Summer has arrived and so has the dreaded, prickly presence of yellow starthistle, Centaurea solstitialis. Reports indicate that yellow starthistle infests between 10 and 15 million acres in California, making it the most wide-spread noxious weed in the state.
The foliage of this plant ranges from gray-green to blue-green; once flowering begins it can easily be identified by its bright, thistle-like yellow flowers, which have sharp spines surrounding their bases. This weed grows from 6 inches to up to 5 feet.
Yellow starthistle forms dense infestations and rapidly depletes soil moisture, preventing the establishment of other species. It is also poisonous to horses, causing a nervous disorder called “chewing disease” (nigropallidal encephalomalacia), which is fatal once symptoms develop. Horses are the only animal known to be affected – they should not be allowed to graze on yellow starthistle.
Yellow starthistle can be spread as a contaminant in grass seed and in all classes of hay, particularly grass hay. Hay used as mulch along roadsides or disturbed areas can be a source of yellow starthistle. Livestock that have fed in areas infested with yellow starthistle can be transporters of the seed.
Because the seeds germinate during the rainy season, a single cultivation after the rainy season, when soils are dry, effectively controls yellow starthistle seedlings and rosettes. Timing is critical: this cultivation must take place in late spring after the last rains but before seeds are produced.
Mowing can be used to manage starthistle. Be sure to mow well before starthistle is in full flower or has set seed. Mowing is most effective when soil moisture is low and no irrigation or rainfall follows mowing. If rain occurs after you've removed existing plants, seeds will continue to germinate and the eradication process must be repeated. Pre-emergent and post-emergent herbicides can also be helpful in controlling growth and infestation of starthistle.
Lastly, effective management of starthistle includes revegetation. Planting affected areas with desirable plants discourages starthistle regrowth by creating competition.
For information about herbicides and for photos of yellow starthistle's growth cycle, see the UC IPM Pest Note on Yellow Starthistle.
UC Master Gardeners of Butte County are part of the University of California Cooperative Extension (UCCE) system. To learn more about us and our upcoming events, and for help with gardening in our area, visit our website. If you have a gardening question or problem, email the Hotline at mgbutte@ucanr.edu or leave a phone message on our Hotline at (530) 538-7201. To speak to a Master Gardener about a gardening issue, or to drop by the MG office during Hotline hours, see the most current information on our Ask Us section of our website.
By Michele Carter, Butte County Master Gardener, July 12, 2013
Yellow starthistle seeds germinate from fall through spring, corresponding to the normal rainy season in California. After germinating, the plant allocates most of its resources to root growth and by late spring, roots can extend over 3 feet. The plant exists as a short rosette amongst other weeds until it begins to flower.
The foliage of this plant ranges from gray-green to blue-green; once flowering begins it can easily be identified by its bright, thistle-like yellow flowers, which have sharp spines surrounding their bases. This weed grows from 6 inches to up to 5 feet.
Yellow starthistle forms dense infestations and rapidly depletes soil moisture, preventing the establishment of other species. It is also poisonous to horses, causing a nervous disorder called “chewing disease” (nigropallidal encephalomalacia), which is fatal once symptoms develop. Horses are the only animal known to be affected – they should not be allowed to graze on yellow starthistle.
Yellow starthistle can be spread as a contaminant in grass seed and in all classes of hay, particularly grass hay. Hay used as mulch along roadsides or disturbed areas can be a source of yellow starthistle. Livestock that have fed in areas infested with yellow starthistle can be transporters of the seed.
Because the seeds germinate during the rainy season, a single cultivation after the rainy season, when soils are dry, effectively controls yellow starthistle seedlings and rosettes. Timing is critical: this cultivation must take place in late spring after the last rains but before seeds are produced.
Mowing can be used to manage starthistle. Be sure to mow well before starthistle is in full flower or has set seed. Mowing is most effective when soil moisture is low and no irrigation or rainfall follows mowing. If rain occurs after you've removed existing plants, seeds will continue to germinate and the eradication process must be repeated. Pre-emergent and post-emergent herbicides can also be helpful in controlling growth and infestation of starthistle.
Lastly, effective management of starthistle includes revegetation. Planting affected areas with desirable plants discourages starthistle regrowth by creating competition.
For information about herbicides and for photos of yellow starthistle's growth cycle, visit http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7402.html
- Author: Kathy Keatley Garvey
Honey isn't always amber-colored.
It can range from white to dark brown, depending on the flowers the bees visit.
Back in 1971, a group of UC Davis bee specialists wrote a booklet, Fundamentals of California Beekeeping, published by the "University of California College of Agriculture." Although now 37 years old, it's still a good source of information in many respects.
The authors included UC Davis faculty members Harry H. Laidlaw (for whom the bee biology facility at UC Davis is named), Robbin Thorp, Norman Gary and Lee Watkins. UC Davis Extension apiculurist Ward Stanger served as the editor, consulting with Len Foote, then supervisor of apiary inspection for the State Department of Agriculture.
"Hundreds of species of California plants yield pollen or nectar, but the most important plants for commercial nectar are alfalfa, oranges, cotton, beans, sages (black, sonoma, white and white leaf), yellow starthistle, wild buckwheats, manzanita, eucalyptus and blue curls," the authors wrote. "Extensive use of herbicides to control yellow starthistle has decidedly reduced its pasturage in California. Alfalfa, oranges, cotton and beans present a hazard for bees because of pesticides used on them."
The book also mentions the toxicity of California buckeye (Aesculus californica). It blooms in May and June and is very attractive to bees.
"...bees feeding on its pollen are believed to produce larval food (pollen and honey) which results in malformed adults," the authors pointed out.
Yellow starthistle (Centaurea solstitialis) deserves special mention. Growers hate it and beekeepers love it. It's an exotic, invasive weed that's well established in California. It blooms from May to October.
The honey? It's white to extra light amber and delicious.
So, buckeye is attractive to bees but bad for them, and yellow starthistle is bad for farmers but good for beekeepers.
That's something to think about when you're spreading honey on your freshly baked roll or dribbling it over your pancakes.
