies pinsapo, you would find out from SelecTree's full tree record that both of these have moderate root damage potential. Calocedrus, Picea orientalis, Sequoiadendron and Cryptomeria are also rated as moderate. Cephalotaxus fortunei is rated low, as are Pseudotsuga menziesii, and several Chamaecyparis species. The following conifers rated as having high root damage potential:




Pinus nigra caramanica CRIMEAN PINE


Date 2019-07-17
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Plant Answer Line Question

Keywords: Tree roots, Ficus carica

I have a fig tree cut down to just above the ground and want to know how to stop it from growing, i.e. putting up suckers. I plan to pave over that area and am afraid, because of its vigorous growth, that it will find a way up and out.


I recommend renting a stump grinder, or hiring a tree service to grind the stump and the larger roots. You can water the area well to soften the soil, and try digging up the remaining roots. You could try applying full-strength vinegar to kill any shoots, or you can wait and continue to cut them off as soon as they emerge. Over time, this should weaken any part of the tree that remains in the ground, and it will eventually die. You could also cover the area with black plastic once the stump and major roots have been removed. This should suppress any growth coming up from what is left of the roots. There are also chemical treatments which should only be used with extreme caution, and exactly according to directions on the product.

If you look at page 4 of this document on controlling invasive species from the Hawaiian Ecosystems at Risk project, you will see the different approaches used to remove fig trees and their roots from a natural area (in Central California), including physical and chemical methods.

Date 2020-10-17
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Plant Answer Line Question

Keywords: Tree roots, Landscaping drain fields

I have several sewer pipes that are getting plugged by tree roots on my grounds. I have used a rooter to remove the majority of the roots and know would like to detour their return by using a chemical called Root Free. It is a Copper Sulfate Pentahydrate. Is this product safe for my trees if used according to label?


Here is a link to information on this chemical from the Pesticide Action Network's database. This link leads to the Material Safety Data Sheet for Copper sulfate pentahydrate.

This product is highly toxic to humans and aquatic life, but should not harm the trees. My question would be whether it makes more sense to remove entirely any trees with invasive roots, and replant with other plants whose roots will not cause trouble with the sewer pipes, rather than use copper sulfate. See information below from UGA College of Family and Consumer Sciences Cooperative Extension Service:

Tree roots can enter sewage and drainfield lines and cause plugging of the lines. Lines should not be placed near trees, and trees should not be planted near lines. Remove tree roots mechanically or flush copper sulfate crystals down the toilet to help discourage or destroy the roots where the solution comes in contact with them. Some time must elapse before the roots are killed and broken off. Recommended dosage rates are two pounds per 300 gallons of tank capacity. No more than two applications per year are recommended. Time the application of copper sulfate to allow minimum dilution and maximum contact time. Copper sulfate will corrode chrome, iron and brass, so avoid contact with these materials. Used in recommended dosage, copper sulfate will not interfere with septic tank operation. Neither mechanical removal nor copper sulfate contact is a permanent solution for tree roots. Remove the trees for a permanent solution to the problem.

Here are some links to more information tree roots and sewer lines and about planting on drain fields:
Tree Roots vs. Sewer Lines from the city of Paso Robles, CA.
Choosing Sewer Safer Trees
Planting on Your Septic Drain Field

And here are some suggestions for alternative plantings:
Landscaping Your Drainfield

If you do decide to go ahead and use the Root Free, by all means follow the directions to the letter, as it is required by law. You may want to check with Seattle Public Utilities Drainage and Sewer Maintenance to make sure that use of this chemical in the sewer system is permitted: 206-386-1800

Date 2019-06-12
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Plant Answer Line Question

Keywords: Acer circinatum, Tree roots

I moved into a house built in 2001 on a cement slab (slab height = 18"). Around 1 foot from my foundation is planted a Maple tree. I have been told that it is a Vine Maple. It is around 15 feet tall. My neighbor told me to pull out the tree because the roots will crack the foundation of my house. I don't want to get rid of the tree unless this is true. I went to a nursery today, and they said that it is very unlikely that the tree will damage the foundation (unless the foundation is already cracked and the roots make these cracks worse). What do you think? I have no idea if the foundation has cracks that the tree could exacerbate or if a Vine Maple in general would crack a foundation like this.


