It’s summer and it’s dry. You can’t remember the last time it rained. The warm wind blows past your ears, the fallen tree leaves and withered blades of dead grass crunch beneath your feet, and you’re thinking about fire, and maybe getting out of town, at least for a little while, maybe to a cool, rushing Sierran creek, lined with lilies and ranger’s buttons, hearing the water burble and a nutcracker cry instead of that worrying crunch.
But that’s because you’re an animal. If you don’t like the way things are, you can move. Plants don’t have that option. When things get too dry, or too hot, or too cold, or too wet, plants can’t skip town and rent a place somewhere nicer. They need ways to survive in place. In our dry summer and fall, many of them just… die. Oats, chia, goldfields, and scores of herbaceous plants have children in the form of seeds and drop them onto the soil or throw them to the wind and then just kick the bucket. This is why our grassy hillsides turn such a disappointing shade of taupe around June and become so disturbingly crisp a month later: they live fast and die young, but not before having a ton of hardy kids that will replace them when the rains return. Almost all of our favorite wildflowers are like this too. It’s called the annual lifestyle.
The other way is that of those oaks and their crunchy leaves: invest in roots to tap water deeper in the soil or even in rock, seal in your moisture with thick stems and waxy leaves or shed everything aboveground when things get dry, and just tough it out. Live long and prosper! Manzanitas, death camas, buckeyes, hound’s-tongues, and countless other sturdy plants follow a similar path: the perennial lifestyle.
But what most local annuals and perennials have in common is that they do their most important work, growing and breeding, in spring and early summer when times are good and water is abundant. That’s when the oaks are filling the air with pollen (much to the chagrin of some humans) and when our phacelias and mule’s ears and thistles are at their most ebullient. But when the party’s over, the annuals set seed and die, and the perennials start living on a budget while they wait for rain. Many humans choose to live in lowland California because we don’t have a harsh and freezing winter, when ice would destroy a tender leaf and snow could blot out the sun, but the aridity of our dry season can be just as brutal for our photosynthetic neighbors. Better to retire until the rains return.
Except, deep in the hot, dry doldrums of the Bay Area’s death sleep, a flower blooms.
Actually, a lot of flowers bloom. Seemingly in defiance of all good sense and logic, some plants of our scrub and grassland not only can perform their most difficult and essential metabolic work at the most oppressive time of year, they only do it at that time, with gusto and good cheer. Annual or perennial, they grow roots and leaves during winter and spring like their fellows, but then instead of dialing it back and retiring to the soil as roots or seeds, they double down and spend like maniacs, growing even more aboveground and blooming and reproducing long after the sane plants have called it a year. It’s like buying a house with the last of your savings, like running an ultramarathon after running a regular marathon, all with a smile on your face, every single year. Some species turn hillsides yellow, others provide flashes of crimson and sapphire, many scent the air with a perfumer’s panoply of aromas, an entire show that you might miss while driving east toward your Sierran creek.
So who are these gonzo flowers of fire season? How do they achieve the seemingly impossible? And why?
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See Summer’s Extreme Blooms
Where can you find all these summer blooms in the Bay Area? Here’s an iNaturalist map with summer sighting updates.
Coyote brush: the party perennial
Coyote brush (Baccharis pilularis) probably doesn’t top your list of iconic plants of the Bay Area, but it should. In grasslands or in scrub, if you’re looking at a shrub in this part of the country, it’s probably coyote brush, and that’s because it’s hardy, prolific, aggressive, and tough to eradicate. Sound like a weed? In some situations, it is! It can invade grasslands and transform them into shrublands, displacing grassland species and creating more fuel for fire. It also creates habitat for brush rabbits, wrentits, rain beetles, earthtongues, and countless other plants, animals, and fungi that depend on it for shelter, feed on its shoots and roots, or grow from the duff of its decaying leaves and branches. In a volatile, postcolonial, climate-changed world, coyote brush is a native plant that isn’t just surviving. It’s thriving and helping others thrive too.
