Caring for and Healing the Earth

Alien Plants



Please do your part in controlling the spread of invasive alien plants.
Don't import invasive alien plants into your area.
This is an important part of caretaking, in that we can help to prevent a problem before it occurs.
If they are already there, please take the opportunity to use proper caretaking methods to help reduce, replace and eliminate them.



How to determine whether a plant is a "native", "alien" or "invasive alien"

What is a "native" plant?
A native plant is one that has evolved in an area over time, naturally, without recent human intervention.  What is "recent" human intervention?  Well, that can be debated, but it is generally regarded as being within historic times ("historic" meaning within the time period of written history).

What is an "alien" or "introduced" plant?
An alien/introduced plant (these terms are interchangeable) is a plant that has been imported into an area as the result of human activity or intervention, either intentionally or accidentally.

What is an "invasive alien" plant?
    An invasive alien plant is one that tends to take over an area, ecosystem, or habitat, and tends to exclude and crowd out, "outcompete" other plants.  It tends to lower the biodiversity of the area that it is growing in.   In some cases, native plants become completely obliterated from an area, and all that is left are the invasive alien species.
    Not all alien plants are invasive.
    Naturally, this can have a significant impact on the local ecology, which has evolved (or was created, depending on your point of view) with all of the plants, birds, animals, and so on, in complete harmony with each other.   Each functions as part of the whole, in balance.  When an invasive alien plant is introduced into an area, there may not be any natural control on it.  For example, none of the local animals or insects may find it edible.  So the plant can grow unabated, provided the local climate is accommodating.  Even worse, the plant may be one that tends to crowd out others.  Thus, over time, sometimes a very short time indeed, the alien plant becomes the dominant plant form of the area, and native species are excluded, crowded out.
    A plant may be alien to an area, yet not be invasive for some time, perhaps even as long as 100 years.  Then some factor could change in the local environment or ecosystem which enables the plant to then take on invasive characteristics.   For example, a disease that kills off other plant species enable the alien to spread.\


How does one determine if a plant is an alien (invasive or otherwise) or a native plant?

Human values
    First of all, human values often cloud this issue.  Plants which are deemed to be "nice" or "useful" to humans will not as likely be the target of alien plant crusaders and laws.
    Beauty:  Many varieties of grass that make up lawns are not native to the area in which they are grown.  They are alien varieties of grass, but are not regarded as "bad" because people like lawns.   Similarly, people often don't want to regard their beautiful Norway Maple trees as "aliens".  On the converse side of this, "ugly" plants, or those lacking pretty flowers, are regarded as "useless" and are more likely to be the target of removal, whether they are native or not.
    Herbal uses:  Mullein is an alien plant, but herbalists extol its healing properties, and wouldn't dream of pulling it up wherever it grows.
  "Weeds":  Many plants are deemed to be "weeds", both in common practice as well as in law.  Yet many "weeds" are native plants.  Plants are usually classified as "weeds" in relation to their perceived negative influence on human food crops.
    Names:  Names of plants can fool us too.   The well-known "Canada Thistle", which is common in parts of Canada, is not native to Canada at all, but was introduced from Europe.

Natural migration of plants
    Another thing to consider is that in the normal and natural course of things, plants move around, even without any interference from humans.  Plants adapt gradually to new climates and environments, which can cause them to gradually migrate into new climate zones.  For example, a plant may gradually, over many years, adapt to a cooler climate and thus extend its range northward.  Or vice versa, adapt to warmer climate and move southward.  For the same reason plants may migrate up or down in elevation.
    Again, in the normal and natural course of things, birds and animals spread plant seeds as they move about on the landscape, or during migrations.
    Wind and water also do their share in the normal and natural dispersal of plant seeds.

