While horseweed and hairy fleabane have been in California basically since farming began, the weeds became an obvious problem around 2003 particularly in tree and vine systems and non-crop areas. Now new control solutions are recommended.

According to Kurt Hembree, University of California Cooperative Extension farm advisor in Fresno County, the traditional use of combinations of pre-and-post-emergence herbicides and cultivation managed the weeds in the past. However, recent changes in environmental regulations, economics, herbicide use patterns toward more post-emergence-only programs, treatment timing, and glyphosate-resistant biotypes have contributed to the problem.

Other factors contributing to the spread include high seed production, wind dissemination, lack of seed dormancy requirement, the preference of undisturbed areas like tree and vine rows, and the adaptability to moist and dry soils. To gain control of the weeds, Hembree encouraged growers, managers of non-crop areas, and other landowners to do their part to resolve the issue. Preventing new seed production is a must to be successful.

A good understanding of the weeds’ growth period is important. While considered summer annuals, horseweed and hairy fleabane can also emerge in early October in the southern San Joaquin Valley, he noted.

The weeds go through an over wintering stage where root growth seems more important than leaf production. By the time spring emergence occurs in mid-February, plants that actually emerged several months earlier may appear the same as those that just emerged. This could help explain why some late-winter or early-spring applications of post-emergence herbicides are not as effective.

Relying strictly on post-emergence products can make horseweed and hairy fleabane very problematic. Consider using effective soil-residual herbicides where possible, Hembree suggested.

Once under control, apply treatments every second or third year to maintain control. Also consider making split-applications in October/November and again in January/February if weeds emerge during this time.

For farmers in groundwater protection areas (GWPA), a permit to use some products will be required so talk with a county agricultural commissioner for local GWPA regulations. While most of the effective materials fall under GWPA regulations, most can still be used.

Also consider spray additives like citric acid, ammonium sulfate or spreaders if approved on the label to improve activity. Tank-mixing various post-emergence products can also work well.

Many products sold in California contain glyphosate, but not all contain the same amount of active ingredient. Read the label carefully to ensure the correct product amount is being used to provide at least 2 pounds of active ingredient per acre.

If using recommended label doses, herbicide timing, and properly calibrated and operating spray equipment still leaves some weeds uncontrolled, contact a local farm advisor and chemical representative to make sure a herbicide-resistant biotype is not being used. If resistance does exist, changes to the weed management program will be needed immediately.

In addition to appropriate herbicide selection and use, cultivation can also play an important role in horseweed and hairy fleabane management. Use shallow cultivation to dislodge plants less than four inches from the soil.

Cultivating in moist soil will improve control. Where in-row equipment like Bezzerides can be used, control can be excellent. Mowing does not control these weeds and should not be used.

Studies have indicated that the weeds’ seeds do not germinate well in soils that have been disturbed through cultivation. The small, light seeds do not emerge from the soil if buried more than a few millimeters deep and are only viable for two-three years after production. Scrapping or disturbing the soil lightly before weed germination can be an effective means of control.

Other weed species may also be waiting for an opportunity once horseweed and hairy fleabane are history. It’s a good idea to routinely monitor fields after each herbicide application and check for any weed escape or a change in the types of weeds present.