But its craziest trick: It blooms in September and October. It carries out a blush-inducing bacchanalia of reproduction at the most bone-dry time of year. Entering a flowering stand of coyote brush can smell like stepping into a chamber of vaporized honey and sound like you’re nearing a hive for all the buzzing bees, flies, and other insects zooming around to feast on one of the only nectar sources around. When the party’s over and it’s time to set seed, the female plants can produce such a fleece of white, wind-dispersed seed-tufts as to look practically mammalian.
Coyote brush hosts an interrupted hornet fly (Spilomyia interrupta), yellow-faced bumblebee (Bombus vosnesenskii), Acmon blue (Icaricia acmon), and a Cylindromyia fly. (Illustration by Laura Cunningham)
So how does coyote brush pull this off? First, deep roots. Almost all summer blooming plants employ this technique, whether perennial or annual, because the most consistent and persistent source of water is underground, the deeper the better. Gardeners will recognize that even in the driest months, digging just a foot or more can often reveal some damper soil. Andrew Wright, a master’s student at UC Berkeley in 1928, painstakingly dug up a few coyote brush individuals “with a small ice pick” and found that they had sent down roots deeper than nine feet, even when the aboveground plant was hardly taller than three feet. Given that a mature coyote brush can be twice as tall or more, these roots are probably extending one to two NBA basketball players beneath the surface in search of water. There’s more of the coyote brush belowground than above.
Another of Wright’s findings was that in addition to those deep roots, coyote brush maintains a network of shallow roots. These help it exploit another important source of summer moisture in coastal California: fog. Working on a thesis at UC Berkeley almost a century after Wright, plant ecologist Allison Kidder proved coyote brush supplements its water supply with fog, particularly as seedlings, suggesting fog may be important in the plant’s ability to invade grasslands. She also found, amazingly, that coyote brush loses water at an increasing rate from spring through fall, culminating in its bloom in September and October. The less water it has, the more it spends, a spendthrift party animal almost until the rains come again.
(Illustration by Laura Cunningham)
Vinegar Weed: Toxic Avenger
After traversing a beige grassland as dun as the dullest pair of hiking pants at REI, the sight of vinegar weed (Trichostema lanceolatum) is a revelation: shining verdigris leaves speckled with shocks of reaching purple-blue flowers. Summoned, you approach, reach to touch this crystalline living gift amid a mat of death, stoop to inhale its swee–HOLY CRAP WHAT IS THAT SMELL?! Vinegar weed is a looker, but it is also a stinker, emitting a bouquet of organic compounds equivalent to an odiferous roar: DO. NOT. EAT.
Much like coyote brush, vinegar weed bolts and flowers fairly late in the year, blooming at a time when there’s less competition for pollinators and the sunlight is plentiful. But this poses a problem: If you’re the only green growing thing around, you might look attractive to pollinators, but you also look awful tasty to herbivores desperate for a juicy mouthful. Some summer-blooming plants dissuade such attention with a medieval armament of spikes (like star thistle and spikeweed), but vinegar weed has chosen better living through chemistry. It literally exudes acetic acid, better known as vinegar, and if you pretend to be a deer and stick your face in it, you will immediately understand why deer don’t do that. Its effervescent pungence will startle you awake, but it doesn’t exactly whet the appetite. In addition to vinegar, vinegar weed emits an array of interesting volatile compounds, including terpinen-4-ol, the primary component of tea tree oil and a powerful toxin acting against bacteria, fungi, mites, and even other plants. This compound has been shown to inhibit the gut microbiota of deer, suggesting that in addition to simply exposing them to an arresting smell, eating vinegar weed might arrest digestion in those desperate mammalian herbivores.
Curiously, the chemical armory of vinegar weed has been shown to inhibit the germination of other plant seeds as well, leading some experts to theorize that it may stunt or stop the growth of other plants around it, but field experiments have shown that despite that toxicity, it has no significant effect on its neighboring plants in the real world. Terpinen-4-ol has also been shown to be toxic to mosquito larvae, so it seems probable that while these defenses may not do much against competing plants, they do probably guard against caterpillars in addition to deer and cattle.
Turkey Mullein: Setting a table for the enemy of its enemy
Let’s be frank: When it comes to California’s many botanical wonders, no one is wondering much about turkey mullein (Croton setiger), also known more poetically (if not more admiringly) as doveweed. This relative of poinsettia has little of its Christmas cousin’s pizzazz, opting instead for muted blue-green foliage, demure and tiny flowers of a similar color, and at best ankle-high stature. If you find yourself charmed by this gray tribble of September, it might be for lack of other options.
