Q. My property backs up to open space. There is
a plant with yellow flowers that is in bloom now. When my dog runs
through it he gets covered with dark green sap and smells. What is this and how
do I get rid of it?
A. This time of year many of the hills in Contra Costa and Alameda counties are filled with the ‘fragrant’ yellow-white blossoms of a plants called tarweed, tarplant, gumweed, gum plant or rosinweed. These native plants are members of the Sunflower family, and well adapted to our climate, with long taproots that search out water reserves deep in the soil. Toward the end of our long, dry summer, the composite flowers bloom in shades of yellow to creamy-white, varying in number from 3 to many per flower, attracting many of our native pollinators with abundant pollen. The plants can reach as tall as 45 cm (1.5feet), just the perfect height to brush against a dog’s fur. The ‘gum’ is a sticky resin formed on the leaf surface and stems. This sticky material then collects dust and dirt and can make an animal need a good rub down with an alcohol-based solvent to remove the coating. This resin helps these late-season plants retain water and repel pests.
In the fall high protein, hard, dry seeds called achenes, fall from the plants, germinating with the first winter rains. They are dispersed by wildlife, wind and rain. By the end of the rainy season, a deep tap-root anchors a rosette of broad leaves, from which shoots emerge and branch during the summer months. Tarweeds are eaten by livestock when they are young, but are avoided when older and covered in resin. They are an important source of protein for ground squirrels.
This plant has thrived in the recently changed environment of our grasslands. Several hundred years ago the grasses that covered the hills were longer-lived with deeper root systems than the annual grasses which replaced them, and therefore more competitive with the tarweeds.
You have several options if you want to discourage tarweeds.
If mown in late May, populations can be reduced by as much as 20%; in
July mowing will give you a 90% reduction. Mowing will need to be
repeated yearly for a long time, as the seedbank in the soil will be viable for
many years. Rose clover, ( Trifolium hirtum), fertilized with single
superphosphate, overseeded onto sites has provided good competition as should
other legumes.