Invasive Plants and Noxious Weeds

To be categorized as invasive, plant species normally must have a high capacity to reproduce, grow in dense populations, out-compete native species, and be unpalatable to livestock and wildlife.

Bull (Cirsium vulgare), Milk (Silybum marianum), and Italian (Carduus pycnocephalus) thistle are all common on the Mitsui property, but have different distributions. All three produce large volumes of windborne seeds that tend to collect in the lee side of trees, rocks, logs, and tall vegetation. From these concentrated areas, the next generation of seeds spreads downwind, and local infestations grow outward from where the original seeds settled. All three species are well armored with spines and unpalatable to most livestock, although goats will eat all three.

Bull thistle is relatively new to the Mitsui property and is somewhat localized to the moister areas around springs or in swales. It flowers late, from June through October.

Butterfly on flowering Bull thistle ©Jeff Wilcox

Butterfly on flowering Bull thistle ©Jeff Wilcox

Milk thistle germinates with the first rains and flowers from March through June. This thistle probably came in with contaminated feed, and is now widespread in patches throughout the property. The largest infestations are at Rose Spring, near Dolcini Pond, and in 85-Acres Pasture.

Italian thistle germinates in late winter and flowers from May through July. It is widespread wherever it can find bare ground to get established. The presence of this plant can be an indicator of overgrazing, and has been a problem on Sonoma Mountain for decades.

Fennel (Foeniculum vulgareFennel has the potential to form massive stands where most other plants are excluded. Many roadsides in California are covered with dense populations of it. On Mitsui Ranch, fennel has been found and removed near the outflow of Copeland Creek in Middle Pasture.

Himalayan blackberry (Rubus discolorHimalayan blackberry tends to stay localized, and exists in most of the springs and streams on the Mitsui property. In some cases it forms a large, impenetrable thicket to the exclusion of other plants, but it also provides shelter, refuge, and food sources for certain wildlife species. It has become naturalized on the property.

Periwinkle (Vinca majorPeriwinkle grows only in the copse of California bay trees immediately north of the ranch house and hasn’t spread in the past 30 years. Care must be taken to keep it out of the streams and shaded forest areas where it could get established and take over.

Yellow starthistle (Centauria solstitialisYellow starthistle is a problem statewide, as witnessed along many roads, highways, and open pasturelands in dry areas of California. In dry lands, this plant exploits the presence of annual, shallow-rooted grasses and limited rainfall. It germinates in winter, growing a small rosette on the ground at the base of the fast-growing annual grasses. While grasses are growing up, yellow starthistle is using its energy to grow a long taproot that can access moisture far below the roots of the annual grasses. When the grasses have exhausted the moisture in the ground surface and begin to die back, yellow starthistle begins its growth spurt, producing seeds long into the late fall and sometimes early winter.

Yellow starthistle is widespread on Sonoma Mountain and the Mitsui property, and has been for decades. Grazers have attempted to control the infestations of both yellow and purple starthistle by spraying annually with Milestone (Aminopyralid), but control has been limited to the road and places where an All-Terrain Vehicle can easily reach. The infestations extend well beyond the areas being treated.

Purple starthistle (Centauria calcitrapa) and puncture vine (Tribulus terrestrius) are new invaders to the Mitsui property, probably brought in on contaminated agricultural equipment or feed in the last 15 years. Both are becoming established in the southeast area of the High Knolls Pasture, especially where barren ground offers an open place to germinate.

Medusahead (Taeniatherum caput-medusaeMedusahead was introduced into California in the late nineteenth century through contaminated alfalfa seed imported from Turkey. Since then, it has invaded most dryland pastures on the West Coast of the United States. Pastures on the Mitsui property are heavily infested with medusahead, especially in drier upland areas.

Of all introduced species, medusahead has the most deleterious effects on quality forage in pastures everywhere in California, including Sonoma Mountain. This grass is nearly completely unpalatable to all livestock. Some animals will eat it before it begins to mature in the early spring, but they must be confined in order to do so. The high silica content of the cell wall makes medusahead unpleasant to chew and renders it nearly indigestible to livestock. Furthermore, the silica in the cell walls doesn’t break down quickly, so dead plant material persists on the ground, forming dense mats that don’t allow other plants to grow and encouraging conditions for hot-burning grassland wildfires.

Medusahead © Stefan Gutermuth

Medusahead © Stefan Gutermuth