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Irish Shamrocks and Other Plant Confusions

Happy St. Paddy’s Day to ya. After our recent cold and snowy weather, it’s nice to be celebratin’ a bit o’ th’ green. My grandmother told me we were “part Irish.” But then, as an adult, I realized her family name was Stewart, the 7th most common Scottish surname. I’m now confused about how Irish I really am.

Likewise, many people may be confused about Irish shamrocks. The original shamrock is a three-leafed clover identified as white clover, Trifolium repens. The plant that we now call a shamrock, with purplish-colored leaves and delicate flowers, is an Oxalis. Most types of Oxalis are invasive and should not be planted outdoors. Even in isolated containers they will “spew” their seeds into areas we cannot control.
One well-known, weedy oxalis is Oxalis corniculata, creeping woodsorrel. It has lovely, trifoliate leaves with broad, heart-shaped leaflets and small yellow flowers. A native of Europe, it’s now widespread throughout the southwest. It grows in lawns, flower beds, gardens and greenhouses. I have it sprouting in the cracks of my driveway!
The University of California Integrated Pest Management describes it like this:
“Creeping woodsorrel is a major weed in turf, ornamental plantings, and nurseries. Infested container stock can contaminate uninfested landscapes. As seedpods mature and expel seeds, creeping woodsorrel spreads from container to container, from flower bed to flower bed, or across ornamental plantings. Creeping woodsorrel can establish rapidly in semishaded areas of new or established grass lawns or low-growing perennial ground covers. It spreads during mowing and other cultural operations.” Vigilant hand weeding is one way to try to control creeping woodsorrel.
Yellow flowers and “spreads during mowing” brings to mind another plant not native to California. However, this plant is much tougher than the low-growing sorrel. It, also, can take the form of a low-growing mat, if mowed too early and too frequently. Or it can grow, in the presence of tall competition, to be five or six feet tall.
At lower elevations this yellow-flowered plant has just formed innocent-looking rosettes reminiscent of dandelions. At mid elevations free of snow, it’s just beginning to germinate. That tiny four-leafed plant looks so innocuous with its rounded, softly-fuzzy green leaves.
For the next few weeks, there will be little top growth. The yellow-flowered plant is putting all its energy into growing a tap root that will eventually extend as much as two meters into the soil. With that hefty taproot, it is able to out-compete blue oaks and native purple needlegrass (Nassella pulchra) for summer water. It can create drought conditions, literally stealing soil water in mid to late summer.
Its bright yellow flowers are the only color in the late summer landscape along roadsides. Bees love those bright yellow blooms and, I’ve heard, create tasty honey from them. However, the plant is poisonous to horses and, once those bright inflorescences are protected by the ring of stiff, protuberant spines, no animal will graze it. It’s painful to animals and humans alike.
Have you guessed the bright yellow flower yet? It’s yellow starthistle, Centaurea solstitialis. NOW is the time to do something about it. In the four-leaf stage, it can be hand pulled or sprayed with herbicide. At the rosette stage, it can be hand-pulled, hoed or sprayed. Once the plants have bolted-sent up tall stalks-in summer, it can be grazed by sheep, goats, cows, mules or donkeys. Just before the flowers open, it can be effectively mowed. Standing skeletons from last year (“Q-tip phase”) can be gathered and burned.
The goal is to prevent seed formation. The University of California is hosting informational workshops about yellow starthistle control in local neighborhoods. The next workshop is scheduled for Thursday, March 24, at 6:00 p.m. at the Evangelical Free Church on Ferretti Road in Groveland. Workshops are being planned for Columbia/Gold Springs, Ridgewood, Phoenix Lake Road/Cedar Ridge, and Twain Harte. If you’d like to schedule a workshop for your neighborhood, please call the UCCE Natural Resources Program at (209) 533-6993.

Erin go bragh!

Rebecca Miller-Cripps is currently working on the Yellow Starthistle Leading Edge Project. If you would like additional information about Mother Lode-specific gardening practices, including weed control, please consider purchasing a copy of the Tuolumne County Master Gardener book, “Sharing the Knowledge: Gardening in the Mother Lode.” Copies are available from the UCCE office at 52 North Washington Street in Sonora. Cost is $25 including tax.