But you wouldn’t be alone. In addition to moisture-retaining, sun-reflecting hairs and potentially toxic foliage (it was used as a fish poison by California Indians and cattle tend to avoid it), turkey mullein charms an unexpected cadre of admirers: spiders. Researchers at UC Davis noticed that the hairy leaves of turkey mullein often collect pollen that may be blowing around in the hot fall winds. They theorized collecting pollen might attract generalist insect predators that feed on other insects that would otherwise eat the turkey mullein. To test this, they coated some wild turkey mullein with pollen, left some others alone, and counted the bugs on both. Weirdly, while they didn’t see too much difference in the abundance of insect predators on the plants with added pollen, they observed a marked increase in the number of tiny spiders on those plants. This is a bit of a mystery, because almost all spiders are obligate predators that only eat other animals. Why would they care about pollen? But apparently, even though these researchers didn’t observe it in their experiment, some spiders do supplement their diets with pollen, so perhaps turkey mullein adds to its defenses against insects by setting out an appetizer for arachnids that then, for their main course, protect the plant by devouring aphids, caterpillars, and other hungry, hungry marauders.
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Tarweeds: All that and more
Are you noticing some patterns here? From deep roots to chemical defenses to luring in useful thugs, the flowers of fire season often adopt the same strategies to survive in dry times. But tarweeds? They do it all, and more.
Tarweeds belong to a tribe within the sunflower family containing many faces familiar to the California flower fancier, from the shrubby seaside woolly sunflower of the coast (Eriophyllum staechadifolium) to the hill-coating golden carpets of Monolopia to be seen in Southern California’s Carrizo Plain and elsewhere. Believe it or not, the gargantuan Hawaiian silverswords you may have seen on top of Haleakalā or Mauna Kea are also tarweeds, the descendants of a Californian species that probably hitched a ride across the Pacific on (or in) a bird about five million years ago. Californian tarweeds bloom at different times of year, but it’s the summer- and fall-blooming species that help define the look and smell of our dry season flora, from the superabundance of white (or yellow) hayfield tarweed (Hemizonia congesta) in areas with fog to Heermann’s, San Joaquin, and virgate tarweeds (Holocarpha heermannii, obconica, and virgata) of the baked interior.
Common madia tarweed with an assassin bug
(Pselliopus spinicollis) and Heliothinae caterpillar (Illustration by Laura Cunningham)
There’s so much to talk about with tarweeds. Similar to other annuals like vinegar weed and turkey mullein, the late-season tarweeds put down deep roots to tap water long into the dry months. They tend to germinate in the winter alongside their spring-blooming relatives, but instead of growing up, they grow down. When June rolls around and most other annuals are calling it quits, the late-season tarweeds start to bolt, ascending among the desiccating grasses without too much fanfare, only to unfurl in July through October, using that springtime investment to dapple our grasslands with pops of white and gold.
Let’s talk about the tar. Most tarweeds exude a sticky substance from glands that are often borne on specialized hairlike structures called trichomes. If you walk through a field of hayfield or Heermann’s tarweed, your clothing will bear the evidence of this. So what’s it for? It probably dissuades large herbivores by feeling gross to the touch and (probably) tasting gross, but just like the turkey mullein’s pollen offering, tarweed tar can indirectly ward off smaller herbivores, albeit in a more sinister way. Like a Pleistocene mammoth getting stuck in a Los Angeles tar pit, many insects that try to feed on tarweeds get stuck in their glandular secretions and die of starvation. If you take a close look at coast tarweed (Madia sativa) or common madia (Madia elegans), you’ll often find they’re covered in tiny corpses of gnats, aphids, and other minuscule insects. The same UC Davis researchers that sprinkled turkey mullein with pollen noticed this and that other insects seemed to navigate the tar just fine, including one of tarweeds’ most abundant predators, the caterpillars of moths in the subfamily Heliothinae. The researchers tried adding extra dead fruit flies to some individuals of common madia and leaving others alone, and they found that adding more dead flies both attracted more predators and resulted in fewer buds damaged by caterpillars. So one important function of the tar is probably to attract scavenging, caterpillar-slaying defenders like assassin bugs and stilt-legged bugs with a charcuterie board of insect cadavers. Tarweed species that bloom later in the season tend to exude more tar, so perhaps these are enhanced defenses at a time of year when tarweeds might be the only edible plant around for caterpillars to munch.
