Oh joy, it’s January and the acacias are blooming.
You’d think it would be pure joy, all these showy yellow flowers when it’s still winter. I used to like them in a uncomplicated fashion myself, but now I have a double grudge against them. Unlike most flowers that are this showy, acacia blossoms are allergenic, and I’m allergic to them. We’ve had a few nice days, the winds are relatively lazy, and ow, my sinuses; oh, my lungs.
My bigger grudge is with just a few species—the same ones that are annoying me personally, for the most part. Acacia dealbata, Acacia decurrens, and a smaller, shrubby, thorny relative, Acacia paradoxa, have all escaped cultivation here and invaded wildlands. I believe Acacia melanoxylon is out there too, though the state officially calls it “rare” as an invader.
The problem with letting exotic trees replace natives is this: Wildlands aren’t ornamental gardens. Whether they look pretty to us is a minor matter. They’re workplace and pantry, as well as home, to every wild species we have—bugs and birds, fish and frogs, lizards and lions, and other plants as well. What looks like another green thing to us looks and tastes and acts very different to anyone who has to eat it, and many of our wild neighbors find an acacia, like many other exotics, completely useless. It’s as if we’d replaced everything in the grocery store with that spooky plastic reproduction food you see in the windows of sushi bars. The fact that little or nothing eats them is one reason invasives manage to invade, and often look healthier than they do in their home ranges. Biologically, they don’t pay their way.
They aren’t standard common street trees, at least. There are a few senior specimens left along Martin Luther King, Jr. Way, just across the border into Oakland. Most of the ones I notice are in parks or yards. A couple of the species here have one oddity: when they’re young, they have the feathery compound leaves that mark acacias, but as they age they grow simple, leathery blade-shaped “leaves” which are actually phyllodes, expanded petioles. (Petioles are the stem bits that hold leaves onto twigs.) If you cut a branch back far enough that it sends out sprout from its oldest wood, it might revert to the juvenile compound leaves. Go ahead, torture them, I don’t care.
Aside from the leaves, you can tell most of the local acacias by their yellow flowers. Some of them even smell nice, if you dare to get close enough. Bees like them. Hmph.
The reason acacias are here is that they’re handsome plants. You’ve doubtless seen that PBS photograph of an African plain with some large animal and a graceful flat-topped tree against a rosy sunset. That’s an acacia. Maybe a periodic giraffe drive from the Oakland Zoo along Route 13 would serve as a control, and they could sell tickets to the parade route. Koa, the mighty Hawai’ian canoe tree, is an acacia. (That’s one of those that have bladelike phyllodes.) The wattles of Australia are acacias. There are about 800 species of them around the world.
There are several Central American and African species, the swollen-thorn acacias, that house symbiotic species of ants in those swollen thorns. The ants defend the tree from insect and even animal predators by rushing out and biting the bejabbers out of them with their formic-acid-laden jaws; in turn, the acacia gives them fortified homes in the hollow, swollen thorns.
Acacias tend to be nitrogen fixers—they have associated bacteria that take nitrogen from the atmosphere and add it to the soil. Koas even have nitrogen-fixing nodules above the ground. Making fertilizer is a good quality in a farm crop, not so much in a wildland plant here because it favors the growth of other invasives. Like many nitrogen fixers, some acacias have nutritious, relatively high-protein seeds that are traditional human foods in their home ranges. Some are being considered for cultivation, because they’re drought-tolerant, generally tough, and because tree crops use less in the way of resources in the long run.
All very well if we were farming or even eating them. Inhaling them, though, makes it hard to appreciate those cheerful yellow posies.