Tom's Blog

Saturday, August 21, 2010

Buckthorn underground

As those following these posts know, we have been battling buckthorn (Rhamnus cathartica) for many years. In the early years, we eradicated lots of large buckthorn shrubs (really small trees) using basal bark herbicide (Garlon 4 in oil). However, getting rid of the large buckthorns did not "eradicate" this pesky species. New growth, presumably from the seed bank, kept coming up. For many years, we assumed that all the new growth we were seeing arose from this seed bank. However, as the years went by, we began to doubt this idea, especially since the available research suggested that buckthorn seeds don't survive long in soil.

In several areas of Pleasant Valley Conservancy, we have been working to eradicate buckthorn for at least ten years, and although buckthorn is a lot less than it once was, it has still not been eradicated. In one experimental area, we have been working consistently for three years. This is an area just north of the Rocky Overlook, where small buckthorn shoots are quite common. This is an interesting area, because when we started there it had a small remnant population of lead plant. As we knocked back the buckthorn, the lead plant has expanded, and is now one of the major species.

But each year we still have numerous small buckthorn plants. These are almost all multi-stemmed plants, with each stem arising from a small area at the base. Why multi-stemmed, and why do they keep on coming?

Today I dug up the small buckthorn plant with two stems, shown in the photo here. The typical fibrous root system is visible, but above this is a very thickened underground stem from which the two shoots arise. When Kathie looked at this root mass, she pointed out that there were at least three or four small dead shoots which had probably been killed by fire some earlier year. But the green shoots must have come from living buds that were below the surface and hence protected from fire. Although invisible in the spring, these shoots grew up during the summer and if not killed have the potential to make a fairly substantial plant.

If I kill these shoots, will I kill the whole root mass? Possibly not. There may be more dormant buds.

It seems evident that rather than dealing with a persistent seed bank, we are dealing with persistent living root masses, left over from the original buckthorn infestation that we removed years ago.

This situation is not unique to Pleasant Valley Conservancy. I have seen numerous sites in Dane County where buckthorn has been cut and treated with herbicide, but new shoots have arisen a year or two afterward. These shoots are not arising from a seed bank, since they are much too vigorous to have arisen from tiny seedlings. They most likely are coming from an underground viable root mass.

The moral here is that if we are to eradicate buckthorn from a site, we must return year after year, killing all new shoots with herbicide, until we finally exhaust the capabilities of these underground root masses.

I think we are up to the task. This year we have been using a more efficient and more benign herbicide treatment, spritzing a couple of leaves on each buckthorn shoot with Garlon 4 in oil, as described in an earlier post. I have been quite happy with this approach, especially since with reasonable care the now extensive lead population is not affected.

In order to find out how many more years I have to continue spraying buckthorn, I have staked out an area and am systematically traversing it, making sure that I spray every plant. Since not all buckthorn shoots are large enough to see at the same time, I have been returning every week or two and spraying every newly visible shoot. Fortunately, the buckthorn shoots are fairly easy to recognize, because of their shape, their dark green color, and their shiny appearance.

Most of the shoots are knee high or less, although a few quite small ones may be hidden beneath the larger ones. If these are missed in the next few weeks, they should definitely be visible in early October, when all of the native vegetation is senescing. At that time, they can be basal barked with Garlon 4, which should kill them permanently.


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