Invasive Thistles & Native Plants

Thistles in our area Thistles are annual or biennial plants which are part of the sunflower family (Asteraceae). The purple or yellow flowers turn in to furry seeds that are dispersed by the wind. Controlling Invasive Plants Control of invasive plants can be a very Invasive Thistles & Native Plants daunting task. There are a variety of ways to go about control: manual, chemical, mechanical, There are many varieties of thistle, unfortunately most of the thistle species in and biological. San Luis Obispo are non-native and invasive. These thistles displace and out compete native plants for space, water, nutrients and sun light. Since these Manual an be extremely hard work and very thistles are relatively new to this region the native plants have not had adequate expensive in labor costs. time to develop strategies to compete with them. For example, the keystone, late In some cases, biological control can be an succession plant species in this area is the Coast Live Oak. It takes hundreds of effective and low cost solution, although extreme years for this tree to fill its niche in the ecosystem. It holds its place by creating care must be taken. a canopy that native plants can’t survive under. It drops acorns and leaves, creating a thick layer of duff that native plants can’t sprout through. But In Canada the Weevil (Rhinocillus Italian Thistle can grow fine through the thick duff and canopy shade conicus) was introduced to combat of the live oak, and when that oak tree finally falls, it might not foster the their invasive thistle problem. This little next generation of Oak saplings but a field of thistle instead. beetle burrows into the thistle, eating the Unchecked invasive thistles can dominate the flora developing seeds before they can mature. in some areas, minimizing biodiversity and making The weevil population surged as they ecosystems more vulnerable to disease and less ate the invasive thistles and when their hospitable for wildlife. food source ran low, the weevils moved on to the native thistle population. This effectively turned the biological control Invasive Thistles & Agriculture into an invasive species. Not only do invasive thistles adversely affect our natural environment but also Mechanical San Luis Obispo’s long tradition of agriculture. This region’s history of grazing means such as mowing are not land has shaped the landscape, perhaps more than any other human activity. very effective because thistles can regenerate As invasive thistles move into grasslands, cattle eat around them looking for new seed heads every time they are cut. Thistles more palatable plants, leaving the thistles free to seed in the newly disturbed need to be cut off 4 inches below ground level area. In time this can render large areas completely unviable for grazing cattle to effectively kill the plant. Thistles can produce and change the historic and cultural landscape of San Luis Obispo. viable seed heads at only 3 inches tall. Chemical application can have many draw Some of the most common invasive thistles in backs as well, including water contamination San Luis Obispo are: and adversely affecting other plants and animals. Italian Thistle (Carduus pycnocephalus), Yellow Star Thistle (Centaurea solstitialis), and Woolly Distaff (Carthamus lanatus). Not all thistles in San Luis Obispo are non-native. In fact, the Chorro Creek Bog Thistle (Cirsium fontinale var. obispoense) is an extremely rare plant which grows exclusively in San Luis Obispo County. The bog thistle is listed as a federal endangered spices. It has very specific habitat requirements, growing only in and around serpentine springs and seeps. In wet years their habitat can expand out in to the moist fringes of the seeps but in dry years their habitat range is restricted..

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