Tom's Blog

Monday, December 15, 2014

Corylus americana: an invasive shrub?

Hazel is a native shrub that is widespread in Eastern U.S. It is found in every county in Wisconsin, and was occasionally reported by the surveyors in the original public land survey as part of the plant understory. Although not considered an invasive plant in Wisconsin, it is a serious problem in the red or jack pine forests of Minnesota.  It is also considered a problem in other areas, as the following quote from the Flora of North America attests: “Corylus americana is a weedy species, sometimes considered a pest in carefully managed forests.” 

During our early savanna restoration work, based on the prevailing sentiment in southern Wisconsin, we either ignored hazel or allowed it to grow, in some cases even encouraged it. Now we realize that this may have been a mistake.

Although it grows rhizomatously, its rhizomes only grow a short way before they send up new shoots. Thus, in contrast to sumac, gray dogwood, and prickly ash, hazel often grows as tight, compact bushes. A single bush like this may remain compact for a number of years, although its duration in this well-behaved capacity is unpredictable.

Under some conditions a single hazel bush may turn into what is called a “hazel thicket”.
In his pioneering paper on the relationship between forests and fire, Gleason noted that the margin between prairie and forest consisted chiefly of hazel, “a shrub which is not seriously affected by repeated burning…. There is one record known to the writer of a large hazel thicket, covering several acres at the western edge of an upland forest, and several such records of scattered thickets of hazel in the middle of the prairie.” (Gleason, Henry A. 1913. The relation of forest distribution and prairie fires in the Middle West. Torreya 13: 173-181.)

A survey of the literature reveals a number of mentions of “hazel thickets” associated with either prairies or savannas. Thus, hazel “can” become a problem, although the conditions leading to this situation are not clear. At any rate, it seems obvious to me that it would be foolhardy to allow hazel to “run amok”.

At Pleasant Valley Conservancy I have been seen several sites where hazel seems to be getting out of hand. At the south side of Toby’s Prairie, in a former aspen zone, hazel has been spreading even in the face of annual fire. It has also spread extensively in some areas of the White Oak Savanna (Unit 12A), also burned annually. Both of these areas seem to have the potential of becoming bad, so we are killing them this winter by Garlon basal bark.

On the other hand, there are several hazel bushes along the margin between the Ridge Prairie and the East Basin that seem to be well behaved. They were left ten years ago when we cleared all the honeysuckles away, and although they have bulked up they have not spread.

These are savannas. How about prairies? Here I think it depends upon the history of the site, what I call the “legacy”. At Black Earth Rettenmund Prairie, where hazel was present before restoration work began, there is a large hazel clone on the Saddle that has survived dozens of prescribed burns. Also, on the knoll at the top of the North Unit, where the woodies formerly consisted of prairie willow and sand cherry, both relatively benign, in 2012 many clones of hazel appeared. Even more disturbing, they were not only spreading rhizomatously, but were sending up suckers (see photo). In this clone I counted over 350 shoots.

Given these conditions, it seems reasonable to control the hazel, and this is what we are doing.

New shoots of a hazel clone spreading away from the main shrub.


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