Before you plant BEWARE! What you plant may be detrimental to the local environment. Did you know that New York State has a prohibited and regulated Invasive plants list, https://www.dec.ny.gov/docs/lands_forests_pdf/isprohibitedplants2.pdf? Prohibited invasive species cannot be knowingly possessed with the intent to sell, import, purchase, transport or introduce. In addition, no person shall sell, import, purchase, transport, introduce or propagate prohibited invasive species. An example of a plant on this list are the swallow-worts (Cynanchum spp.). Swallow-wort is in the milkweed family and monarch butterflies will sometimes lay eggs on it to the detriment of their larvae. Since this plant is unpalatable to deer and livestock it outcompetes native milkweeds and other meadow flowers. Japanese knotweed (Reynoutria japonica) is another undesirable that once established can overwhelm and outcompete native plants. It is so aggressive that it’s nearly impossible to eradicate once established.  In the United Kingdom Japanese knotweed decreases property value and makes it difficult to sell and buy land. It flowers late in the season and attracts lots of pollinators but it quickly out competes other plant in an area creating a monotypic or single species community to the detriment of the pollinators. A healthy and diverse pollinator community is dependent upon a diversity of plants to ensure a continual bloom from early spring to late fall.  

Monarch caterpillar on common milkweed. Swallow-worts can be disruptive to the monarch lifecycle.

Invasive species are non-native plants that when introduced into a new ecosystem overwhelm and outcompete native plants.  In a nutshell, invasive plants reduce biodiversity.  This can have far reaching negative impacts on native bees and other pollinators, especially for specialist pollinators, pollinators that visit one or few plant species. Approximately 30% of the 450 species of bees native to the Mid-Atlantic and Northeastern United States are pollen specialists. These specialists are found in 6 families, 28 genera, and 142 species of specialist bees.  It can be argued that certain non-native plants such as butterfly bush (Buddleja) attract pollinators but take a closer look, who are they attracting? Are they attracting the hundreds of native bees to the region or lots of a few generalist pollinators such as the common eastern bumblebee and non-native honeybees? Over the course of time, native plants have forged a unique partnership with native bees and other pollinators that in their absence can diminish pollinator diversity. Invading invasive plants often outcompete native plants diminishing an ecosystem that once supported a diversity of flowering plants that inevitably supported a diversity of insects. The detriment to butterflies is not that they only need a nectar source but they also need their preferential host plant to support their larvae. Kudzu (Pueraria montana var. lobata) is an invasive vine that has been dubbed the “plant that swallowed the south”. The plant grows quickly and shades out the plants it covers. The diversity of suffocated plants could have benefited pollinators and a diverse food web ranging from insects, birds and mammals. In comparison, an oak trees support over 500 species of native butterflies and kudzu only one, the silver spotted skipper.  The loss caused by invasive plants has a ripple effect leaving behind degraded ecosystems.

A spring mining bee (Andrena spp.) frequent the flowers of Japanese barberry.

A recent study conducted in Acadia National Park paired three invasive species and three native counterparts to learn what impact the presence of invasive plants pose on native plants and their associated pollinators. Japanese barberry was paired with low bush blueberry, alder buckthorn was paired with wild raisin and purple loosestrife was paired with meadowsweet. The study found that native bees visited the invasive counterparts more frequently than the native plants though native fruit set was not significantly affected by their presence. The study also found that there was a greater abundance of native bees in study plots where invasives were present. The data suggests that invasive plants attract more species of pollinators. The study was short term and did not address long term affects to native plants nor if over the course of time the invasive counterparts would eventually overwhelm and replace the native plants. This is only one study that addresses the impact of invasives on our native pollinators. Future research will study impacts on invasive species on pollinator nutrition, disruptions in native pollination, impacts of invasive alien pollinators and other threats from invasive species. Research validates or invalidates observations in the meantime it is important to take action from a land management perspective. The longer we wait to do something the harder the problem will be to control.

Leaf cutter on thistle

Planting native plants is an important step in helping native pollinators but controlling the spread of invasive species is the single most effective means to aid the ecosystem. One reason invasive species are so prolific in urban and suburban areas is due to forest fragmentation.  In highly disturbed habitats such as construction sites, road corridors, parking lots, backyards, abandoned farm fields invasive plants have a competitive edge. Many of these plants prosper in full sunlight and thrive in edge habitat. Intact undisturbed forests hold invasive plants at bay, the forest interior is too dark and the interlocking root system of native plants leaves little opportunity for invasives to get a foothold. Unfortunately invasive insects such as wooly adelgid, hemlock scale, emerald ash borer and gypsy moth caterpillars can cause catastrophic damage to their host trees and when they succumb and die they leave gaps in the forest that allow invasive plants to take hold. In some cases, the invasive plants may have been subsisting at a nominal level waiting for such an opportunity to present itself.

Flowers of the Norway Maple are attractive to native bees but its aggresive nature outcompetes native maples.

What causes invasive plants to grow so aggressively? Non-native plants have no natural enemies in their newfound home. After three hundred years of growing in North America only 5 species eat phragmites compared to their homeland where 170 species consider phragmites food. Phragmites outcompetes native cattails and other native grasses. Almost unanimously, deer will eat native plants long before they will eat a non-native plant.  Hence the allure of non-native plants as ornamentals. Without their natural checks and balances put in place by nature the plants run rampant. Nature adapts over millennia not years or decades leaving the invasive plant populations free to run rampant.

Planting native plants along stream. Once native plants are established there just needs to be monitoring to keep invasives from taking over.

In many cases, the horticulture industry is responsible for the introduction of many invasive plants that were chosen for their attractiveness and resistance to deer browsing. They look nice around a house and are easy to maintain. Since they are not native there are few if any natural predators. On the other hand, native plants are edible to native insects and wildlife and this is not a desirable trait to homeowners and gardeners looking for low maintenance plants. Insects are the ecological bank account that keep ecosystems functioning and the goal of every gardener should be to create a diversity of plants that attract a diversity of insects that attract a diversity of birds and other predators that eat them.  Invasive species destroy this equilibrium leaving the ecosystem poorer because of them. New York State not only banned certain plants for sale but encourages native substitutes to grow in their place. It is in the best interest of every homeowner, landscaper, land manager, gardener and outdoor enthusiast to fight the invasion of invasive plants, the effort is rewarding to see harmony restored between native plants and their pollinators.

Spurge is an invasive of open spaces and meadows. Here it attracts a non-native honey bee.

Sources:

NYS Prohibited Plant List – https://www.dec.ny.gov/docs/lands_forests_pdf/isprohibitedplants2.pdf

Vanbergen, Adam, Espíndola, Anahí, and Aizen,Marcelo A..  January 2018. Risks to Pollinators and Pollination from Invasive Alien Species.  Nature Ecology & Evolution.  Vol 2. 16–25.

Jarrod Fowler and Sam Droege.  Specialist Bees of the Mid-Atlantic and Northeastern United States.  https://jarrodfowler.com/specialist_bees.html 

Stubbs, C. S., F. Drummond, and H. Ginsberg. October 2007. Effects of invasive plant species on pollinator service and reproduction in native plants at Acadia National Park. Technical Report NPS/NER/NRTR–2007/096. National Park Service. Boston, MA.