Nonnative Invasive Garden Landscape Species

According to the written Conservation Policies of The Virginia Native Plant Society, alien or exotic plants are those that humans have introduced from other places, either deliberately or inadvertently. The majority of these species originate from Europe or Asia. They may thrive in the North American natural environment, but they are not adapted to play the ecological roles of native species.

Undesirable Characteristics of Nonnative Species

Prolific reproduction and aggressive spreading are typical traits of nonnative species spreading throughout North America. If these species escape from built landscapes, these characteristics easily become out-of-control threats to natural environments. A safe alternative is to investigate and install native plants that do not tend to become invasive.

Often these same reproduction and spreading characteristics plus natural attractiveness make these plants appealing to landscape gardeners. They are easy to come by through swaps with friends and neighbors, sales and trades sponsored by organizations like garden clubs, and some retail and wholesale plant markets.

Invasive Potential

The Canadian Botanical Conservation Network ranks nonnative species in the following manner:

• L = low level invasive,
• P = potentially invasive, and
• H = highly invasive.

The Network also lists more characteristics that determine whether species are potentially invasive:

• Rapid growth within a variety of conditions e.g. Japanese Knotweed, Polygonum cuspidatum (Photo #1)
• Lack of pests or pathogens acting as natural controls e.g. Lesser Celandine, Ranunculus ficaria (Photo #2)
• Ability to out-compete native species e.g. Goutweed, Aegopodium podagraria ‘Variegata‘ (Photo #3), and
• High costs to remove and control these plants e.g. Purple Loosestrife, Lythrum salicaria (Photo #4).

Seven Methods to Remove and Control Plants

1. Hand pull each stalk by roots at ground level before seed begins to set. This is the most environmentally friendly technique, but requires diligence in removing all stems and roots to prevent resprouting or reseeding.
2. Grub up (using spade or shovel) root system. This may cause site disturbance because of digging and resprouting if entire root system is not removed.
3. Shear by cutting stems close to ground level with shears or chain saw. Trees may sucker if cuts are not treated with herbicide.
4. Girdle young trees by removing bark and phloem layer from 10-cm-band around trunk. This method may also encourage suckering. Girdle should be checked for regrowth of bark.
5. Spray with foliar herbicide, but using caution. If possible, spray undesirable and exotic species with squirt bottles or backpack sprayers when species are dormant.
6. Combine cutting and herbicide to eliminate suckering. Monitor and retreat sites if necessary.
7. Change environment to increase drainage, to remove species liking wet soil. Grade soil to create alternate water drainage pathways.