Description

The Norway Maple is a tree that usually grows to 40-60 feet in height, but can reach heights of 100 feet. The bark of the tree is grayish and regularly and shallowly grooved. The palmately lobed leaves are opposite and have 5 to 7 sharply acuminate lobes (with large but few teeth). These leaves are 4-7 inches wide. The leaf petioles exude a white sap when broken. The leaves are usually green in color, but there are some cultivars that have dark red leaves. The fall color of the green leaves is yellow. The flowers appear in April and May and are yellow-green in color. They are borne in erect, pedunculate, rounded corymbs. Each flower 0.25 inches wide. The pendulous fruit measure 1.5-2 inches in length. The fruit are samaras that are green when young and turn yellow, then brown, with age. The samara wings are divergent, reaching nearly 180 degree angle to each other.

Habitat

The Norway Maple has a wide distribution throughout New England, in part due to its extensive planting as a street and ornamental tree. It has naturalized to nearby woods such as urban woodlots, forest edges and fragmented forests, and from there has moved on to less disturbed habitats.

History and Introduction

The first documented introduction of Acer platanoides to the northeast was by John Bartram of Philadelphia, in 1756. Bartram later offered it for sale in his garden catalogue in 1762. Multiple sources of seed from Europe were available shortly after this initial introduction.

Threats

This tree is able to shade out native understory vegetation such as spring ephemerals, and eventually outcompetes native tree species in the forest canopy. Thus, it can reduce native species diversity and change the structure of forest habitats.

Dispersal Methods

Norway Maple seeds are contained in winged samaras that are dispersed by wind.

Control

Don’t plant Norway Maple. Prevent or limit seed production by targeting mature trees. Prevent saplings from growing to maturity. Avoid soil disturbance, which promotes seed germination. Kill any sprouts at the end of the growing season, and annually thereafter. Light gaps left by cut or girdled canopy trees may result in rapid growth of Norway maple seedlings or colonization by other invasive species. Monitor these gaps carefully and remove invasive species and/or plant native species to compete. 

Mechanical

Digging up or other manual removal is suitable for Norway Maple seedlings and saplings as long as the roots are completely removed, or adult trees felled close to ground level. Girdling, by removing the bark and phloem layer from 10 cm around the trunk is also suggested.

For small trees, try the “cut and cover” method to prevent stump sprouts: cut several inches above ground level and cover the stump with something that completely blocks light for 1-2 years.

Once seed sources are removed, seedlings may either be monitored every few years to remove saplings, or hand-pulled annually until the seed bank is depleted. Soil disturbance from hand-pulling results in an increase in new Norway maple seedling density the following year, but the seed bank is short-lived. 

Pull seedlings when soil is moist. Dig out larger plants, including the root systems. Cut down large tree. Grind out the stump, or clip off re-growth. Girdlethe strangling of a tree branch or tree trunk by something wrapped around it, which chokes off the flow of nutrients the tree by cutting through the bark and growing layer (cambium) all around the trunk. Girdling is most effective in spring.

Hand Pulling

Can be effective for young plants. Care should be taken to remove the entire root system. A weed wrench can be used on saplings if soil disturbance is not an issue. 

Cut Stump

Cut stem 2-3 inches above the ground and apply a pre-mixed 50% solution of a water–soluble triclopyrherbicide used to control both broadleaf and woody plants amine with water to the stump immediately following cutting. Avoid run-off onto the soil. Continue to cut re-sprouts.

Basal Bark

The most effective method for control of this species. A 20% solution of Triclopyr ester in an oil-based surfactant should be applied in a 1 foot band completely around the trunk of the tree in summer. Follow-up foliar spray of root suckers may be necessary.

Foliar Spray

When tree and stand size allow, is one of the most effective treatment methods for seedlings. In situations where non-target native species will not be effected use a 2% solution of glyphosatea widely used herbicide that can kill certain weeds and grasses, it works by blocking an enzyme essential for plant growth with a 0.5% non-ionic surfactant.