Native, adapted, introduced, and invasive plants.

Previous

What is a “Native” plant?


On the surface this seems like a simple question but in reality it is more complicated than you might expect. 

Amsonia tabernaemontana

Amsonia tabernaemontana is native to the lower 48 United States and is naturally found in Pennsylvania. We would therefore consider it a PA native.


In general “native is used to refer to plants that are endemic or indigenous to a region or ecosystem. Webster’s dictionary defines indigenous as”existing, growing, or produced naturally in a region or country..” 

In contrast endemic is defined as “Native to a particular country, nation or region ... restricted to and constantly present in a particular country or locality...”. 

So a plant may be indigenous to more than one area. 

Endemic on the other hand is used to describe a plant that is exclusively native to a specific place. 

Note also that the definitions describe the plant as being naturally found or naturally introduced to the native habitat. This does not necessarily mean that the plant species originated in that place, just that it naturally got there sometime in the past. 

So the first widely accepted requirement for a plant to be native is that it got there without people moving it. But this is interesting because ancient peoples moved plants around long before any records were kept. 

Amsonia hubrichtii

Amsonia hubrichtii is also native to the lower 48 United States but its natural distribution is probably restricted to Oklahoma and Arkansas. We might then include it in a native plants garden in Pennsylvania if we were restricting our list to plants native to the US but might not include it if we were restricting our planting list to Pennsylvania natives.

If a plant was introduced by humans to a place 100,000 years ago and has since become naturalized and is an integral part of the local plant community is it native? Can we even distinguish between a plant that was moved by our stone age ancestors and one that arrived in the same area 100,000 years ago by natural processes?  

Time in a region thus enters into the formula of what is generally considered native and you have to define how long a plant needs to have been in a place to become “native” and the uncertainty of how the introduction occurred also must be considered. 

Although is is not always clearly stated most native plant definitions for North American native plants use the arrival of Europeans and their introductions as the starting point for what is native or rather non-native.

Introduced and naturalized plants.

Plants that have been brought to a region by man are said to be introduced. 

Most introductions are deliberate as in the introduction of Salvia officinalis which was introduced as a culinary herb but is now found in much of North America. 

Plant introductions can also be accidental. 

When an introduced plant becomes established in the natural landscape with self perpetuating (reproducing) populations they are said to have become naturalized. 

A naturalized introduced population that has been present in a region for a long time may eventually be considered native - especially if the method of introduction is not clear. 

Many of our landscape plants are introductions but are related to local species. For example there are many Galium species that are native to North America, but the one we use most frequently in the landscape is introduced. G. odoratum is introduced, it is however present in the flora of Pennsylvania and thus is naturalized.

Invasive plants.

Invasive plants are defined as introduced species that can thrive and spread in areas where they are not indigenous. 

They frequently have very aggressive growth habits and dispersal mechanisms and high reproductive capacity. They have the ability to spread rapidly and displace other species. They frequently have a lack of natural enemies or predators in the new environment.

Beyond the common definition there are legal definitions for invasive species. 

Some common landscape plants are invasive species and can not be used or sold in some parts of the country. 

Plants may be invasive in some parts of the country but not in others because of climate or other limiting factors. For example many plants that are invasive species in Florida are not problems in areas the have consistent freezing, frost or snow.

Executive Order 13112

On Feb 3, 1999, Executive Order 13112 was signed establishing the National Invasive Species Council. The Executive Order requires that a Council of Departments dealing with invasive species be created. Currently there are 13 Departments and Agencies on the Council.

Executive Order 13112 of February 3, 1999 - Invasive Species (PDF | 67 KB)
Federal Register: Feb 8, 1999 (Volume 64, Number 25)

Miscanthus sinensis is an example of a commonly used introduced ornamental that is potentially invasive. It is for example listed as potentially invasive, but not banned in Connecticut. In State College, Pa. some cultivars are more invasive than others. The difference lies in how long the cultivar takes to produce viable seed. Some Miscanthus cultivars have a long enough growth to seed maturity period that the growing season in State College is usually too short to produce viable seed.

DSC_0113

Solidago canadensis is a native plant that can be weedy or invasive. Just because a plant is native doesn’t mean it won’t be invasive or noxious.

A plant can be native and still be a noxious weed or an invasive species.

Solidago canadensis is native and is listed as a weedy or invasive species. 

Geranium carolinianum (Carolina cransebill) is another native plant that can be invasive or weedy in many places, but has botanical variants actually listed as endangered or threatened i.e G. carilinianum var. sphaerospermum which is listed as Threatened in New York.

Some states have extensive noxious weed lists while others contain only a few plants.

Pennsylvania for example has only a few listed noxious weeds on the USDA website. 

Massachusetts in contrast has a large number of state listed noxious weeds including a few common landscape species like Acer platanoides, Euonymus alatus and a groundcover we use a lot of on the University Park Campus Lysimachia nummularia

Iris psuedocorus is an introduced wetland iris that can be invasive and has been banned or prohibited in a number of states Including Connecticut, Massachusetts and New Hampshire and is on the state noxious weed list for 46 states (Not in Pennsylvania)

Landscape use of Adapted vs. Native plant species

There are a number of common arguments for the greater use of native plant species in the landscape and many have some validity. 

It is for example frequently argued that planting natives helps preserve species diversity and existing habitat. This is clearly true and commercial production and landscape planting of commercially produced endangered or threatened species in appropriate landscape settings is to be encouraged. 

Natives are argued to be better adapted to the region but from a Horticultural, or landscape use, point of view whether or not a plant is native, endemic, indigenous, or naturalized is less important than whether the plant is adapted to, or for, the location and use you intend. 

Many introduced species are very useful in the landscape as are many native species.

In the managed landscape it is arguably more important that the plant fit the site and needs of the project than that it be native to that location. 

DSC_0882

Plants on a green roof on the Penn State University Park Campus. Most of the species used on this roof are not natives but are well adapted to life on a roof in central Pennsylvania.

It is probably more useful to think of plants with regard to the physical and environmental conditions of the project site and to pick species that are well adapted to thrive in those conditions. 

It is also important to consider the social aspects and aesthetic sensibilities that relate to a managed landscape. 

Many urban or suburban environments are totally un-natural and selecting only native plants makes very little sense. For example there is nothing natural about the rooftop environment and it is quite unlike most natural environments found in Pennsylvania, so the use of Pennsylvania woodland native plant species on a green roof probably makes very little sense. It makes far more sense to use species that are adapted to the full sun, shallow soil profile and droughty conditions that exist on the roof. While this arguement probably holds for most green roofs in Pennsylvania, there are some native habitats like the Scotia barrens near State College where soil and environmental conditions are more similar to those on a roof. Some of the species found in these habitats might in fact be good choices for a green roof on campus.

It is of course undesirable and irresponsible to use potentially invasive species without proper control practices, but some potentially invasive species can be very useful in the landscape if used responsibly. An example would be the Bamboo allay in the PSU Arboretum. This is a very attractive and desirable feature in this garden and the location and bed preparation make the likelihood of this planting becoming invasive very small indeed.

Exercise: Complete this exercise using the Canvas Assignment located in the Week 4 module.

Using only plants from this weeks plant list:

Which species of Amsonia is native to Pa?

What other species on our list are native to the lower 48 states?

Which genera on our plant list are listed as both introduced and native depending on species

Identify one plant that is potentially weedy or invasive.



© Penn State Horticulture  2019