Frank Landis' Blog

Wow, 3.5 inches of rain in the last two days! Yes, yes, I know anyone outside southern California wouldn’t even bother with an umbrella for that little, but around here, the news media are howling about THE FLOOD. Worse, according to at least one outlet, we environmentalists have caused the flooding (!), because “we” stopped San Diego from pulling on its 1950 tin hardhat and running bulldozers through all the creeks and storm drains so that they’ll drain straight to the ocean. Of course, “THE FLOOD” has occurred where water backed up into the flood plains of the San Diego and Tijuana Rivers, areas that flood during every major wet event, but that doesn’t get much attention. Worse, if all the crap in the storm channels went into the Pacific Ocean, it would trash San Diego’s famous beaches and drive off the tourist. This is the real reason why the city is having trouble implementing this plan, as it runs contrary to the Clean Water Act. But that doesn’t really bubble up to the media surface either. Flooded roads! Drama! Media exposure!

Where was I? Oh yes, there are in fact plants growing inside the flood channels despite the glowers of the SD Storm Water Department, and they are the topic for this entry. The plants? Willows and seep willows, two plants that grow great together. Both of them like water, whether it’s in seasonally flooded arroyos or on the shores of rivers and lakes. Or in the city’s undeveloped canyon bottoms.

To start with, “seep willow” is an alternate name for mulefat, and it isn’t a willow. Instead, Baccharis salicifolia is one of the biggest Asteraceae in California. It’s often mistaken for a true willow (genus Salix), but the white Baccharis composite flowers usually give it away. Mulefat blooms in fall, but the dead flowers and buds are around most of the year. The smell is distinctive too, resinous and easier to remember than describe.

As for telling the true willows apart, that’s a bit harder. My favorite system for southern California is still the one Tom Chester published a few years ago. Go to that site to learn the five willows that are common in San Diego. Actually, there’s only three that are common (four in the desert), so they are not hard to learn.

Why do willows and mulefat look so much alike? The reason is convergent evolution, which is a technical way of saying that living anywhere that can flood is a bit dangerous, and very different species have figured out the same formula for surviving a flash flood. If you’re a plant, you don’t get to run away from danger, and where it floods, you’ve got to deal with surges of water, along with the gravel, rocks, and other debris that come with the water. Both willows and mulefat have worked out a strategy that includes being able to resprout from a stump or branch (useful if a branch gets broken off and carried downstream), having very flexible branches, and long, streamlined leaves. It’s fun to play with the branches of willows and mulefat. They flex all over the place, and both are useful for basketry (think wicker) because of this. These are plants that bend before the water rather than breaking, and if they do break, they can resprout if the flood leaves them in suitable soil. This last property is beloved by restorationists, who propagate these plants from cuttings, because they will grow rapidly and keep banks from eroding.

Sexually, willows and mulefat are also similar. Mulefat blooms in the late fall, willow blooms in winter, and both species have fuzzy seeds that float on the breeze. This is probably convergence again: releasing seeds during flood season is a way to colonize sandbars and other areas scoured bare by floods. I still can’t explain why both mulefat and willows have separate male and female plants, but they do. The easy way to spot the females is to look for unreleased seeds and the resulting fuzz, and it’s easier with mulefat than with willows. The fact that each plant is a single sex can be a problem for restorationists, as they need to plant both male and female plants to insure that they will have breeding partners. It’s a minor but important detail.

Finally, if you look around, you will find mulefat away from the seasonally flooded arroyos it prefers. Often it has been planted, but still, it is common to find mulefat in areas that seldom, if ever, flood. What is it doing there? The simple answer is it’s growing. Happily. The thing to remember is that the mulefat is adapted to surviving floods, but it doesn’t particularly need floods to survive. If there’s a bit of water in the soil, mulefat can happily grow with other dryland Baccharis species (such as coyotebrush or broom baccharis). However, if the site floods, I’d bet that mulefat would be the only survivor. Everyday living isn’t an emergency, despite what the Storm Water Department might want us to believe.