I do think that planting anything one foot or less from the house is not ideal, especially a tree. However, vine maple roots have a low potential to cause damage, according to the database of the Urban Forest Ecosystems Institute (see full tree record for Acer circinatum).

On the question of how close to a house a tree can be planted, I found the following from New Mexico State University Extension site, in answer to a question like yours about root damage potential:

"A more important consideration is keeping the branches from rubbing against the house and damaging the stucco, siding, or paint and shingles. By planting the plant a distance greater than the expected mature crown radius from the house, you will avoid damage to the house by branches. You will still benefit from shade if the tree is properly positioned.

"Many trees are planted so that their branches are trimmed to be higher than the roof and then grow over the roof. Remember, if one of these large branches breaks in a wind storm, it can damage the roof, so distance from the house is the best protection from such damage. Learn how widely the branches spread from the trunk when the tree is mature and plant at least that distance from the house. Yes, you can break this rule-of-thumb, but the hazards increase when you do."

You may want to consult a certified arborist to evaluate the situation, and see if you can keep the tree where it is. To find a certified arborist, contact Plant Amnesty or the Pacific Northwest Chapter of the International Society of Arboriculture.

Date 2019-05-03
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Plant Answer Line Question

Keywords: Magnolia grandiflora, Tree roots

I recently built a 2-foot tall boulder retaining wall in my front yard. I have a small landscape bed along the top of the wall. I want to plant a small evergreen tree in the landscape bed to provide privacy from a busy intersection at the corner of my property. I'm pretty much settled on a Magnolia grandiflora 'Little Gem.' Do you think the roots of this tree will interfere with/knock-down my boulder wall, if the tree is installed in a location where the trunk of the tree is approximately 2 feet behind the wall?


Two feet from the edge does not sound like enough space to me. Although it grows relatively slowly, Magnolia grandiflora 'Little Gem' will reach at least 20 feet, and dislikes root disturbance. From the tree's point of view, the boulders might be a problem. Below is a link to general information about the tree, from Urban Forest Ecosystems Institute.

The following, from University of Florida Extension, describes the root system of this tree:

"The root system is wider spreading than most other trees, extending from the trunk a distance equal to about four times the canopy width. This makes it very difficult to save existing Magnolia trees on construction sites."

It is possible your tree might coexist peacefully with the retaining wall, but my recommendation would be to plant it as far away from the wall as you can, and leave the bed at the edge for perennials and small shrubs. Here is general information on the needs of tree roots, from Brooklyn Botanic Garden's Tree Care Primer by Christopher Roddick (2007):
"Out of view, and usually out of mind, roots make up almost a third of a tree's mass. Trees need a serious amount of underground real estate. Unfettered by subterranean obstacles, their root zones easily spread far beyond the tree's dripline, the perimeter of the tree's branches. If roots are curtailed by obstacles that inhibit their spread, the amount of water, nutrients, and oxygen to which they have access will be limited."

Date 2019-08-07
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Plant Answer Line Question

Keywords: Tree roots, Magnolia

Does a magnolia tree root system cause cracking on the house foundation if planted too close, especially on clay soil?


What kind of magnolia are you growing? Is it deciduous or evergreen? Is it large species or one of the smaller varieties?

According to SelecTree, a website of the Urban Forest Ecosystems Institute, most Magnolias have a low root damage potential. A few, such as Magnolia delavayi, are rated as moderate. I am not an arborist, so I cannot conclusively tell you that a Magnolia will not be problematic for the house foundation. However, it is always a good idea to plant trees far enough away from structures so that you do not have to do a lot of pruning to keep them from conflicting with windows, entryways, the roofline, etc. From what I have read, most tree roots that harm foundations are not themselves causing the cracks, but are exploiting preexisting weak points in the foundation. Roots do expand in size as the tree matures, and they do seek out water to some extent, depending on the type of tree.

