Invasive Plants

Invasive plants grow on many of our private and public lands. If left unchecked, they will continue to degrade our woods, meadows, wetlands, rivers, and yards. Non-native invasive plants are those introduced to a new place by humans—either intentionally or accidentally—or by other means, and cause harm to our environment, economy, or human health. For example, invasive garlic mustard and glossy buckthorn shade native plants and suppress tree regeneration in forest understories, patches of Japanese barberry harbor more lyme disease-carrying ticks, and Japanese knotweed takes over roadside edges. Invasive plants have several qualities that enable their aggressiveness: they lack natural predators, produce lots of seeds, and are fast growing, allowing them to out-compete native plants for space, moisture, and nutrients.

The State of New Hampshire passed the Invasive Species Act, which makes it illegal to “collect, transport, import, export, move, buy, sell, propagate, or transplant any living or viable portion of any plant species, which includes all of their cultivars and varieties, listed on the New Hampshire prohibited invasives species list.” Currently there are 35 invasive plants on this list, including garlic mustard, barberry, buckthorn, burning bush, and Japanese knotweed. No one should be buying or selling any of the plants on this list.

Removing invasive plants from conservation areas, shorelines, roadways, and backyards will take patience and persistence, and is worth the effort to restore a healthy native plant community to these places. Best methods for removal sometimes vary by species and proper disposal of pulled plants is important. Never compost seeds, berries, or viable parts of invasive plants. And research sources of loam, gravel, mulch and other materials as these can carry and spread invasive plant parts.

The Town of Durham has several ongoing projects to control invasive plants on conservation areas, including:

• Doe Farm
• Milne Nature Sanctuary
• Oyster River Forest
• Thompson Forest

KNOTWEED CONTROL PROJECT

The Town has embarked on an effort to control invasive Japanese knotweed along Mill Pond Road and around the Town Hall. Doug Cygan, Invasive Species Coordinator with NH Department of Agriculture, Markets, & Food, is assisting the Town with this pilot project. Japanese knotweed (Reynoutria japonica) is an aggressive and highly invasive herbaceous to somewhat woody perennial originating from eastern Asia (Japan, Korea, China and Taiwan). One of the plant characteristics is the noticeably jointed stems, much like bamboo. It was first brought to the United States in the late 1800s for ornamental and horticultural purposes. It quickly became popular in the nursery trade and has been planted in landscapes throughout North America. It was also planted for erosion control and as a forage crop; little did they know at the time how damaging these practices would be.

Invasive characteristics of Japanese knotweed:

• Fast growing, ~ 8” per day
• Large woody rhizomes that penetrate the ground up to 10’ deep and laterally can exceed 40’
• Allelopathic properties (chemical compounds that are released by certain plants to eliminate vegetative competition) allowing it to displace native vegetation
• Forms dense clonal communities
• Regenerates from rhizome (root) fragments as small as ½” in length
• Rhizomes can remain dormant for up to 20-years
• Cut or mowed stem fragments can regenerate from nodes
• Out-competes native species and reduces or eliminates native plant diversity
• Grows through concrete and pavement causing issues with infrastructure, utilities, drainage, septic systems, walls, and foundations

The most effective control method is herbicide application by a NH licensed applicator. Japanese knotweed is unlike most plants in that the flow of nutrients/carbohydrates is in one direction, moving upward during the growing season until flowering and then the process reverses to deliver the nutrients/carbs back down to the rhizome system for overwintering. Therefore, chemical application is most effective just after flowering up until the first killing frost (September – November). To increase effectiveness of the treatment, the stems can be cut to the ground in June. The re-growth will be reduced by the time of treatment in the fall. We’ve done this at two sites—behind the town hall and at the College Brook bridge. These piles will be left to dry out to ensure they are non-viable before being taken to the town transfer station for burning. Doug Cygan will be treating the plants in the fall and other invasive plants along Mill Pond Road (not on the Milne Sanctuary) as well.

--------------------

The removal of invasive plants is coupled with restoring these areas to healthy, natural plant communities. Sometimes this involves augmenting the site with plantings, but often requires just letting the native seed bed sprout and dominate the site again.

Invasive plants cross property boundaries. By working together, we can better restore and maintain native plants, which benefits people too. Native plants are more suited to our soils and climate and are essential for native butterflies, bees and other pollinators. The good news is that native plants are becoming more available at local nurseries.

Integrated Pest Management: read more about Durham's approach to managing invasive plants here.