Is it an Invasive Species?

Is it an invasive Species?

What is an invasive species? I often ask this question at the outset of any discussion on invasive plants. Common answers usually include familiar plants like Japanese knotweed, round leaf bittersweet (pictured), poison ivy, sumac, or even dandelion. We will get into the specifics of each of these plants later, although you may have already noticed that not all of these are invasive plants. So, what makes a plant invasive? For starters, the term invasive species is grounded in both science and policy. 

Round Leaf Bittersweet. Photo by Chris Evans, University of Illinois, Bugwood.org.

From a science perspective, invasive plants display certain common traits that allow them to outcompete other species. These traits include things like early and prolific seed production; leafing out earlier in the year and holding leaves longer into the fall (extended phenology); releasing chemicals into the soil that inhibit the growth of other plants (allelopathy); reproducing from small plant fragments called propagules; and tolerating disturbance. Additionally, one of the biggest advantages is that these plants have been released from limiting factors such as fungi, viruses, and insects that evolved over thousands of years to feed on them in their native range. 

On the policy side, The N.H. Invasive Species Law (RSA 430:52)and N.H. Administrative Rules (AGR 3800) define an invasive species as "an alien species whose introduction causes or is likely to cause economic or environmental harm or harm to human health." These species come in a variety of forms, including trees, vines, shrubs, and grasses, terrestrial herbaceous and aquatic. State law prevents the sale, distribution, or transport of invasive species. RSA 430:51-57 states, “no person shall collect, transport, import, export, move, buy, sell, distribute, propagate or transplant any living and viable portion of any plant species, which includes all of their cultivars and varieties, listed in Table 3800.1, New Hampshire prohibited invasive species list.” A few key words here are living and viable portions of any plants. This means any plant parts capable of producing a new plant are included in the above restrictions. For example, the movement of viable seeds or fruits in the treads of heavy equipment or the transport and use of fill with invasives, violates state law.

Getting back to the original question, an invasive species must first be non-native to the ecological area being considered. The plant must also pose some form of ecological, economic, and/or human health threat. These considerations along with the biology of the plant are used to determine a specific plant’s “invasiveness”, and this drives policy. Two common invasive species in NH, Japanese knotweed and round leaf bittersweet, are both non-native plants that display a host of invasive characteristics. These traits can lead to economic and ecological impacts to both agriculture and forest and habitat management and productivity. For knotweed, in addition to ecological concerns, it creates visual safety issues in rights-of-way and can cause considerable damage to property. Individual states in the United States maintain official lists of invasive plants and watch species. Prohibited invasives found in Table 3800.1  are those where state invasive species laws apply.  Those listed as “watch” are not regulated. However, they exhibit some invasive characteristics that over time could allow them to spread rapidly and become invasive.   Prohibited and watch list plants can be found on the NH Comprehensive Invasive Plant List, along with aquatic species regulated by the NH Department of Environmental Services (NHDES).  

Poison ivy and sumacs are native plants therefore not invasives. Both plants also have tremendous wildlife benefits from their fruit (mast) production. Poison ivy & poison sumac are certainly plants that pose a human health concern and could easily be classified as weeds (plants that are unwanted in a particular space). Sumacs, staghorn & smooth sumac in particular are both harmless but frequently colonize disturbed sunny sites and can cover large areas, a reason they are mistaken as invasive species. Poison sumac on the other hand is a plant relegated to swamp edges and wet woods and is more closely related to poison ivy than other more commonly encountered sumac species. Again, love them or hate them, these are all native plants and therefore not technically invasive species. 

Lastly, Dandelions are native to Eurasia, but do not really pose any widespread economic, ecological, and/or human health concerns. Sorry, lawn weeds do not measure up here. However, in the White Mountains of New Hampshire, there are globally rare tundra ecosystems found on our highest peaks above tree line. In these fragile alpine communities, dandelions have been found to be the most invasive plant species present, displacing rare native plants and altering the ecology of these fragile communities. So, in the case of dandelion, it is not an invasive plant from a policy standpoint, but it definitely is from an ecological standpoint when looking at a very specific ecological community.    

Based on what we know about invasive plants, we must carefully consider them in all our land management decisions, so we can slow their spread and stay legally compliant. Being aware of invasive species, monitoring for their presence, and practicing early detection rapid response (EDRR) are some of our best defenses against invasive species. When it comes to invasive species introductions, spread is rapid and control can become increasingly difficult, costly, and less effective over time. We apply the EDRR concept to plants that are not yet widespread, where quick action can lead to eradication. The EDRR concept can be applied to specific plants that are not yet widespread in the state or to any plant that is not widespread in a particular piece of land you are managing. 

To help with your invasive species management efforts, there are many resources available to assist in identification, planning, and implementing invasive species management projects. You can find a variety of resources on species, control options, and project planning through the NH Department of Agriculture and UNH Cooperative Extension. To assist with identification and learning more about NH’s prohibited upland invasives species, this Digital Field Guide to NH’s Upland Invasives is available as a resource.  For information on aquatic invasives such as Eurasian milfoil, NHDES has resources. You can also request a woodlot visit with your local UNH Extension Forester to discuss your forest stewardship goals, invasive plant concerns, and options for management.