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Invasive Plants

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Invasive Plants

If you drive down the road, take a walk or horseback ride on Willistown’s local trails, or go fishing in your favorite stream, there is a good chance that you will see many invasive plant species such as multiflora rose, oriental bittersweet, or Japanese honeysuckle. 

Before going any further, however, there are several terms we should define.  A native plant is one that has grown in a particular geographic area for long periods of time.  A non-native (alien, exotic, or introduced) plant is one that occurs outside of its natural range and was brought to a new area by people either accidentally or on purpose.  For example, a non-native plant in Pennsylvania may have originally come from Asia, but also could have come from the West Coast of the United States.  Non-native plants have many uses in our lives such as food sources, medicines, and gardening.  However, when a non-native plant exhibits certain characteristics, it’s considered invasive.  

Found both on land and in water, invasive plants threaten our native flora and fauna because they grow quickly and aggressively.  When invasive species take over large areas of habitat, ecological processes are changed.  Their phenomenal growth allows them to out-compete native plants by shading them out and producing huge numbers of seeds.  For example, the Norway maple’s dense canopy withholds light from shorter plants.  The dense, shallow roots of this European native make it difficult for native forest-floor plants to find space, and consume more than their share of water, weakening neighboring trees.  Also, the Norway maple’s huge numbers of wind-blown seeds thrive almost anywhere.   Once these plant species are well established, it is sometimes impossible to remove them.  Invasive species degrade many of the habitats that other species, such as birds and butterflies, depend upon for reproductive success.  Therefore, it is important that landowners recognize that what they plant and what they allow to grow on their property can have far-reaching impacts.

Multiflora rose is a common invasive plant that will take over open fields in the Willistown area.

Why are some plants invasive? 

A plant is invasive in an area if it grows rapidly and is difficult to remove or control once established.  Many of these immigrants leave behind the pests and predators that had held their numbers in check in their native habitat.  Furthermore, some plants develop into invasive species when they hybridize with relatives they meet in their new environment. 

Characteristics of invasive plants that allow them to alter natural environments are the following: 

•   Plants physically take over the space around them, preventing native plants from growing in the same area.

•   Plants change their immediate environment by altering the amount of sunlight, water, and nutrients that are available to other plants and organisms.
•   On a larger scale, invasive plants can actually decrease the variety and number of native plants that are available to wildlife and other organisms, for food, shelter, and breeding or nesting sites.

Warning Signs 

1.  Bird-dispersed berries pose the greatest risk of spreading non-native plants and gardeners should recognize that the production of attractive non-native berries could be a red flag for potential long-range dispersal by birds.
2.  High seed production and/or germination rate, or the ability to spread rapidly by their rhizomes or root systems are common features of invasive plants.  A single Chinese “Tree of Heaven” can spawn 350,000 seeds per year, most of which can germinate and establish taproots within 3 months!

Managing Invasive Species

The list on the following page and the color photographs provided in Appendix A will help you identify those plants that are most invasive and cause damage to native plant communities.  Early detection can prevent these exotics from becoming established. 

What can you do?

Preventing plants from becoming established is the most effective, economical, and ecologically sound approach to managing invasives. However, if you already have an infestation, more than one tool may be necessary to control your problem.  Here are some basic guidelines to follow:
•   Understand a plant’s identity and habits so non-toxic, preventative measures can be used first.  If you are unsure of the identification of a plant, take a sample to a university, arboretum, department of agriculture office, local nature center, or native plant society for assistance. Find out if it is known or thought to have invasive tendencies. If the exotic plant is closely related to an invasive species, it is likely to have similar tendencies. To be on the safe side, if you don’t know it, don’t grow it.
•   Where feasible, pull plants up by hand in the fall or spring before flowering and fruiting to prevent the production and dispersal of seeds.  If the plants are pulled while in fruit, the fruits should be bagged and disposed of.   If hand-pulling is not practical, remove them entirely by cutting, mowing, or applying an herbicide to prevent flowering and seed dispersal.  These measures will also help reduce their spread outside your property. 
•   Chemical control in combination with manual and mechanical methods is effective and may be necessary for large infestations.  The systemic herbicides triclopyr (e.g., Garlon 3A and Garlon 4) and glyphosate (e.g., Roundup and Rodeo) can be used successfully to control many invasive plants.
•   Do not purchase or use invasive species in your landscaping.  Instead use plants that are native to your local region as much as possible or use those that are known to not be invasive.
•   Avoid disturbance to natural areas, including clearing of native vegetation and dumping of yard wastes.                        

From Your Land, Your Legacy, the Willistown Conservation Trust Landowner's Management Guide. Download the complete management guide.