How do we determine if a given plant is a native or an alien?
    Ok, so you find a plant somewhere and you want to determine whether it is a native plant (perhaps newly discovered) or an alien plant that has been artificially introduced.  Perhaps you want to know because you want to do your part in stopping the spread of introduced plants.  How do you go about figuring this out?  The methods listed below are ways that botanists use to try to decide if a given plant is a native or an alien.  You can also use these pointers yourself if reference materials are not present.
    Obviously, each of the following signs or types of evidence does not stand in isolation.  Many of these factors need to be looked at together.
    Fossil evidence:  If the plant is present in fossils from hundreds or thousands of years ago, depending on the context, then it can be considered a native to the area.
    Historical evidence:  Review of historical records will help to determine whether the plant was present in the historical past in a given area.  However, the plant may have been introduced not too long prior to the historical record in question.
    Where it grows:  If a plant tends to grow mainly around areas of human settlement, that would point towards it being an introduced plant.   For example, its range might be centered around a major seaport, or certain urban areas, or generally around areas of human activity.
    Geographical distribution:  If the plant grows only in isolated patches (geographically speaking), such as 100 square miles in southern Ontario, and 50 square miles in Pennsylvania, then this suggests an introduced plant.   However, these patches may of course be the natural areas that a native plant grows in.  Or, it may be the last remaining areas that an endangered native plant is growing.
    Known naturalizations:  If a particular plant is known to have been introduced elsewhere, then that increases the probability that it will have been introduced into the area in question.  In other words, this is something that has been done before somewhere else.
    Genetic diversity:  If the genetic diversity of the plant is low, that is, if there are very few varieties of it in a particular area, then it is likely to be an introduced species.  The reason for this is that plants will evolve over time, but gradually, branching out into different varieties to suit various local habitats and conditions.  This takes a long time, however. Therefore, if there are very few varieties or only one variety present, the plant has likely not been in the area for very long, and has therefore been introduced.
    Reproductive pattern:  If a plant has specialized reproductive strategies that rely on specific local insects and animals and climate, then it is less likely to be an introduced plant.  Again, these specific strategies take considerable time to evolve.  If, for example,  it uses a more general strategy for its seed dispersal such as fruit that appeals to a wide variety of animals, or the wind, or just plain lets its seeds fall to the ground, then it is more likely to be an introduced plant, since these are more generalized seed dispersal methods.
    Mode of introduction:  Have there been any possible modes of introduction of the particular plant in the area?  For a plant to be introduced, there has to have been a means to get it to the area.  Has there been the particular type of human activities and involvement in the area that could result in a plant introduction?  For example, let's say that there is a plant that usually grows alongside wheat.  One would expect that such a plant could accidentally be introduced into another country along with wheat or wheat seed.  Therefore, we would expect to find this introduced plant growing in the new country alongside wheat, since that is how it arrived.  If it is instead found in the "new" country in an area far away from any wheat growing areas, and not near wheat at all, then it is more likely to be native to the area.
    Predators:  Many plants have predators that eat, or are dependent on, specifically one type of plant.  If these specialist predators are present, the plant is more likely to be native.  Again, the reason is that it takes time for these predators to evolve together with the plant to form the interdependency that exists.  If instead the plant is preyed upon by "generalist" predators (predators that eat a wider variety of plants) then the plant is more likely to be introduced.
    In general, we see that the more specialized a plant is, the more likely it is to be a native plant.  This is because it takes a considerable amount of time for plants to evolve specialized ways.  Similarly, if the plant is the object of specializations in other plants, insects, or animals, then it is more likely to be a native plant.  The same is true of climate, soil type, ecosystem type, and so on.  The more specialized and dependent the plant is on these factors, the more likely it is to be native to the area.

Common characteristics of invasive alien plants:

Generally, these plants have the following characteristics.  These are attributes that enable them to grow and reproduce quickly.

  • They produce a lot of seeds, and the seeds are viable for long periods of time
  • They produce seed early in the year, ahead of other plants.  This gives them extra time to get established in the same season.
  • They grow quickly
  • They flourish in a variety of habitats
  • They can reproduce without seeds, using vegetative reproductive methods such as by means of runners (rhizomes).
  • They may not require the use of a specific pollinator
  • They fix their own nitrogen
  • They are good at not being eaten (ie, they do not appeal to animals or insects).   This is accomplished by chemical (disagreeable taste, poison) or mechanical (thorns) means.


Habitat factors

Some habitats are good at "defending" themselves.  In other words, the habitat does not lend itself to alien plants coming in and invading.  What are the factors that contribute to this?

