More Garlon for Glen Canyon Park?

We’ve been observing the issue of pesticides at Glen Canyon Park at a remove; but here’s what we understand. The place falls under the Natural Areas Program (NAP), which plans to use Garlon, one of the most toxic herbicides San Francisco permits, to attempt to get rid of oxalis, theoretically to benefit native plants. Garlon works on broad-leafed plants, but not on grasses or iris or conifers.

Glen Canyon Park is a popular one, and it really is a canyon, with steep slopes above a little creek and a pathway through tall eucalyptus to a clubhouse and play area. Small children — including preschoolers — play and hike there. People walk their dogs in the park. A major investment in the park is planned for the near future. So unsurprisingly, when notices went up that the NAP intended to spray Garlon on the canyon slopes, neighbors objected. The doyen of the nativists, Jake Sigg, weighed in on the issue in his newsletter. We discussed that here.

The Garlon spraying was postponed for a week, and we heard a rumor that NAP was considering using Roundup (glyphosate) which is somewhat less toxic. (It’s a Tier 2 Hazard, while Garlon — triclopyr — is a Tier 1 hazard, i.e. Most Hazardous with a High Priority to Find an Alternative.)

Then a sign just said “Postponed” and rumors went round that NAP were going to abandon the use of toxic pesticides in Glen Canyon altogether, in deference to neighborhood concerns.

Not so. Someone just forwarded us a letter from Dennis Kern, Director of Operations at SF Rec and Park, addressed to a citizen. (As an official letter from a city employee, this is we believe non-copyright and can be published here.)

Thanks for your e-mail.  Any rumor that we are discontinuing our controlled use of herbicide in Glen Canyon is incorrect.  As you may know, the Glen Canyon parkland is one of our Natural Areas.  There is no way that we can protect biodiversity and control invasive species in Natural Areas without the informed and controlled use of herbicides as one of our tools.  In particular, Oxalis is not controllable by hand nor any other non-herbicide means.  If we were to discontinue herbicide use at Glen Canyon we would soon lose the biodiversity that our Natural Areas Program is designed to protect.  All of our limited herbicide use is in strict compliance with all City regulations and is performed in partnership with SF Environment.

I hope that this information is helpful.


Yes, it is indeed legal. Recently,  SF’s Department of the Environment, which regulates pesticide use on city-owned properties in San Francisco, permitted an exception to allow Garlon 4 Ultra to be sprayed. (Before that, it was only allowed to be daubed or injected, but NAP sprayed it anyway.)  People applying it must wear respirators (though on Twin Peaks, at least on one occasion they have been seen spraying without one). They have to post notices at least 3 days in advance, which we understand they do; and the area is not safe to enter until the chemical is dry. We’re not sure how that’s ascertained, but anyway, if you are visiting sprayed areas, please allow a sensible amount of time from when the spray is completed. This should also be posted on the notice (though sometimes they forget).


If it’s legal, why do we worry?  Here’s why:

When we looked into Garlon, we found that much of the research and information that minimized the risk came from sources such as programs that wished to use the chemical, or from industry sources. The existing research on Garlon is incomplete, and more focused on acute risk than the risks of long-term exposure. In April 2010, we summarized what we read in the Marin Municipal Water District’s excellent chapter on Triclopyr, the active substance in Garlon. Here’s an excerpt from our article of ten months ago.

What stood out, though, was how much is not known, particularly about the effects of repeated low-level exposure. There simply isn’t that much research out there, and few human studies. “Although triclopyr has been registered in the US since 1979, there are still very few studies on triclopyr that are not part of the EPA registration process.” Most of the research that exists is on Garlon 4. What is used on Twin Peaks [and in Glen Canyon] is Garlon 4 Ultra. It’s similar but isn’t mixed in kerosine. It’s mixed in a less flammable but apparently equally toxic methylated seed oil.

What is known makes uncomfortable reading.

  • Garlon “causes severe birth defects in rats at relatively low levels of exposure.” The rats were born with brains outside their skulls, or without eyelids. “Maternal toxicity was high” and exposed rats also had more failed pregnancies.
  • Rat and dog studies showed damage to the kidneys, the liver, and the blood. It’s insidious, because there’s no immediate effect that’s apparent. If someone’s being poisoned, they wouldn’t even know it. In a study on six Shetland ponies, high doses killed two ponies in a week, and two others were destroyed.
  • About 1-2% of Garlon falling on human skin is absorbed within a day. For rodents, its absorbed twelve times as fast. Too bad for the gophers…
  • It isn’t considered a carcinogen under today’s more lenient guidelines, but would have been one under the stricter 1986 guidelines.
  • Dogs may be particularly vulnerable; their kidneys may not be able to handle Garlon as well as rats or humans. “The pharmacokinetics of triclopyr is very different in the dog, which is unique in its limited capacity to clear weak acids from the blood and excrete them in the urine.” Dow Chemical objected when EPA said that decreased red-dye excretion was an adverse effect, so now it’s just listed as an “effect.”
  • There was insufficient information about Garlon’s potential effect on the immune system, or as an endocrine disruptor.
  • It very probably alters soil biology. “There is little information on the toxicity of triclopyr to terrestrial microorganisms. Garlon 4 can inhibit growth in the mycorrhizal fungi…” (These are  funguses in the soil that help plant nutrition.) No one knows what it does to soil microbes, because it hasn’t been studied.
  • It’s particularly dangerous to aquatic creatures: fish (particularly salmon); invertebrates; and aquatic plants.
  • It doesn’t [ETA: generally] kill adult honeybees, but there are no studies of other insects. [ETA: Some studies show slight “acute toxicity” to honeybees.]
  • Garlon can persist in dead vegetation for up to two years.

Given all the information we do have on this chemical (and all the information we don’t have ) we have to question why native plant restoration is worth spraying poisons on some of the highest points in our city. Garlon must be used when the weather is wet; if the plants don’t have water, they will not grow and the chemical won’t work.

In Glen Canyon, it’s being sprayed in the watershed of a creek, and upslope from the play areas of the park. Since it can remain in vegetation for months and years after spraying, it could be contaminating the whole area at low levels.


We’re not sure when the last spray was (we are enquiring of Rec & Park, and will report the response here). Neighbors tell us that they heard that one-quarter of the canyon was sprayed some time in January or February 2011.

We also don’t know when the next spray is planned. We presume it depends on weather conditions; we hope neighbors’ objections will be taken into consideration.

We are observing this process with some trepidation. We fear that in less than a year, [ETA: should the Plan be implemented to slash 90% of the trees and all the understory of the forest, and use herbicides to prevent regrowth], this could be the story of Mount Sutro, where two-thirds of the forest is currently pesticide-free — perhaps the only wildland to be so.

The Sharp Park Golf Course is also pesticide-free [ETA2: Since August 2010, when some Roundup was used there], by the way.  The city’s “Natural Areas”, on the other hand, are regularly sprayed with Garlon 4 Ultra, Roundup and other glyphosate-based chemicals, and occasional doses of Imazapyr — which wasn’t a permitted chemical at the time, but they used it anyway.

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