Buckthorn eradication: results of a five year study

Because of buckthorn's importance, there have been many earlier posts from Tom's Blog. A list of these posts, with URLs, can be found at this link.
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Buckthorn is probably the most difficult shrub to get rid of in prairie and savanna restoration (it’s at the top of IPAW’s list). Because of its strong allelopathic nature, if unchecked it can form extensive monocultures.

Removal of buckthorn requires the use of an herbicide, either as a basal bark or cut stem application. Although these treatments are very effective, they do not eradicate an infestation for two reasons:

•  There is almost always an extensive seed bank. This is fairly easy to control, and only persists for several years.
• There are numerous dormant root masses capable of sending up new shoots, and these are a much more difficult problem. Experience has shown that these viable structures can remain alive for at least 15 years, perhaps longer, even if any new shoots they form are mowed or top-killed by annual fire.

What I am reporting here are the results of a long-term study I carried out at Pleasant Valley Conservancy. The area used was a small part of a much larger bur oak savanna which was first cleared of buckthorn in the 1999-2000 period. The initial fairly large-diameter shrubs were killed by basal bark treatment with Garlon 4 in oil. After the shrubs had died (which took about two years) they were removed when the savanna was restored. Since then, the site has been burned annually.

Despite annual burns, new buckthorn shoots kept appearing every year. Each shoot was part of an extensive underground root mass. Five years ago I decided to see how long it would take to get rid of them. I think I have finally succeeded.

The preliminary work for this study was described in a 2011post, which should be read for details on this study. 

Starting in 2010 I canvassed the site carefully and sprayed with Garlon each buckthorn shoot, counting as I proceeded. In the three early years, when there were lots of shoots, I returned over and over again at 2-3 week intervals. At the end of the season, I made sure that there were no live buckthorn shoots left.

The table shows the results. I should emphasize that the effectiveness of my spraying is not in question. Thus, a shoot found in 2012 is not one that had been sprayed in 2011 and not killed. My conclusion is that in this small area there were many dormant root masses, but not all of them left dormancy the same year.

Year	Buckthorn shoots sprayed in study area	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

Another conclusion is that it is possible to eradicate a site of buckthorn, but the site must be revisited in multiple years.

One might ask what effect this multiple spraying might have on the natural flora. That’s a lot of Garlon! I should emphasize that I was careful with my spraying, making sure that nearby “good” plants were not treated.  The table below is the species check list I made this year, when no spraying had been done. The diversity is satisfyingly high! Note the presence of Gentiana alba, a State Threatened species. There are actually two separate patches of this species, one of which has numerous stems, and both patches flowered.

Latin name	Common name
Actaea rubra	Red baneberry
Agrimonia gryposepala	Tall agrimony
Allium cernuum	Nodding wild onion
Amorpha canescens	Lead-plant
Andropogon gerardii	Big bluestem
Anemone virginiana	Woodland thimbleweed
Asclepias syriaca	Common milkweed
Asclepias verticillata	Whorled milkweed
Aster ericoides	Heath aster
Aster novae-angliae	New England aster
Aster sagittifolius	Arrow-leaved aster
Campanula americana	Tall bellflower
Desmodium glutinosum	Pointed tick-trefoil
Desmodium illinoense	Illinois tick-trefoil
Elymus hystrix	Bottlebrush grass
Eupatorium altissimum	Tall boneset
Eupatorium purpureum	Purple joe-pye weed
Gentiana alba	Cream gentian
Gentianella quinquefolia	Stiff gentian
Heliopsis helianthoides	Ox-eye sunflower
Hieracium kalmii	Canada hawkweed
Lactuca canadensis	Tall lettuce
Monarda fistulosa	Wild bergamot
Ratibida pinnata	Yellow coneflower
Rudbeckia hirta	Black-eyed Susan
Schizachyrium scoparium	Little bluestem
Silphium integrifolium	Rosinweed
Silphium terebinthinaceum	Prairie dock
Solidago rigida	Stiff goldenrod
Solidago speciosa	Showy goldenrod
Sorghastrum nutans	Indian grass
Sporobolus heterolepis	Prairie dropseed
Toxicodendron radicans	Poison ivy
Tradescantia ohiensis	Common spiderwort
Verbena stricta	Hoary vervain
Zizia aurea	Golden Alexander