Experts tackle readers’ garden questions.
Q: Could you please identify this plant/weed? My neighbor says this is a new invasive species weed that was not here 10 years ago, but is spreading very fast.
It starts out with soft foliage, but when it gets to the blooming phase it turns to prickly. I’ve never seen honey bees foraging on it. It has a strong tarlike odor in the blooming phase, but I’m sure that smell is coming from the foliage, not the blossoms.
It has spread so fast that I can’t control it by chopping it out. Mowing doesn’t help – it just keeps coming back. I don’t want to resort to glyphosate (Round-up). Is there a weevil that would eat the seeds or compromise the stems?
Rod Hill, Grass Valley
Horticulturist Ellen Zagory: It looks like a tarweed. There are lots of DYC (Darn Yellow Composites) that are called tarweeds and they are natives – not weeds – although they can act weedy.
They are great for pollinators and the foliage is stinky so many people don’t like them.
For a specific identification, (Rod) could press a sample with flower, leaves, stem etc. and send to his county master gardeners or to the UC Davis Herbarium. It might be a lot of work to figure it out exactly.
Editor’s note: Importantly, tarweed is not yellow starthistle, the invasive plant your neighbor likely suggested.
Although a native plant, tarweed can be troublesome, especially in pastures. Livestock kept in pastures with tarweed get its smelly resin stuck to their coats.
As for getting rid of it, tarweed doesn’t like nitrogen fertilizer or water – especially in fall. It can’t stand shade or competition from taller plants.
Herbicides, if used, should be applied in spring before tarweed flowers and goes to seed.
Mowing can help control tarweed, but it needs persistence. When new plants sprout, take a hoe and whack the plants just below the soil line. Cover the area with mulch to smother new sprouts.
Ellen Zagory is public horticulture director for the UC Davis Arboretum.
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