  • Gaps or clearings vs. dense growth.  For example river and stream shorelines often have very dense growth.  This makes them less susceptible to invasion.
  • Shaded woodlands help to inhibit invasions due to lack of sunlight.
  • Degree of bio-diversity.  The more interactions there are within an ecosystem or plant community the better it can "defend" itself.  In other words, there are more varieties of plants to inhabit each niche within the ecosystem.  For example, in a given area there may be 4 different creeping ground cover type of plants. If an alien ground cover plant was able to usurp one of the native ground covers, then it is more likely that the remaining 3 native types would be able to resist the alien.   There's more variety and thus more different types of defenses against alien plants.
  • Climate.  If the climate is "tough" then there are fewer plants that can survive in it.  For example, the arctic and deserts.  Tropical areas are more susceptible to invasion due to the more hospitable climate (warmer, more moisture).   Of course, such a climate is inhospitable to desert plants.
  • Degree of disturbance of the soil, or ground surface.  For example, farmed areas are more likely to suffer from invasions of alien plants.  When the natural ground is broken up it provides more of an entry for seeds.  Native plants are adapted to the type of ground surface in which they grow.  For example, if the ground surface is naturally rather hard in an area, non-adapted plant seeds would have a harder time germinating.  Break the ground surface up and a much wider variety of seeds will then be able to germinate in that area.
  • Fires alter the soil and remove vegetative overgrowth, giving other species a chance to take hold.  Fires also tend to alter the soil balance towards low nitrogen and high phosphorous.  (Of course, native plants colonize burned areas as well).
  • Farming practices. These have a major influence, in many ways. See section on influence of farming below.

Farming practices:
Farming has wide-reaching effects on an ecosystem.  Factors which encourage invasion of alien species are:

  • Soil disturbance.  See section on Habitats.
  • Domestic animals may disturb the soil in ways that allow alien seeds to germinate.
  • Alteration of soil chemistry by use of fertilizers, pesticides and herbicides, and domestic animal scat.

Even after farming has ceased in an area, the effects will be seen.  One of the less obvious ones is that domestic animals will no longer be present to browse the alien species, thus allowing them to grow to maturity and take over.



Timing Factors

A plant may be alien to an area, yet not be invasive for some time, perhaps even as long as 100 years.  Then some factor could change in the local environment or ecosystem which enables the plant to then take on invasive characteristics.  For example, a disease that kills off other plant species enable the alien to spread.
    Generally, a Timing factor comes into play when initial conditions are not optimal for the growth of the alien plant.  Then at some point those conditions change and the alien plant becomes invasive.
    Here are some of the factors that influence the timing of an alien plant becoming invasive:

  • Micro-climate change, perhaps due to human intervention in the ecosystem such as the removal of a forest (will result in more sunlight and warmer temperatures, and quicker runoff of precipitation), encroachment of a city (may result in warmer temperatures), or a stream or pond drying up (results in lower humidity and warmer temperatures)
  • The introduction of other plants to the ecosystem.  These may encourage the growth of the original alien plant, or the two alien plant species may cross-pollinate with each other, resulting in a third plant more able to invade.  As well, the introduced plant may crowd out existing native vegetation that has until that point inhibited the growth of the first alien plant.
  • Predator removal, either by natural migration or human influences.  There may have been a native animal that ate the alien plant, keeping it under control.  Its removal then allows the alien plant to spread unchecked.
  • Introduction of a pollinator species.  The alien plant may not be doing very well due to a lack of natural pollinators (a certain fly or bee, for example).  When this pollinator is introduced (again, either through natural spreading or human introduction) the plant is then able to produce much more seed and thus spread more readily.
  • Introduction of a seed-spreading species.  Again, the introduction of a new animal or bird species to the area may encourage the spread of seeds of the alien plant.


How do they get here?

The usual way that alien plants gain a foothold is through their seeds.   These are introduced into an area either deliberately or by accident.

  • Human methods:
    • Gardens are the single most important source of alien species.  People usually deliberately import plants that they like for their gardens.  From there they "escape" in to the wild.  The most invasive alien plants of all that plague natural ecosystems are from this source.
    • Farmers, through the use of crop seeds, which occasionally naturalize in the surrounding wild areas.  However, most crop plants do not naturalize very readily.  More frequently it is the accidental introduction of other seeds that are inadvertently mixed in with crop seeds that escape and invade an area.
    • Deliberate introduction of plants for other reasons.  In an example unrelated to plants, there was a club in the USA around the turn of the century whose goal was to introduce every bird known in Europe into North America.  Not because these species were needed in North America, but just because it would be "nice".
    • Other "accidental" ways, such as ship ballasts (in another non-plant example, famous for introduction of zebra mussels into the Great Lakes), airplanes, motor vehicles (mud), mud on shoes, and so on.
  • Natural methods:  These usually are not considered the initial cause of an alien plant being initially introduced into an area.  However, these factors facilitate the spread of the alien plants in the new area.
    • Water: seeds are carried along by stream and river currents
    • Wind: lightweight seeds are blown on the wind
    • Birds will eat in one area, fly to another area and defecate there, leaving behind seeds.
    • Other animals, especially seed gatherers such as squirrels.


What problems do they cause?

Depending on the alien plant's characteristics, there can be many negative effects from the introduction of an invasive alien plant into an ecosystem.

