Image Credit: Cobwebby Thistle by Eric Hunt on Creative Commons
Many of us have fought an ongoing battle trying to eradicate yellow star thistle from our properties. Introduced by accident in imported alfalfa in the 1800s, star thistle (Centaurea solstitialis) has become one of the most noxious weeds in the state. Dispersing over 75,000 seeds per plant, star thistle outcompetes native species destabilizing entire ecosystems. The spiny heads can also injure livestock. It causes a neurological disorder in horses leaving them unable to eat or drink.
Hand pulling seems to be the only effective way to control star thistle. The Santa Lucia Conservancy recommends planting or seeding native vegetation after hand pulling to compete with the following season's emergence. May the Force be with you.
Another invasive thistle that is particularly rampant this year is the purple Italian thistle (Carduus pycnocephalus). This is the one that jams up your weed whacker with its fibrous stems. Like the star thistle, the Italian thistle crowds out native species and is particularly dangerous as a fire ladder into tree canopies.
With their prickly stems and spiked flowers, you might conclude that all thistles are bad. Unlike introduced thistles, the cobwebby thistle (Cirsium occidentale var. californicum) is a native thistle of strategic importance to local diversity. The blood-red to purple flowers are surrounded by long spines laced with fibers resembling cobwebs. Don't pull this one! It serves as a larval host to several butterfly species including the painted lady.
Some invasive plants are dangerous not only to livestock but also to humans. Not to be confused with native desert parsley which it resembles, poison hemlock (Conium maculatum) is deadly. All parts of the plant are poisonous; ingestion can lead to respiratory paralysis. Seeds spread quickly so hemlock should never be mowed and burning can release toxic fumes. The best control is achieved by digging up the entire plant, wearing protective gear, and composting in sealed bags.
Some plants, harmless in their native habitats, become a problem when proliferated indiscriminately. Birds, wind, or inadvertent human activity can spread invasives into areas where they compete with native species. Yarrow (Achillea millefolium) is a welcomed flower in North America and Europe while in New Zealand and Australia, it can wreak havoc. In British and Irish superstition, yarrow was thought to attract a mate. Girls would place yarrow under their pillow and recite before going to sleep:
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