Alternatives to Invasive Landscape Plants

Many of the most familiar plants in our gardens, fields, and even our forests are not native to New Hampshire . Over the last 400 years or so, immigrants have brought plants from their countries of origin, importing such specimens as Japanese barberry for hedges, purple loosestrife for color in the garden, and Norway maple trees for shade. Conservation organizations have encouraged the planting of autumn olive for erosion control and wildlife food. The nursery trade continues to import exotic species, promoting new varieties and colors for gardening and landscaping. The vast majority of plant imports that are common in our landscape do not cause problems, but a small percentage of them (such as the examples above) have proven to be such good competitors in their new environments that they have become recognized as invasive plants.

 

According to the definition accepted by the New England Wild Flower Society, in order to qualify as an invasive plant, a species must be non-native. So staghorn sumac, an aggressive colonizer of disturbed sites, would not qualify since it is native to our state. An invasive plant must also be able to spread into an intact ecosystem and cause economic or environmental harm by developing self-sustaining populations and becoming dominant or disruptive to those systems. So the lawn grass in your backyard, though non-native and dominant, would not qualify either since it is a desired plant and is not self-sustaining (rather, it requires a lot of maintenance!). In addition, in the agricultural world, many so-called “weeds” qualify as invasive plants, since most are non-native and all cause economic harm to the farmer whose fields or gardens they have infested. For example, spotted knapweed, a problem invasive plant in western rangelands, has reduced livestock and elk forage up to 90% in some areas.

 

Invasive plants share several characteristics that help them thrive and dominate in new environments. Most invasive plants produce a lot of seed—a single purple loosestrife plant can produce 2 million viable seeds per year, have aggressive root systems, colonize in disturbed areas, and are habitat generalists. Imagine the vines of oriental bittersweet in the autumn, with their thick clusters of bright orange berries, overrunning disturbed areas such as hedgerows or old fields, and sending out roots that sucker and sprout shoots when the top of the plant is cut.

 

So why should we care if plants are invasive or not? One might argue that this pattern of one plant out-competing another plant is the natural course of things. How can one plant be “better” than another? The answer to this question lies in the high value we place on native plants, endangered species, and the biodiversity of New Hampshire .

 

When an invasive plant comes to a natural area (dispersed by a bird or the wind, or planted by a gardener), native plants suffer. The diversity of the site usually plummets as the invader eventually dominates the area. Cattail marshes invaded by purple loosestrife do not support the same diversity of wildlife and fish species. The vines of oriental bittersweet encircle trees and kill them like a boa constrictor. Invasive plants also disrupt natural succession, as they form self-sustaining populations that alter the landscape indefinitely. For example, dense stands of common buckthorn shrubs can dominate the understory of forests, preventing the establishment of the next generation of tree seedlings, while new, buckthorn seedlings replace those older plants that die off. Forty-two percent of all species on the Federal Endangered Species List are listed in part due to invasive species (and for 18%, invasive species are the sole reason for their listing), and invasive species are destroying public natural areas at an estimated rate of 4,600 acres per day. Invasive species also impact forestry and farming to the tune of $37 billion per year, jeopardizing seedling regeneration and damaging crops. Finally, invasive species may be partly responsible for the present decline in migratory songbird populations.

 

In 2004, the Invasive Species Committee finalized a list of 18 plant species to be immediately prohibited from sale, transport, distribution, propagation or transplantation in New Hampshire . These species are in addition to 14 species already prohibited since 1998. The following three species (including all their cultivars and varieties) will also be prohibited as of January 1, 2007: Euonymus alatus (Burning bush), Berberis thunbergii (Japanese barberry), and Acer platanoides (Norway maple). Existing stocks of these species are allowed to be sold until that date. For information on the laws and criteria regarding invasive species in New Hampshire , please see the State’s webpage.

 

Euonymus alatus, Berberis thunbergii, and Acer platanoides are ornamental plants currently prominent in the regional landscape plant palette, and their loss will potentially have a large economic impact on nursery and landscape businesses. The lists of potential alternatives for these three species, presented here, have been developed with input from nursery owners and growers, landscape architects and designers, landscape contractors, arborists, and concerned citizens. While recognizing that no one plant can directly substitute for all the functions and aesthetic qualities of the invasive plant of concern, the lists provide suggestions suitable for a range of site conditions and plant functions. The plants listed are known to be adaptable to New Hampshire conditions (within the appropriate hardiness zones) and are currently available or can be brought into production in sufficient quantities to meet future demand.

 

Alternatives for Euonymus alatus, Burning Bush: American cranberrybush viburnum, Highbush blueberry, Redvein enkianthus, Fothergilla and Red chokeberry.

 

Alternatives for Berberis thunbergii (Japanese barberry ): Weigela, Slender deutzia, Common ninebark and Shrub roses.

 

Alternatives for Acer platanoides (Norway maple) Red maple, Northen red oak, Gingko, Japanese tree lilac, European beech, Flowering crabapple (Malus species), plum and cherry (Prunus species).

 

For more information about these alternative plants please call the UNH Cooperative Extension's Family, Home & Garden Education Center 's Info-Line toll-free at 1-877-3984769 for "Practical Solutions to Everyday Questions." Trained volunteers are available to answer your questions Monday through Friday from 9 am to 2 pm .

 

By: Catherine Neal, Extension Specialist and Malin Ely Clyde, Coverts Project Coordinator University of New Hampshire Cooperative Extension, Hillsborough County