Tarweeds also survive the dry times with . . . jam. Making most jams involves adding a packet of pectin, a kind of plant-derived sugar that binds strongly to water and thickens your jam into a lovely, wobbly goop. Tarweeds are packed with pectin-like sugars, and the later they bloom, the more they have. In tarweeds, the pectin-like sugars attach to the outside of their cell walls, between the cells, where they bind to water, and the resultant goop insulates the cells against water loss under drought conditions that would wilt another plant. Researchers have compared leaves of hayfield tarweed (Hemizonia congesta) containing higher concentrations of pectin-like sugars to those that have lower concentrations, and gooier leaves retain water longer under conditions of drought stress. The ancestor of all tarweeds probably evolved in a montane setting like the Sierra (the tarweeds that seem to compose the most ancient lineage are the raillardellas, which are exclusively montane), a similarly dry environment where this gooey “mucilage” may also serve as a defense against formation of ice crystals inside the cells. Jam to keep you moisturized, jam to keep you from freezing. Are the tarweeds on to something?
It’s also hard to ignore the many wonderful (and not so wonderful) smells of the tarweeds, due to the variety of terpenoid organic compounds they release from their glands. These also probably play a role in dissuading herbivores, especially in the dry months when the plants become more of a target, but for us humans they are fascinating, and diagnostic. While not nearly as arresting as vinegar weed, small tarweed (Madia exigua) can hit you with a Jolly Rancher–like pop of fruity tones when rubbed between your fingers. Coast tarweed (Madia sativa) smells like a mix of pavement and gasoline, while the similar-looking grassy tarweed (Madia gracilis) is pleasantly lemony. I swear common hareleaf (Lagophylla ramosissima) sometimes smells like pickles, and to me, the astringent yet rich and warm smell of Heermann’s tarweed (Holocarpha heermannii) is the scent of summer, a reminder of many pleasant afternoons around Mount Diablo. Bruce Baldwin, a world authority on tarweeds, describes pale yellow layia (Layia heterotricha), a rare species from the southern Coast Ranges, as “simultaneously evocative of freshly cut apples, pineapple, and banana” and the common Fitch’s tarweed (Centromadia fitchii) as ranging from “eucalyptus-like” to “urinal deodorant cake.”
I hope I’ve convinced you to spend a little time with our flowers of fire season, and that in reading about them you’ve noticed some common themes. Many have independently evolved similar strategies for dealing with drought and with herbivores. Some of their chemical innovations have been noticed and used by people for millennia, both as medicines and as fish poisons (vinegar weed, turkey mullein) and even as hallucinogens with religious significance (as with sacred datura, which—warning—can also be fatally toxic). Almost all the plants I’ve discussed have the word “weed” in their name, indicating a somewhat adverse relationship with English-speaking peoples (searching for information about tarweeds inevitably brings up agricultural advice on how to kill them) but also their ability to flourish under disturbed, challenging conditions. We often lament the decline of California native plants in the face of wave after wave of newly introduced weeds from elsewhere, but despite incursions by drought-adapted introductions like yellow star thistle (Centaurea solstitialis), many of our late-blooming natives seem capable of holding their own. Indeed, some of our summer bloomers are themselves invading other parts of the world, like turkey mullein in Australia and panicled willowherb (Epilobium brachycarpum) in Europe. Their hardiness can also make them excellent low- to no-water garden plants, like common madia and California fuchsia. Much of California is becoming hotter and drier as climate change continues to transform the world, but these plants seem ready for it. Their ancient ancestors survived ice ages in the past, their recent ancestors survived colonization, and the descendants of these gritty, spiky, sticky, perfumed, exuberant creatures will undoubtedly survive this round of upheaval. So if you’re driving up to the mountains this summer, take a pit stop in the searing foothills and sniff a tarweed. It’s the smell of success.
