Escaping Ornamentals: A Threat to Natural Area Biodiversity

Essay by: Miriam Owsley, Outreach Assistant, Northwest Michigan Invasive Species Network

Northern Michigan boasts some of the most beautiful natural landscapes in the country, where we explore and recreate and where native plants play a vital role in providing habitat for the wide range of insects, birds, and animals. Despite some conservationists’ best efforts, some invasive terrestrial plant species have the potential to overrun the natural areas that characterize the region’s beauty if left unchecked.

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Invasive Phragmites invades wetlands, impedes wildlife movement and degrades habitat. (Photo Credit: Great Lakes Phragmites Collaborative)

Invasive plants arrive in Northern Michigan via a number of vectors, bringing with them potentially devastating effects on the environment, infrastructure, human health, and the economy. Some are accidental hitchhikers that take advantage of the free ride on ships, vehicles, and equipment, such as invasive Phragmites. Others are imported for the well-intentioned use for soil erosion prevention, like the autumn olive shrubs that were distributed by conservation organizations  before the plant’s invasive tendencies were known. Many invasives spread faster than natives because they tend to have higher seed dispersal or reproduction rates, such as purple loosestrife and garlic mustard. But perhaps the most deceiving are those that are introduced as ornamental plants, which escape the garden environment and quickly take over the natural landscape. These ornamental invasives tend to be the most difficult to convince people not to plant because they are attractive and easy to grow, and may hold sentimental value to some gardeners.

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Purple loosestrife, a wetland invader, is pretty, but troubling. (Photo Credit: Saffron Blaze)

Beloved by many gardeners as a quick-growing and attractive ground cover, Vinca minor, aka periwinkle or myrtle, is commonly found in natural areas adjacent to landscaping features. Vinca poses a risk to woodland understories, as it overwinters and shades out spring ephemerals, (think trillium, Dutchman’s breeches, and trout lilies), which provide some of the first pollen resources for dwindling pollinator populations. Similarly, dame’s rocket, a member of the mustard family, competes with native plants for light, moisture, and nutrients, which may inhibit tree seedling germination and growth. Dame’s rocket is commonly included in woodland seed packets, spreads quickly, and is easily mistaken for native phlox.

Some invasive ornamental plants are more trouble than others. Of the more heinous offenders, Japanese knotweed registers high on the list. Japanese knotweed was introduced as an ornamental plant from east Asia, and is enjoyed for its clusters of creamy white flowers, unique bamboo-like stalks and as a quick growing privacy buffer between neighbors. The trouble with knotweed lies in its aggressive tendencies – knotweed has adapted in its native environment to be one of the first plants to repopulate areas after a volcanic event. In other words, sidewalks, roadways, and building foundations are no match for this exotic invader, costing property owners, municipalities and taxpayers in expensive repairs as the shoots push their way through anything in its path to sunlight. Japanese knotweed is legally prohibited in Michigan. It is illegal to possess, introduce or move this species without a permit from the DNR. Tall clonal shrubs, like red osier dogwood and American hazelnut are ideal replacements for this formidable invader.

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Japanese knotweed costs property owners in expensive repairs and remediation. It’s best to contact a professional to treat this species. (Photo Credit: O’Donovan AGRI)
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Japanese barberry quickly takes over natural areas. Infestations have been linked to an increase in ticks that carry Lyme disease. (Photo Credit: Leslie J Mehrhoff, University of Connecticut, Bugwood.org)

Just as insidious, Japanese barberry has the potential to wreak havoc in natural areas after it escapes the confines of landscaping installations, where is it commonly planted by those who appreciate its long lasting foliage and plentiful attractive red berries. Like many non-native shrubs, white-tailed deer won’t browse on it because of the sharp thorns, but birds aid in the spread by dispersing the seeds, allowing Japanese barberry to form dense thickets, thereby shading out native plants and severely impeding wildlife movement. According to the Michigan DNR, barberry has been linked to an increase in ticks that carry Lyme disease. The shrub is host to deer mice, the larval host, and also provides habitat for adult deer ticks. In areas of barberry infestations, populations of adult Lyme disease carrying deer ticks increase, posing a threat to human health. An excellent native alternative for barberry is ninebark, with interesting foliage, white flowers in summer and clusters of nutlets in fall.

Mounting evidence suggests that the effects of climate change, including warmer temperatures, longer growing seasons, and milder winters, are aiding non-native species in their invasions. Due to warming temperatures, some ornamental plants introduced from warmer regions no longer require overwintering indoors for survival. Of concern in Northern Michigan is the arrival of water hyacinth and water lettuce, two aquatic invasives introduced into the wild after escaping aquascapes and water gardens. These two plants are already an issue in southern Michigan, where images of waterways clogged by huge swaths of green have been captured on Google Earth. Water lettuce and water hyacinth populations have struggled to survive the freezing Michigan winters, until recently, which can be attributed to generally shorter, more mild winters, allowing these plants to survive from one year to the next.

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Water hyacinth takes over in a slow moving waterway after escaping from a residential water garden. (Photo Credit: Kim Starr, Bugwood.org)

Despite the varying degrees of destruction caused by these ornamental invasive plants, there is something they have in common. Phragmites, garlic mustard, autumn olive, Vinca, dame’s rocket, Japanese knotweed, Japanese barberry and many others are listed on the Northwest Michigan Invasive Species Network’s Top 20, Early Detection, and Ornamental Invasive lists, which can be found on ISN’s website, HabitatMatters.org.

The Northwest Michigan Invasive Species Network (ISN) is a collaboration of over 40 partners in Benzie, Grand Traverse, Leelanau,  and Manistee counties working to manage the invasive terrestrial plant populations that threaten Michigan’s natural communities. ISN was formed in 2005, and has been cooperating with partner organizations, municipalities and private property owners to remove and treat invasive species since. ISN’s Go Beyond Beauty program serves Northwest Michigan by educating garden industry professionals, municipalities and gardeners about the harmful potential of ornamental invasive species.go_beyond_beauty_logo

In an effort to slow the spread of invasive ornamental plant species, the Northwest Michigan Invasive Species Network administers the Go Beyond Beauty program, which exists to provide positive recognition to Garden Professionals who exemplify business practices that benefit and protect our region’s natural areas. Retail and wholesale nurseries, landscapers, garden clubs, lake and neighborhood associations, municipalities, and concerned individuals who buy, sell, distribute or install any kind of landscaping or gardens are invited to participate in this voluntary program. Participants of Go Beyond Beauty have committed not to sell high-priority invasive plants, including those listed on ISN’s Top 20 and early detection species lists.  The program offers support to participants in removing high-priority invasive plants from trade in a business-positive manner, provides science-based information to staff, and helps businesses gain new customers through free publicity and outreach. Participants also receive additional recognition for further steps they may choose to take, such as selling native plants, participating in Buy-Back and Trade-Up programs, or organizing community workshops.

For more information on Go Beyond Beauty, visit ISN’s website, HabitatMatters.org, or contact Emily Cook, Outreach Specialist, ecook@gtcd.org.

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