Spraying herbicides on water hyacinth in the Delta | Photo: California Department of Boating and Waterways

An explanatory series focusing on one of the most complex issues facing California: water sharing. And at its core is the Sacramento-San Joaquin Bay Delta. Stay with kcet.org/baydelta for all the project's stories.

The Bay Delta region is pretty much a paradise for plants. There's a lot of water, even in a historic drought. There's moderate temperature with hardly any freezes, and plenty of sunshine almost year-round.

That's true for native plants, and it's also true of those plants that we brought into the state in the 19th, 20th, and 21st centuries.While the vast majority of introduced plants more or less behave themselves, there are a bunch that multiply out of control -- and they cause both ecological and environmental damage as a result.

This is by no means an exhaustive list of invasive plant species in the Delta, but it's a start. Some of them are pests that we brought here by mistake, and others we imported deliberately. All of them bear close watching.

Water hyacinth

This plant might just constitute the single most-destructive species introduction in California history. Commonly called the "fastest-growing plant in the world," a single small water hyacinth -- Eichhornia crassipes -- can grow to cover 6,500 square feet of open water in a single growing season.

Water hyacinth was reported from the Delta in the 1940s; the species was spreading in the the Sacramento River basin farther north as early as 1904, imported by an avid water gardener. And it's not hard to understand why: it's purple blossoms are showy, and it grows with very little effort on the part of the gardener other than tossing it into the local pond.

There, it disperses itself by means of the little float in each leaf stem. The plants propagate by budding and by setting abundant seed -- and those seeds fall to the bottom of the water, where they can stay viable in the muck for many years. Even if they didn't set seed, the plants would be a menace: they can form floating mats six feet thick, shutting off light and depriving native organisms of nutrients, then turning the local water quite acidic when they decompose.

State, federal, and local agencies charged with keeping waterways clear -- headed up by the California Department of Boating and Waterways -- spend a huge amount of money each year combating water hyacinth, removing tens of thousands of truckloads from sloughs and aqueducts in a typical year. While other invasive plants might still be eradicated from California, the state's weed control experts pretty much admit they have all but given up on doing anything but ongoing control of water hyacinth. The situation is so hopeless, apparently, that you can still buy the plant and take it home: aquarium shops sell it as a nice houseplant for your fish tank. If you happen to have some in your home aquarium, do not dump it in your local storm drain.

Brazilian waterweed

This plant, known botanically as Egeria densa, is a good example of why you shouldn't empty your freshwater aquarium into a storm drain: it made it to the Delta as a result of being sold in aquarium shops. Now it covers thousands of acres of the Delta's open water, floating at or just below the surface. No one knows exactly when the stuff first showed up in California waterways, but it got to the Delta about a half century ago, and it's been growing like mad since. Outside the Delta it's widely distributed in Sierra Nevada lakes below 7,000 feet, along the Central Coast, in the Central Valley and in spots in the Peninsular Ranges.

Brazilian waterweed may be implicated in the decline of the Delta smelt: it restricts water movement and traps floating organic matter, which cuts down on the amount of food available to the Endangered smelt.

Able to root in as much as seven feet of water, Egeria quickly forms thick, ropy mats that can foul propellers and pumps, posing a management hazard for marinas and ports. And if a powerful metal propeller has trouble chopping through the stuff, just imagine the obstacle Brazilian waterweed poses to the Delta's fish. The only feasible method of control in the Delta is herbicide use: mechanical removal poses a danger to small fish, and while big fish have been introduced to Southern California lakes to eat Brazilian waterweed, that approach in the Delta would be a little like bringing in wild dogs to eat your feral cats.

Spongeplant

This is a new entrant in the field of invasive aquatic plants in the Delta. First confirmed in the state a little bit more than a decade ago, spongeplant (Limnobium laevigatum) wasn't detected in the Delta until 2008, but there are a few things about this plant that have sent the Departmemnt of Boating and Waterways and other agencies into high alert.

There's its growth rate, close to that of water hyacinth. Like water hyacinth, spongeplant can create large mats of vegetation from a single floating plant. But spongeplant is smaller that water hyacinth, each individual plantlet about an inch tall, which means that spongeplant can withstand frosts that would kill back water hyacinth by sitting beneath the hyacinth's four-inch leaves for shelter.

Spongeplant's seeds float, as do the tiny seedlings that emerge from them, which makes it even easier for a plant to disperse its progeny downstream.

The potential effects of spongeplant are much like those of the other floating aquatic plants discussed here: crowding out native species, depriving fish of nutrients, changing the pH of the local water, and fouling equipment.

And given that the tiny floating seedlings can hide effectively in native vegetation such as tules and cattails, eradication will probably prove quite difficult.

Giant reed

Scientists sample tissue from a giant reed stand in the Central Valley | Photo: USDA /Flickr/ Creative Commons License

So far we've been talking about wholly aquatic plants that can choke out habitats by their sheer mass, but which are generally rather small. Giant reed, Arundo donax, is different: It grows on land, and it's huge: clumps of this tall grass that are just getting by in less-than-optimal conditions routinely reach nine feet in height. Given ideal conditions, Arundo donax can top out at 30 feet, its hollow stems an inch and a half thick.

