Willow (Salix spp.)

DESCRIPTIONS
Willow: The many species of native willow range widely in shape and overall size--from the 10-foot marsh willow to the 65-foot black willow. All willows have simple leaves that are alternately arranged on spindly stems. Leaf margins vary from smooth to serrate. The plants are dioecious: pistillate and staminate flowers grow on separate plants. Both male and female flowers appear as catkins; the male catkins contain 2-5 stamens. Fruits are bivalved capsules. In winter, leaf buds are covered by a single, nonresinous scale. Several species can be aggressive in certain habitats.

DISTRIBUTION AND HABITAT
Willow: At least 30 different species of willow shrubs and trees grow in the United States and Canada. Twenty-two species can be found in Wisconsin yards, roadsides, and wetlands. Willows can withstand many climatic extremes, making them well suited to live in temperate regions like Wisconsin. These species prefer moderately alkaline soils, and grow poorly in either extremely acidic or extremely alkaline conditions. Willows commonly occur in forested wetlands, especially riparian and flood plain habitats. They are particularly prominent in early successional stands where seasonal flooding occurs regularly.

LIFE HISTORY AND EFFECTS OF INVASION
Willow and Cottonwood: Female flowers of the willow and cottonwood are pollinated in the springtime, giving rise to many-seeded capsules that ripen between early summer and late fall. The seeds are usually dispersed by wind and water. Asexual reproduction can take place by suckers, sprouts, and root shoots. Saplings flourish in damp, well-lit areas. Cottonwood and willow are shade-intolerant, and do not compete well under dense forest canopies. Some willows and cottonwood are occasionally considered problem plants that alter wetlands by limiting herbaceous undergrowth. Once in the sapling stage, these woody plants are difficult to eradicate.

CONTROLLING WILLOW, COTTONWOOD, BOXELDER, AND DOGWOOD
When choosing a control strategy for willow, cottonwood, boxelder, or dogwood, it is important that the habitat manager carefully observe the life-history responses of the trees or shrubs within the target region, because responses may vary from region to region, affecting the success of management manipulations. For instance, in northerly regions, willow can be controlled by mowing seedlings and saplings and subsequent flooding; farther south, mowing and flooding may actually induce more aggressive plant growth.

Mechanical Methods: The complete drawdown of water in an impoundment during the hottest summer days may prevent the establishment of extensive root systems by woody plant seedlings. Combined with discing, drawdown can terminate newly established seedlings and destroy the root systems of older plants. It should be noted that in southern latitudes, woody species of plants may experience an enhanced germination rate if drawdowns expose great expanses of mud flats before seed dispersal; exposing mud flats after seed dispersal can reduce and confine establishment of woody plant growth.

Discing is an economical technique that can destroy both aerial and subterranean woody plant growth. A double cross-disc is most effective for disturbing especially dense stands of trees and shrubs. Discing 2-3 times during the growing season is the best method to impair seedling growth. Discing becomes ineffective when plant stems reach more than three inches in diameter. After plants reach four inches in diameter, bulldozing is the only feasible method to physically remove them from the landscape en masse. The method is rarely advised due to the obvious resulting alteration of wetland topography.

Cottonwood can be eradicated through girdling. Boxelder, however, is resistant to girdling and sends up multiple shoots below the point where its phloem is severed. Large-diameter trees can be removed with a chainsaw. Resprouts must be recut, or herbicides may be applied to the cut stump (see below).

Chemical Control: Because new shoots may sprout from severed trunks, cut stems and stumps must be treated with an herbicide, such as glyphosate licensed for use in wetland environments.

Glyphosate applied to foliage at 7pts/acre in August has been found to kill dogwood and willow, although this herbicide will harm most anything green. Notice must be posted before spraying due to possible fish contamination. Hand-held or tractor-mounted wick applicators can be very effective in the application of foliar herbicides to relatively low dogwood and willow stems.

Grey dogwood can be successfully controlled by cutting stems in summer and fall, and then carefully applying an herbicide to cut stems using a spot applicator. Basal bark application may also be effective.