Alliaria petiolata
Native range: Europe, Asia, Northwest Africa
Invasive range: Much of the Lower 48, Alaska, and Canada. (See map.)
Habitat: Moist, shaded soil of floodplains, forests, roadsides, edges of woods, and forest openings. Often dominant in disturbed areas.
Description: Biennial herb. First-year plant has a rosette of green leaves close to the ground. Second-year flowering plants 2 to 3 feet in height with small white flowers. Four petals in the shape of a cross. Coarsely toothed leaves have odor of garlic when crushed.

 
 
 
 

Garlic mustard is a Eurasian native likely introduced to North America by early European colonists as a food and medicinal plant—which then hopped the garden fence and went wild. In 1868, it was recorded “outside cultivation” on Long Island, flourishing in what field guides call “disturbed ground”: the edges of roads, railroads, trails, fields, and abandoned lots. From there, like most invaders, once it establishes in a new location, it invades undisturbed plant communities and becomes the dominant species. The insects and fungi that feed on it in the Old World aren’t present in the New. Because white-tailed deer rarely feed on garlic mustard, they may encourage it by consuming native plants instead, and their trampling as they browse disturbs the soil, encouraging seeds—which can germinate up to five years after being produced—to grow. Its roots inhibit the growth of below-ground fungi that some native plants require, reducing the ability of tree seedlings to survive in a sea of garlic mustard.

Garlic mustard has headed west from the northeastern coast, taking the eastern and midwestern US, crossing into Canada, mounting incursions into western states, including the Pacific Northwest. Reproduction is entirely by seed, and each plant produces about 350 seeds, which means about 100,000 seeds per square foot. Its spread in the East has been exponential, nearly 4000 sq mi per year, making it the dominant under-story species in woodland and flood-plain habitats, where eradication is most difficult.

The plant spends its first year as a rosette, then bolts in early spring of its second year, sending up a stalk over 3′ tall, so it’s hard to miss. Leaves are dark green, coarsely toothed, and deeply veined. All parts of the plant smell like garlic, though less so towards fall. The basal rosettes stay green all year, even under snow.

We’re the only ones who will eat it—even white-tailed deer won’t touch it—so forage away, as long as you’re at least 1/4 mile from the side of a busy road. Avoid plants that may have been treated with weed killer or other pollutants. Make sure there’s no poison ivy growing in with the garlic mustard. Make sure you have the right plant—the rough-toothed leaves and garlic odor when crushed are giveaways—then pull it up by the roots. Don’t scatter any seed as you bag up the whole thing. The roots will keep it fresh until you’re ready to cook. Then cut off the leaves, and discard the stalk and roots in a sealed bag for disposal. Do not plant or compost!

Wash the leaves. Young plants, with their mild mustard-garlic flavor, can be used raw in salads. Cooked, the flavor’s even more subtle. Older leaves, fresh or dried, come through stronger, tending toward bitter, and are great in soups, marinades and dry rubs, especially with meat. If the plant is in full flower or has produced seeds, it will be more bitter.

Ava Chin, urban forager at the New York Times, recently reported on her success with garlic mustard pesto:

“Back in my kitchen, I cleaned the garlic mustard by soaking the leaves in a bowl of water before drying them off. I then blended them in a food processor with extra virgin olive oil, walnuts, salt and pepper, and a teaspoon of apple vinegar. That evening, we had a wild-tasting seasonal pesto pasta with shaved Parmesan that even our toddler enjoyed.”

 

 
Undaria pinnatifida
Native range: Japan Sea
Invasive range: Southern California, San Francisco Bay, New Zealand, Australia, Europe, Argentina
Habitat: Opportunistic seaweed, can be found on hard substrates including rocky reefs, pylons, buoys, boat hulls, and abalone and bivalve shells.
Description: Golden brown seaweed, growing up to nine feet. Forms thick canopy. Reproductive sporophyll in mature plants is fluted and ruffled, forming around the main stem or stipe near holdfast of kelp. (Not found in native kelps.) Also known as Asian kelp.

 

Well, I asked for something to eat
I’m hungry as a hog
So I get brown rice, seaweed
And a dirty hot dog
I’ve got a hole
Where my stomach disappeared
Then you ask why I don’t live here
Honey, I gotta think you’re really weird.

“Brown rice, seaweed, and a dirty hot dog”: back in 1965 Bob Dylan, with just 10 words, sent the macrobiotic diet limping off the field in “On the Road Again.”

The “seaweed” in question was Asian kelp or wakame. It wasn’t an invasive species then but it is now, on the coast of California, having stowed away on cargo ships from Japan and China, where it’s native.

Since the 1960s, the word wakame has largely replaced the term “sea mustard” in the US. Originally the popularity of the macrobiotic diet created enough demand for the product to be imported dried from Japan and sold at natural-food stores and Asian-American grocery stores. Demand for wakame, if not for the macrobiotic diet, increased in the 1970s, thanks to the growing number of Japanese restaurants and sushi bars.

Wakame, or Asian kelp, Undaria pinnatifida, is an annual brown algae native to the northwestern coast of the Japan Sea: Japan, Korea, southeastern Russia, and eastern China. It has been recorded in Australia, New Zealand, Italy, Mexico, and Argentina. The likely means of these accidental introductions are farmed shellfish imported from Asia, or ballast-water or fouling organisms discharged by international cargo-ships coming from the Far East. It was deliberately introduced in Brittany as a crop in 1983, and then spread to Spain, the UK, Belgium, and the Netherlands. Wakame has been nominated one of the 100 worst invasive species on the Global Invasive Species Database.

Wakame invaded the east coast of Tasmania in the 1980s, probably introduced from the ballast of Japanese ships. License has been granted to divers for its harvest and its sale, fresh and dried, to restaurants and to home cooks in Australia.

Although it has been a very serious invader in New Zealand––so serious that all harvest of it has been banned––it does not appear, so far, to have serious impacts in California, where it was first recorded in 2000 in several estuaries from San Francisco Bay south to San Diego. It usually can’t compete with the native perennial brown algae, but if native algae aren’t present, it can colonize rapidly and form dense kelp forests. Wakame, like invaders on land, occurs in high densities in recently disturbed areas. The familiar “disturbed areas” that invaders love.

Once wakame has established in an estuary, it spreads in two ways: millions of microscopic spores are released by each fruiting body, and the weeds attach to ship hulls and to aquaculture equipment. Like all good invaders, it’s tolerant and opportunistic, growing on stones, rocks and reefs, shells, ropes, pontoons, buoys, and ships’ hulls. In the early stages of its life cycle, it can grow on other algae and sea-grasses. As an adult, it grows into kelp “forests.” The large canopy it forms modifies the habitats of the species that end up below, reducing light levels and water movement. The fronds may attach their holdfasts to shellfish on the seabed, whether the shellfish or their prey like it or not.

* * *

On the West Coast, scientists and managers have been removing the invasive kelp from infested marinas. This summer, workers in Oregon have been removing wakame and other marine organisms from docks and debris moved by in the Japanese tsunami. Controlling spread is critical to halting its impact.

 
 

Boat owners: The most important thing you can do is inspect and clean your boat of kelp before you move it.

Read more from the California Invasive Plant Council here.