This post is based on 20-years work restoring oak savannas
and tallgrass prairie remnants at PVC. During that period, we were able to
obtain a lot of experience on the eradication of invasive woody plants, of
which buckthorn was the most difficult. See the table at the end for an index to my blog posts that dealt with buckthorn.
Common buckthorn (Rhamnus
cathartica) is the #1 invasive shrub on the IPAW list. When we started serious restoration work at Pleasant Valley
Conservancy in 1997, it was present in very dense patches throughout most of
the savanna areas. Later I discovered that buckthorn is a calciphile, and was growing
mainly in high-calcium high-pH soils derived from or influenced by dolomitic
rocks. In many of the calcium-rich oak savanna areas, the understory was almost a buckthorn
monoculture.
During our major restoration work, buckthorn was removed at
the same time as all the other invasive woody plants. Buckthorn removal mainly
involved cutting the buckthorn thickets with a brush cutter or chain saw, and
treating all the cut stems with herbicide (generally Garlon 3A at 15-20%). Buckthorn
was not the only “bad” plant in our degraded oak savannas, but in the
calcium-rich areas it was the principal one. The mature open-grown savanna oaks
were also being crowded by invasive trees such as walnut, elm, cherry. There
were even some “bad” oaks, such as the more rapid-growing oaks of the black oak
group, and these were removed also.
According to my notes, all of the large patches of buckthorn
were removed during the major clearing that took place in 1998 through 2004.
Once the savanna areas were cleared, fire was introduced as a
principal tool. Although the use of fire was important, it only top-kills woody
plants. The underground rootstock remains alive, and quickly resprouts. In
fact, there is a strong “legacy” of woody plants in any formerly wooded area.
If a cleared area is abandoned, it will quickly become overgrown again.
Although this “legacy” is important for most woody species,
it is especially severe with buckthorn. It took me some years to discover that with
buckthorn there are three legacies:
- 1. Buckthorn produces a toxic chemical called emodin which inhibits the growth of other plants and prevents “good” plants from becoming established. It creates what we have called a “buckthorn desert”. It generally takes about three years for the toxic chemical to dissipate.
- 2. An extensive seed bank from the large number of berries produced. (Although only the female buckthorn plants produce berries, the ratio of female to male plants can be as high as 6:1.) The seed bank only lasts a few years.
- 3. An extensive rootstock (sometimes called a caudex) from which new shoots can arise. It took me some years to realize that the rootstock of the buckthorn can remain dormant from many years.
By this time I had started my first Blog (Tom’s Journal),
and buckthorn was a major topic. The table at the end of this document gives
links to the various Blog posts.
There is a lot of internet material on buckthorn, mostly
focusing on getting rid of it. Most of this work is based on short-term
studies, or on anecdotal evidence. Buckthorn has a complicated ecology, which
enters into a discussion of control methods.
Through the 20-year period that I have been following
buckthorn I determined that buckthorn rootstock can be long-lived, and remain
dormant for many years. Thus, an area that once had had a large buckthorn
infestation can have numerous invisible and dormant rootstocks.
In order to get some quantitative data, in 2010 I chose a
site of about 100 X 100 feet where buckthorn had been removed around 2000 but
kept coming back. I canvassed the site carefully and sprayed each buckthorn
shoot with Garlon 3A (foliar at 4% aqueous). Most of the plants had only single
shoots, but some had more than one. I sprayed all the shoots I could find at
each survey.
In the three early years, when there were lots of shoots, I
returned over and over again at 2-3 week intervals, and sprayed all the “new”
buckthorn shoots that had arisen. At the end of each season, I made sure that
there were no live buckthorn shoots left.
The table shows the results. My conclusion was that in this
small, buckthorn-rich area, there were many dormant rootstocks, but not all of
them left dormancy the same year.
|
Year
|
Buckthorn shoots
sprayed
|
Year total
|
|
2010
|
not recorded
|
>300
|
|
2011
|
12 + 30 + 71 + 35 + 27
|
175
|
|
2012
|
17 +18 + 55 + 5 + 44 + 20 + 82
|
241
|
|
2013
|
6 + 7 + 8
|
21
|
|
2014
|
0
|
0
|
 |
| Buckthorn resprouts, waiting to be sprayed! These could be either from fire-killed plants or from dormant rootstock |
I should emphasize that the original removal of buckthorn
was in either 1999 or 2000. Unanswered is what stimulates these dormant
rootstocks to start growing after many years?
My conclusion is that buckthorn might be eradicated from a
site, but it might take many years.
Note that the area I studied had been heavily “infected” by
buckthorn, and that there was plenty of time for the plants to make extensive
underground rootstocks. Sites recently colonized by buckthorn might be
eradicated much easier. However, I suspect that most sites in southern
Wisconsin that are full of old-growth buckthorn would be similar to my site,
especially if they had dolomitic soils.
 |
| A single buckthorn plant still green after the native vegetation has senesced. Can be sprayed without causing any peripheral damage, with either glyphosate or triclopyr |
Today Pleasant Valley
Conservancy is virtually free of buckthorn! This is because we have worked
very hard to kill the plant when we see it. Every year, we canvass all sites
where buckthorn has been a problem.
We are especially helped by the fact that all of the savanna
areas, where buckthorn had been the biggest problem, are burned each year. Fire
keeps buckthorn at a juvenile status, and prevents any extensive spread by
underground roots or rhizomes. Fire makes it possible for us to spray buckthorn
shoots when they are still small (foliar spraying is done in the spring).
Any buckthorn plants not killed in the spring can be killed when
they are found in summer, autumn, or winter by basal bark treatment. The dark
shiny leaves of buckthorn plants make them easy to spot, especially in the fall
of the year when native vegetation has senesced.
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| Single buckthorn stem that had been basal-barked with 20% Garlon 4 in oil. The leaves are curled and dying.
Blog posts through the years on buckthorn. The ones before 2008 are no longer available
|
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