Some days ago, in a comment on our post responding to Jake Sigg’s defense of Garlon, Wendy Poinsot said:
Garlon is effective on oxalis and other broadleaf weeds like dandelion. Who wants a park full of south african oxalis? Not me. To use Garlon safety you need to have an adequate setback from the creek and other drainages. It would be wonderful to restore a more native plant community to Glen Canyon Park. If it was a just a small area, like my garden, we could just keep pulling the oxalis and replant, but for 30+ acres in Glen Canyon, we unfortunately need to use something that will work. There are time-tested safe methods of applying Garlon to get rid of weeds while allowing subsequent replanting. Garlon-type products are what is added to lawn fertilizer and weed control products that so many people use to control weeds in lawns year after year. Hopefully, one or two applications of Garlon, followed by plantings, will go a long way to controlling the oxalis. In the shade, we could actually replace it with the native oxalis — redwood sorrel.
We understand that some people do want to see non-native plants eradicated and substituted with native plants. (Others don’t.) But is it true that “one or two applications of Garlon, followed by plantings, will go a long way to controlling the oxalis” ?
Probably not, and here’s why:
Oxalis, though it’s a rich nectar source for bees, butterflies and other insect pollinators, doesn’t spread in San Francisco by setting seed. (In effect, it gives away its nectar for nothing.) Instead, it spreads through vegetative reproduction. It forms little bulbs underground (“bulbils”) that then grow into new plants. Sort of like potatoes, but smaller.
According to a UC Davis note, the toxic herbicide poisons the visible part of the plant; it doesn’t kill the bulbil. So, the next year, it grows back. The only way to successfully kill it with Garlon is to keep hitting the oxalis with the chemical, so that the plant cannot form the bulbils and eventually it exhausts them. This means that we’re not talking one or two application, but many of them, year after year after year. Between March and October 2010, the SF Natural Areas Program and Shelterbelt Builders (contractors) made ten applications of herbicides in Glen Canyon, mostly triclopyr (Garlon) or glyphosate (Roundup or Aquamaster).
A TOXIC RISK
What does this mean for groundwater, wildlife, people and pets?
Garlon is known to have a moderate risk of contaminating water. Considering that Glen Canyon really is a steep-sided canyon with a creek flowing at the bottom, it’s probably a lot more than moderate in this location — and on Twin Peaks, or for that matter, Mount Sutro, all of which are steep watershed areas.
Most of the research into Garlon is about immediate effects on people, and some little information about immediate impacts on a few animals, fish, and adult honey bees. There’s little or nothing about the long term impact of exposure year after year. (Most of the categories on the UC Davis data sheet show “no information.”) We’ve several times published our summary of the impacts described in the research thus far.
We’d suggest that we’re not getting the bang for our toxic buck. We’re taking the toxic risk, but with no certainty of effectiveness of this or any other herbicide in any reasonable quantity.
OXALIS IN THE FOOD WEB
We’d also suggest that the presence of oxalis isn’t entirely negative, quite apart from its beauty. Oxalis is already clearly and obviously part of the food web in our wildlife environment. Its flowers provide “copious nectar” [according to a UCLA note UCLA note linked here as a PDF: Oxalis_pes-caprae_UCLA_SantaMonicas] which feeds honeybees, bumblebees and butterflies. Birds eat the insects that feed on the plants and flowers. The bulbils are food to gophers, which are in turn food to owls, hawks, and coyotes. And so on. Moreover, it provides a good ground cover, stabilizing dirt and reducing surface erosion.
We’re not suggesting planting more of it… though where the soil has been disturbed (as for instance along the road above the Laguna Honda Reservoir at Clarendon Avenue) the oxalis tends to sprout from bulbils hidden in the soil, and establish itself. But we do think that a futile attempt to poison it out of existence with Tier I pesticides (the most hazardous allowed on City land) negates the Precautionary Principle that underlies San Francisco’s use of toxins and is bad for the environment.