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Organic herbicides to control yellow starthistle

Can organic herbicides be used to control yellow starthistle in a matrix of grasses? Noxious late-season weeds such as yellow starthistle are a key management challenge in California's grasslands and rangelands-- decreasing plant diversity, and livestock productivity. Because these late-season species remain stunted into mid-spring (when other grassland species are at their peak growth), they are potentially susceptible to organic herbicides (which are most effective in killing plants less than 6 inches tall who have yet to go through their most active growth). We compared controls (no herbicides) to conventional treatment of yellow starthistle (Transline) with two organic herbicides (Avenger and Suppress). These were applied at 3 time points: - early spring (early March) when naturalized annual grass is at its peak growth rate, but yellow starthistle is still a rosette and has minimal aboveground growth - mid spring (mid-April) when yellow starthistle is still at the rosette stage These first 2 time points were meant to kill yellow starthistle before flowering - peak flowering of yellow starthistle (late June) - intended to kill all developing seeds

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Project Results

Applications at peak flowering of yellow starthistle of two organic herbicides, Avenger and Suppress, as well as Transline, virtually eliminated yellow starthistle in the following year. These results were surprising, given that yellow starthistle tends to have an extensive seedbank, and the experimental area was infested with yellow starthistle for years before the experiment. Follow-up testing of the seedbank indicates that there is a very sparse seedbank (potentially due to flooding the year of the experimental trial). This indicates that in new establishments of yellow starthistle (where there is little seedbank), herbicide application (both conventional and organic) effectively minimizes seed production of yellow starthistle.

Herbicide applications were as follows: Control- unmowed, no herbicide Control- mowed, no herbicide All herbicide treatments were mowed to allow for maximum contact of the organic herbicides on the target plant tissues Clopyralid (Transline) - was applied in all 3 time points (early, mid and late spring) at a rate of 2/3 pint per acre at 30 gallons/acre D-Limonene (Avenger) was applied at 20% in 57 gallons/acre for the early and mid-spring applications, and at 33% in 57 gallons/acre for the peak flowering application Caprylic and Capric Acid (Suppress) was applied at 6% in 57 gallons/acre for the early and mid-spring applications, and at 9% in 57 gallons/acre for the peak flowering application.

% cover was measured the year of application, as well as in the spring one and two years after application.

Yes, for one year (and then yellow starthistle cover was low in all treatments due potentially to drought)

Timing for peak-flowering requires monitoring and being able to apply during a quick time-window. It also required spraying at a higher height than usual, requiring some adjustment of methods.

Potentially- it's likely we were lucky with good conditions (e.g. a flood that likely eliminated the seedbank)- so our results were likely better than most would expect.