Fact Check: Nothing Grows UnderEucalyptus? (Allelopathy)

Like most people in the US, we’ve been listening to some of the political speeches and the discussion around them recently. And we’re rather heartened by a newly emerging trend: fact-checking.

In the last few months, one “fact” we’ve heard repeatedly is “Nothing grows under eucalyptus,” and the “explanation” that it’s because of allelopathy, the process by which certain plants exude chemicals that poison the soil and prevent other plants from growing there.

  • Dennis Kern of the San Francisco Rec and Parks Department said it at a West of Twin Peaks Central Council meeting.
  • He repeated it at a Parks and Recreation Open Space Advisory Committee (PROSAC) meeting. (That prompted a member of the public to ask Mr Kern if he’d ever visited Mt Davidson. He said he had.)
  • Glenn Rogers, a candidate for Supervisor in the District 7 race, also spoke of allelopathy, adding that eucalyptus forests are “like a desert” and that no wildlife or birds could live there.

IT’S NOT TRUE

As the fact-checkers have had frequent reason to say: It’s not true.

The internal rain of the Cloud Forest nurtures a lush dense green understory even as Twin Peaks turns brown and city parks need sprinklers. Right now, in Sutro Forest, the trees are green and beautiful – and so is the secondary canopy of acacia trees, and so is the dense understory where it hasn’t been cut back.  [See Mt Sutro Ecosystem and Wildlife Habitat]

The only area that is brown and dry is the Native Plant Garden at the summit. It dries out because there’s not enough tree cover to water it. Some areas now are quite denuded.

In fact, if you do find places in San Francisco with bare ground under eucalyptus, it’s because the Native Areas Program of the SF Recreation and Parks Department, or their volunteers or sympathizers have been pulling up undergrowth with great effort – or poisoning the soil not with allelopathic chemicals but with herbicides like Roundup or Garlon or Imazapyr or “Milestone.”

We understand that not everyone walks in the city’s two wonderful eucalyptus forests at Mt Sutro and Mt Davidson (both, ironically, in District 7). But – surely – they’ve driven by them?  We invite all those who think it’s a desert to drive along Christopher Drive and Crestmont in Forest Knolls, or along Clarendon Avenue between Oak Park and the UCSF Aldea Campus entrance, or down Medical Center Way, to see for themselves. (The picture below is from Clarendon Avenue.)

NO WILDLIFE?

The eucalyptus haters have managed to spread a lot of misinformation about these trees over the last few decades. Chief among them is that they’re an ecological desert where nothing grows and no birds or insects flourish.

That isn’t true either.

  • Some 45 species of birds been recorded in Mt Sutro Forest in a couple of birding session. Mt Davidson is a favorite spot for our city’s birdwatchers, and when they visit it, they’re usually spotting birds in the forest’s trees, not on the bare “native plant” side. [The Mt Sutro bird list HERE and HERE; we welcome any additions from birders, especially with photographs.]
  • Birds from the large Great Horned Owls and Redtailed Hawks nest in eucalyptus trees, and so do woodpeckers and smaller birds like the Western Bluebird and hummingbirds. All kinds of birds use eucalyptus, the ivy it supports, and the understory it sustains, as habitat.  [Related content: The Summer tanager and San Francisco’s non-native plants.]
  • Coyotes use the forest and its dense understory as places to hide and escape people and dogs. [See: Video of a coyote on Mount Davidson.]
  • Bees use the nectar of the winter-flowering eucalyptus when nothing else is blooming. [See: Bees and the Blue Gum Eucalyptus and Herbicides ]

EUCALYPTUS MYTHS AND “CONVENTIONAL WISDOM”

We’ve tried to combat the disinformation by setting up a page listing some of the major eucalyptus myths. Most of the myths can easily be refuted by observers; others have fallen to peer-reviewed published research. The only reason they seem to persist is that those who hate eucalyptus wish the myths to be true, and others accept them.

To be fair, sometimes a myth is so widespread that it’s “conventional wisdom.”

Last year, we joined a guided sound walk of the forest. As we waited on Parnassus Avenue, someone on the tour said that nothing grew under eucalyptus. He’d been to Point Reyes and seen this. “It’s not true here,” we said, but he looked skeptical.

The tour entered the forest at Edgeworth, and climbed up the mountain to the Native Garden, where we paused. There, he sought us out. “I’m amazed by how lush the undergrowth is,” he said. “I had to tell you.” We were impressed that he’d do that. Most people don’t like being mistaken.

But we weren’t surprised. Before we started exploring, we actually believed that birds did not live in eucalyptus forests. Then, the first day we went into the forest, we found it alive with birdsong, and small birds  flitting around the canopy and sub-canopy, hiding in the understory, calling to each other. Later, we heard woodpeckers, and saw the Great Horned Owls that live in the forest.

That’s when we started to question our prior beliefs, and looked for more information. It was there, waiting to be found. We’d had no reason to think about it before.

So we hope this post is taken in the spirit it’s meant: A reason to think again, look for the evidence, and re-consider the myths.

And we’d like to end with an artistic video from photographer and artist Lori D’Ambrosio.

Click on the picture to get the video

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One Response to Fact Check: Nothing Grows UnderEucalyptus? (Allelopathy)

  1. Laura Chariton says:

    We have a wild European honey bee hive that has been in a 40″ diameter and 60 foot high topped Eucalyptus for the last 10 years.
    Every spring and summer we watch the hive swarm and split.
    Not only do bees love Eucalytpus but they live there. The Stellar Jays eat the bees near the hive entrance. The bees pollinate two acres of fruit and nut trees.
    Redwood trees and Tan Bark Oak love Eucalyptus and flourish right next to them.
    Mycellium fed by tree rain keep our property lush, green and moist all year long.
    Our water bills are tiny for the land.

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