Below are some links to information that may be useful to you.
North Dakota University Extension
"How close to the home should trees be planted?
Large trees such as ash, hackberries, maples, lindens and oaks should be kept at least 20' from the foundation. Medium sized trees such as buckeyes, honeylocust and little leaf linden - 15-20 feet away and small trees such as flowering crabs, mountain ash and Canada red cherry - 10 feet from the foundation.
Do tree roots actually crack the foundation?
No, the wetting and drying of the clay soil causes the initial cracks. After these have developed, tree roots will grow into the cracks for moisture."

A personal essay on tree roots and foundations, from Renegade Gardener,a garden blogger

International Association of Home Inspectors
"Tree Roots and Foundations
Contrary to popular belief, InterNACHI has found that tree roots cannot normally pierce through a building's foundation. They can, however, damage a foundation in the following ways:

  • Roots can sometimes penetrate a building's foundation through pre-existing cracks.
  • Large root systems that extend beneath a house can cause foundation uplift.
  • Roots can leech water from the soil beneath foundations, causing the structures to settle and sink unevenly."

Date 2019-10-31
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Plant Answer Line Question

Keywords: Tree roots, Lawns and turfgrasses

I have two mature cherry trees on my parking strip that are about 20 feet tall. Their roots protrude above the sidewalk level and are quite prominent. There used to be grass planted above them but it wasn't well maintained and I took it out several years ago. After trying different plantings, I'd like to put sod back on the strip and wondered if it was best to cover the roots fully with soil and lay the sod on top of that, or if it would be okay to let the roots be partly exposed and lay the sod around them.


I am not sure that it would be wise to reinstall sod over your trees' roots. You may want to mulch them lightly (no more than 2-3 inches, and keep away from the trunk), as described by a document formerly available from North Carolina State University Extension:
"Exposed surface roots can become unsightly or in the way. Roots do not suddenly grow on the soil surface. Roots increase in diameter over a period of years. Soil erosion can speed their exposure. Exposed roots need protection from pedestrian and vehicle traffic including lawn mowers. Mulching exposed roots physically protects them as well as conserves soil moisture and prevents direct sunlight from heating the roots. Cutting off or covering roots with top soil are temporary solutions that can cause long term damage to tree roots."

There is excellent, clear information in this link to the International Society of Arboriculture's page on trees and turf. Ultimately, the best thing for your trees and your grass is to create a grass-free zone under the trees' dripline, mulch that area, and restrict lawn grass to the area beyond the dripline. Here is an excerpt from the link:

"Trees, shrubs, ground covers, and lawn grasses all require sunlight, water, and rooting space for growth. Each plant in the landscape competes with neighboring plants regardless of type or species. Some even produce chemicals that are exuded from roots to restrict growth of nearby plants. For each plant to do well, it must have adequate space. Because perennial woody plants increase in size each year, they require additional space over time. The landscape design should provide adequate space for these plants to mature.

"While shade is the biggest, most obvious problem trees create for turf growth, a tree's roots also contribute to poor turf performance. Contrary to general thinking, most tree roots are in the top 2 feet of soil. More important, the majority of fine, water absorbing roots are in the top 6 inches of soil. Grass roots ordinarily occupy a much greater percentage of the soil volume than tree roots and outcompete them for water and nutrients, especially around young trees. However, grass root density is often much lower in areas where trees were established first. In these situations, tree roots compete much better for water and nutrients and prevent or reduce the success of establishing new turf.

"Competition is especially important when transplanting, seeding, or sodding. The newest plant in the area must be given special treatment and must receive adequate water, nutrients, and sunlight, which frequently means that competing sod should be removed from around transplanted trees and shrubs or that some of the lower branches should be removed from existing trees above a newly sodded lawn. In any case, do not do any tilling around trees.

"Mulching is an alternative to turf around trees, and its use eliminates potential competition. A 2- to 4-inch layer of wood chips, bark, or other organic material over the soil under the drip line is recommended because it

  • helps retain soil moisture
  • helps reduce weeds and controls grass
  • increases soil fertility when mulch decomposes
  • improves appearance
  • protects the trunk from injuries caused by mowing equipment and trimmers that often result in serious tree damage or death
  • improves soil structure (better aeration, temperature, and moisture conditions)

"Maintenance practices for trees and turf are different. Because tree and grass roots exist together in the upper 6 to 8 inches of the topsoil, treatment of one may damage the other. Fertilizer applied to one plant will also be absorbed by the roots of a nearby plant. Normally that is good, but excessive fertilization of either trees or turf can result in tree crown or grass blade growth greater than desired."

