City of Alexandria
Invasive Species Planting
By Jason Zheng
An electronic interview was
conducted with Marion Jordan, the President of Arlington Regional Master
Naturalist, primarily focusing on erosion control in the City of Alexandria,
Virginia to minimize pollutants on water quality. Ms. Jordan was a
previous contact on a group policy, however I was able to further extend my
communications with her. Though Ms. Jordan provided in depth answers on
planting native vs. invasives species to counter erosion, a few of the primary
questions are included below. Permission was granted by Ms. Jordan to use
this interview for academic uses only.
Jason: How long will these native plants last versus invasives
Every species of plant has a natural life span. For example grasses usually live for one
season while perennials and shrubs live for many years and some trees can live
for hundreds of years. Invasive species
are not different in that respect. I am
not sure where you are trying to go with this question, but it seems that it
might be directed toward finding out if natives or invasives are "longer
lasting". If you are considering
the best treatment for a construction site, the real question is which plants
are best suited for the site, and can provide optimal habitat for wildlife as
well as preservation of native plant species, while serving practical needs
such as erosion control and good visibility for drivers. If native plants are properly selected for
the site they will establish themselves and can thrive for many years with low
maintenance. Even though grasses live
for only a season, they put out many seeds that grow the next year so once a
grassy area has been established it will last indefinitely.
Jason: What are the cost of maintenance for these native plants
This is too broad a question to answer without talking about specific
plants and their impact both at the site under consideration and the surrounding
areas. I will use the example for a meadow, as that is usually best suited for
dry sunny areas such as areas adjacent to highways. If a meadow is properly
established it will need minimal care. That care would include cutting down the
vegetation in the early spring to prevent tree seedlings from becoming
established. It would also include
checking in the early years to make sure that invasives do not creep in. You may question why we would want stop tree
succession which is a natural process.
There are several reasons. One
major one is that in Northern Virginia, open
meadow habitat is very scarce. In a
fully natural system, open areas were created frequently by lightning strikes
and resulting fires. Then over many
years, the open area would fill in and return to forest. Meanwhile other open areas were created and
went through that succession process, so that at any given point in time there
were many small meadow areas available for wildlife that need that type of
habitat. Another reason is that some
areas are dedicated to uses that conflict with large trees, such as power
lines. I helped establish a small meadow in an Arlington County
park last year under a power line for just that reason.
The whole question of what to
plant where is very involved and requires really specialized expertise. It is not just a question of natives vs.
invasives, but of what community of native plants will be best suited for a
particular site. The email that I will forward will illustrate some of these
The cost of having invasives
along highways is usually not considered in the cost-benefit analysis, but it
is significant. Invasives travel along
corridors of disturbed land, such as highway construction sites. As the invasives spread, their impact on
natural areas throughout the region, as well as people's individual yards is
Another maintenance factor is
cost of mowing. If turf grass is used
along highways, that must be mowed. That cost should be included in any cost-benefit
Jason: Since erosion fluctuates over time, how adaptable are these
native plants versus invasive species?
Erosion changes when something is done to the landscape, such as
construction. The natural areas in Northern Virginia feature many very steep slopes. Where those slope still have trees and shrubs
with leaf litter protecting the ground you will not see erosion (except for
streambed undercutting, which is another issue). On the other hand, invasive
plants promoted as helping prevent erosion, such as English ivy, do not do
so. I have seen a ravine open under a
thick mat of English ivy. The best
plants for erosion prevention are native plants suited for that area. Those plants will put down deep roots that
will hold the soil.
If you have been told that
erosion fluctuates over time, that is a very unusual claim, unless they are
referring to man- made impacts.
Native plants are very
resilient as they evolved in this area.
Once established they can withstand drought or other adverse conditions
very well. Unfortunately, some
invasives, once they become established like this area very much and can be
very resilient (especially the notorious ones such as porcelain berry, English
ivy, Multiflora rose, Japanese stilt grass, etc.).
Ms. Jordan also stressed that there is
a significant problem with construction projects removing native plants for
their own purposes, and then failing to replant—instead choosing to re-plant
with non-native species, which exacerbates problems already caused by VDOT.