  • Ecosystem issues:
    • A general reduction of biodiversity as the invasive plant takes over and crowds out native species.  This has implications on down the food chain.  Animals or insects that used to eat the native plant decline, and then so do the predators that eat those animals or insects, and so on.
    • Rivers and streams can get clogged up with an invasive water plant that has no natural predators in the area.  Marshes can dry up.
    • Impact on fish populations, again from crowding out of native species, changes in the stream or lake habitat.  For example, a small lake may get grown over by an alien plant, shading the lake bottom and thus changing the ecosystem of the lake bottom.
    • Nutrient levels can change in the ecosystem (depending on the effects of the alien species)
    • Rare plants are often impacted the most.  This is what gets the press.
    • Sometimes an increase in erosion.
  • Human issues:
    • Loss of crop productivity


What can be done about them?

    Fortunately, there are many things that we can do to prevent the introduction of invasive alien plants in the first place, and to control and eliminate them after they are already in an area.
    On a more spiritual note....Please remember, that it is not the plant's "fault" that it is an invasive alien.  All plants have a place where they belong, that they are native to.  Due to non-natural events (human driven) a plant species is now growing in an area in which it does not naturally belong.   Please undertake any plant removal with a caretaker attitude, which includes respect for the life that you are taking.

A. Prevent the problem in the first place:

  • Gardening practices:
    • Create your gardens with native plants.  Do not introduce alien plants into a natural landscape.  If you must use alien plants, do so only under tightly controlled circumstances.  Choose alien plants that will not spread.
  • Farming and vegetable gardening practices:
    • Clean seeds before use to ensure that only the intended crop seeds will be sown.
    • Purchase seeds from a seed grower that sells clean seed.
    • Cycle or rotate crops, to help reduce the effects of monoculture which encourage alien plants
    • Use tilling methods that discourage growth of non-crop plants
    • Utilize companion planting to discourage alien plants
  • Generally:
    • Disturb the natural ground surface and the ecosystem in general as little as possible.   Any kind of disturbance of the natural ecosystem is the key to alien plants colonizing an area.

B.  Manage & eliminate the problem:
Please note that some of the methods listed here are controversial and are not necessarily recommended by Earth Caretaker.  This is just a list.  Decide for yourself what is appropriate for each particular situation that you find.

  • Pull up the invasive plants one by one on an ongoing basis.  This works best for plants that don't have deep roots or long lived seeds.  Sometimes it is the only ecologically friendly alternative that will work.  This method is only effective if followed over a period of years, in order to remove new plants each year that grow from the seeds of the alien plant.
  • Cut or mow an area to keep the alien plant species down.  This approximates the effect of fire (a vital part of many natural ecosystems to which native plants are well adapted), but only somewhat.
  • Allow domestic livestock to graze the area.  For more information on this, please see the Farming section of Earth Caretaker.
  • Farmers usually use herbicides to eliminate non-crop plants from their fields. Of course, herbicide use has other implications.
  • Please don't use pesticides such as Roundup.  The use of this product is controversial in environmental circles.   Because Roundup is said to rapidly degrade into naturally occurring components, many environmentalists applaud its use.  Many others dispute this claim.
Introduction of predators:
    Sometimes efforts are made to introduce some sort of natural predator into the area where the invasive plant is doing its damage.  The predator can be an animal, insect, bacteria or virus.  This is usually only done on a large scale with government funding because it is a very tricky and expensive process, as well as being a very high risk and time-consuming task.
    All too often, the introduction of a predator to control one species unleashes a host of other problems.  This poses a very high risk to the ecosystem in general.  At first glance, bringing in the plant's natural predator sounds like a good logical approach.  However, introducing a second alien species to control the first can easily compound the problem!!

    Generally, the steps that must be followed are:

  • go to where the invasive plant naturally occurs (its native land)
  • find a specific predator there, that eats only that specific plant
  • test the predator to see what else it eats
  • import the predator and test it under controlled laboratory conditions
  • determine what impact the predator will have on other plants and organisms, as well as the ecosystem as a whole
  • test the predator in the open air
  • breed the predator in large numbers
  • release the predator
  • monitor it

    As can be seen, this requires considerable expenditure of time and money.  Once the predator is released, it must be monitored continually to ensure that it is doing what it was intended to do, and on the other hand, that it is not impacting the rest of the ecosystem.



Much of the information on this page was summarized from m notes taken at an excellent session given at the 1999 Federation of Ontario Naturalists conference in Kingston, Ontario, by Adele Crowder of Queen's University.


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