Date 2019-12-27
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Plant Answer Line Question

Keywords: Cryptomeria, Tree roots

I have a question about Cryptomeria japonica 'Black Dragon.' I would like to know the approximate diameter and depth of the root system at full growth. I am trying to convince the local cemetery to permit me to plant one there.


According to Richard Bitner's Conifers for Gardens (Timber Press, 2007) Cryptomeria japonica 'Black Dragon' typically grows to about 6 feet tall. As a dwarf form of C. japonica, I would expect its roots not to be much of a problem. The (non-dwarf) species can grow to 160 feet, in which case, roots would extend a considerable distance. The local website, Great Plant Picks, features the variety 'Black Dragon' and says it grows to about 7'H x 8'W.

Here is general information on trees and their root systems, from Tree Roots in the Built Environment (Roberts et al., Centre for Ecology & Hydrology, 2006):

About root depth:
"A further misconception about tree roots is that they occur typically in significant quantities at substantial depths (i.e. greater than 3 m.) in the soil profile. [...] from numerous studies involving comprehensive root excavations the indication is that typically as much as 90% of the tree root length occurs in the upper metre of soil." According to this same source, conifers usually have about 70% of their roots in the upper 50 cm. of soil."

About root extent:
"[...] large species differences exist but it is also the case that the horizontal extent of tree roots substantially exceeds the perimeter 'dripline' of the crown. [...] there is a good relationship between crown spread and root radius but the relationship tends to be very species specific. Roots extending furthest from the tree trunk are usually found in the soil surface. [...] the maximum extent ot the tree roots is reached before the canopy has completed expanding, suggesting that the ratio of root spread to crown spread may decrease as trees become older."

This same source also says the root extent is highly dependent on soil environment (richness of soil, access to water and nutrients).

To summarize, what all this means is that the width of your tree's crown will give you only some idea of the extent of the roots, and most of those roots will be shallow.

Date 2019-05-22
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Plant Answer Line Question

Keywords: Plant nutrients, Tree roots, Salmon

Does salmon DNA end up in trees?

I saw a show about decomposing salmon being great for riparian ecosystems and that DNA from salmon could be found in the trees. Well, I shared this information with kids and a smart teacher fact- checked it with a scientist who said, no way! Nitrogen and carbon can be found, but not DNA.

It would certainly be cool if there were salmon DNA in trees, but is this true?


It seems obvious that decomposing salmon left by bears around forest trees would leave detectable traces of salmon DNA. Perhaps, along with the nutrients that are being added to the soil and taken up by the trees, there might be a detectable amount of salmon DNA in the trees, too. Certainly there is salmon DNA in salmon carcasses, the carcasses provide nutrients taken up by the tree, but does a test of the tree show traces of salmon DNA? Let's see what scientists in the field have to say. T.E. Reimchen is in charge of the lab at University of Victoria and is in charge of the Salmon Forest project there.
Here is a brief excerpt from their research:
"Conifer trees adjacent to salmon rivers on the west coast of North America incorporate marine-derived nitrogen from the carcasses of salmon carried into the forest by bears and other scavengers. We demonstrated (Reimchen et al. 2003) that small samples of wood (30 mg) extracted from cores of ancient trees contain detectable levels of 15N [nitrogen]. Comparisons among watersheds differing in number of salmon show that 15N levels in wood of trees are directly proportional to the present numbers of salmon entering the streams."
When asked directly about the DNA question, Reimchen said, "I presume that the nitrogen that the tree is sequestering has come from the breakdown of the nucleic acids in the salmon. Have not heard about the incorporation of salmon DNA or RNA into the roots."

Ray Hilborn, Professor of Aquatic and Fishery Sciences at the University of Washington, said, "I don't know anything about the DNA -- I doubt that very much. However what you do find in trees near salmon rivers is what are called 'marine derived nutrients,' that is, nitrogen isotopes that the salmon brought in their bodies back to freshwater."

It certainly sounds catchy to say there's salmon DNA in the trees, but it is probably just shorthand for a more complex concept.

Date 2019-09-27
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