Though it's not a native plant, Arundo has been in California a looong time. It grew along the banks of the Los Angeles River in the 1820s, where it was planted as a raw material for thatched roofing and other forms of shelter, as well as food for livestock.

It's also the preferred source of reeds for woodwind instruments, and at least one California plantation is currently growing instrument-quality reeds for musicians.

Giant reed grows anywhere it can find soil that stays moist for much of the year, and where it has at least a little drainage. In the Delta it forms incredibly dense stands on the banks of bodies of open water, where it can literally crowd out native plants from small wildflowers to mature cottonwoods or elders.

One of the main issues with Arundo is its relationship to fire. A stand a few years old will have a large amount of dried biomass in it, much of it made up of those hollow canes that can act as their own little chimneys. Plants that have managed to survive crowding by Arundo will often succumb to the fires the reeds promote.

Fortunately, there are some uses for giant reed besides facilitating the use of saxophones. The species' startling growth rate makes it a promising candidate for fuel for biomass power plants. It's also been studied as a possible carbon sequestration crop: cutting down the stems and mulching them as a soil amendment could put the carbon dioxide Arundo pulls out of the atmosphere to good use. Giant reed is also used in a traditional form of construction called "cañizo," in which the stems are woven into panels that are then covered in plaster or adobe.

Since Arundo in California reproduces only vegetatively, that means there's no seed bank to keep it going once its rhizomes are thoroughly removed from the soil. And that means that if we come up with enough ways to use Arundo in large amounts, we might be able to limit the future damage the plant causes the Delta.

Yellow star thistle

There are some folks who object to the stigma occasionally placed on invasive plants. They remind the rest of us that there is beauty to be found in many of the plants currently deemed as problems, that blue gum eucalyptus provide perches for hawks and monarch butterflies, that songbirds feast on Himalayan blackberries, that fennel and red valerian and pampas grass and even Scotch broom have their virtues.

I've never seen any of those people say anything good about yellow star thistle.

Known botanically as Centaurea solstitiali, yellow star thistle is thought to have been imported into the state as a contaminant in alfalfa seed sometime in the 1840s. It's been spreading in the Delta region for at least a century. In 1919, botanist Willis Lynn Jepson said that the plant's abundance near Vacaville had grown by a factor of 1,000 in the previous decade.

To read the description on the California Invasive Plant Council's website of yellow star thistle's spread across the state is like watching a 1950s horror movie.The plant covered a million acres of the state in the late '50s. That grew to two million acres by 1965, and to ten or twelve million by the mid-1990s.

Twelve million acres, in case you're wondering, is a little over nine percent of the state's land area. And the plant didn't stop spreading in the 1990s.

Yellow star thistle is a winter annual, which means it grows from seed during the wet season, blooms, makes more seed -- up to 75,000 viable seeds per three-foot plant -- and then dies. Usually. A small percentage of plants survive to make even more seeds in a second year.

Originally hailing from the southern Mediterranean, yellow star thistle came prepared for California's climate regiment of cool, wet winters and hot dry summers. in the old country, the plant has a multitude of insect predators that keep its population in check. That's not so in California: without bugs eating its leaves, flowers, and seeds, yellow star thistle has been able to colonize mile after mile of California grasslands.

Though the plant is palatable to grazing animals early in its life, by the time it blooms it's grown some defenses: namely, a set of fearsome spines up to an inch long on every flowerhead. That deters wildlife and livestock from chowing down on the plant when it's most nutritious. Animals such as horses can suffer long-term health problems from chronic consumption of the plant.

In the Delta, yellow star thistle can be found in infrequently submerged uplands including grasslands and levees. Its thick growth not only deters passage by wildlife reluctant to get stabbed by wicked spines, but can also crowd out other plants. And the weeds' deep taproots allow yellow star thistle to take moisture from the soil, depriving other plants of the water they need.

That's not to say there's nothing good about yellow star thistle. Those same taproots serve to loosen and aerate soils that have been damaged by human activity, and the flowers are a source of nectar for a wide range of native butterflies and other pollinators.

The plant is also a good fodder source for goats, who, being goats, will eat it even when it's full of spines. That happy fact has been put to good use in spot control of thick stands of star thistle. Weed researchers have found success in a combination of mowing, grazing by goats, sheep, and cattle, and overseeding with clover in reducing star thistle infestations on test patches.

There's also the idea of bringing some of the thistle's European pests over to California, which has been tried with limited success. One big issue, of course, is that the biologists exploring this avenue for star thistle control need to make sure the imported insects won't eat the state's beleaguered native plants. So far a few beetles, fruit flies, and a fungus have been brought into the state -- one of the fruit flies by mistake -- many of which have shown limited promise in reducing infestations.

In the meantime, should you have the opportunity to enjoy hiking through a stand of star thistle that's gone to seed, be careful to remove any of the plant's seeds that have stuck to your clothing. Their microscopic bristles can be relatively persistent, and you can't count on laundering to kill them.