University of California at Davis

Trial of a citric acid/acetic acid (vinegar) combination for control of yellow starthistle.

Joe DiTomaso, Extension Weed Specialist, and Guy Kyser, Staff Research Associate.

Spray treatments of commercial vinegar (5% acetic acid) have been found to provide control of seedling annual weeds, and drip treatments with vinegar can control rhizomes and tubers. An acid solution acts as a contact herbicide, breaking down cell membranes where it contacts foliage. In sufficient quantities, it results in quick burn down of the plant, especially in bright sunlight.

Greenergy, Inc. (Crescent City, CA) provided a mixture of 15% citric acid and 15% acetic acid (CAA) for testing on yellow starthistle. Both these acids are edible at lower concentrations, so it was thought this treatment might be useful for annual weed control in organic farm or homeowner situation, e.g., yellow starthistle during winter in organic vineyards.

We applied (CAA) to yellow starthistle preemergence at, early and late rosette stages, bolting, and the spiny stage. All treatments were made with a CO2 backpack sprayer, three 8008 nozzles, at 40 psi.

Preemergence. One thousand yellow starthistle seeds were planted in each of 8 meter-

Square plots 1/15/03. The next day, four plots selected at random were treated with 10%

CAA at 100 gpa. Seedlings were counted and pulled 5 times over 4 months. Counts were not significantly different in treated plots.

Treatment Total seedling Standard deviation T-test

(mean of 4 plots)

10% CAA, 100 gpa 480 +87 0.151

Untreated 577 +21

Postemergence. We planted yellow starthistle seed in 100 (5 ft x 10 ft) plots 21

November 2002. We treated at 4 plant stages – early rosette and late rosette (4 March

2003), bolting (22 April 2003), and spiny (May 2003), at 3 concentrations of CAA (5%,

10%, and 15%), and at 2 spray volumes (50 and 100 gpa). Treatments were made with a

CO2 backpack sprayer with three 8008 nozzles at 40 psi. All treatments were made on sunny days.

All treatments resulted in quick burndown or yellow starthistle foliage. Treatments

Also gave good control of redmaids, shepherdspurse, chickweed, annual bluegrass,

And fiddleneck: later treatments controlled prostrate pigweed, knotweed, and witchgrass,

And gave partial control of lambsquarters and little mallow. However, yellow starthistle plants recovered quickly. Furthermore, starthistle plants in treated plots appeared more robust than starthistle in control plots due to the absence of competition. Approximately one month after treatment in spiny stage, there are no significant differences due to treatments.

At early flowering (12 June 2003) we tested a high concentration, high volume application (full strength 15% CAA, 200 gpa). This again resulted in quick burndown of starthistle foliage; however, some stems remained alive, giving a total plant mortality of 38%. The living stems were producing new flowerbuds by mid-July. In late July we made another application (full strength 15% CAA, 200 gpa to surviving stems. In this case most plants died. The only stems that remained green were ones on the opposite end of the treatment side that were not directly contacted. Under typical field conditions, the population of yellow starthistle would not be as thick and it would be contacted by this high volume treatment. It is also possible that even lower rates of acetic/acetic acid (vinegar) could be effective.

Conclusions. We have found, by testing contact herbicides and mechanical control techniques, that yellow starthistle is remarkably resilient. In some ways it behaves more like a perennial weed than an annual. In the rosette stage, the meristem is protected by the rosette leaves and may even be below the soil surface; after bolting, killing off any of the top portion of the plant stimulates growth of axillary buds. For these reasons, contact herbicides such as CAA are much less effective on yellow starthistle than on many other annual weeds. Starthistle is more susceptible than younger plants and complete coverage is necessary to kill the plants. However, it is possible to get complete kill of yellow starthistle with a late season application of acetic/citric acid at high volume treatments.

Trial of a citric acid/acetic acid (vinegar) combination for control of yellow starthistle:

Treatment list

Treatment Stage Volume Rate Plots

1 early rosette 50 gpa 5% 9 50 61 97

2 early rosette 50 gpa 10% 20 27 69 80

3 early rosette 50 gpa 15% 10 38 55 87

4 early rosette 100gpa 5% 4 37 60 91

5 early rosette 100gpa 10% 6 41 75 99

6 early rosette 100gpa 15% 21 48 63 77

7 late rosette 50 gpa 5% 16 44 66 88

8 late rosette 50gpa 10% 13 30 59 78

9 late rosette 50gpa 15% 1 32 64 76

10 late rosette 100gpa 5% 2 33 58 95

11 late rosette 100gpa 10% 24 39 68 98

12 late rosette 100gpa 15% 3 46 51 83

13 bolting 50gpa 5% 25 26 71 89

14 bolting 50gpa 10% 7 29 72 82

15 bolting 50gpa 15% 18 42 62 86

16 bolting 100gpa 5% 15 43 56 90

17 bolting 100gpa 10% 14 31 52 92

18 bolting 100gpa 15% 5 28 74 79

19 spiny 50gpa 5% 23 35 65 85

20 spiny 50gpa 10% 12 49 70 96

21 spiny 50gpa 15% 22 45 57 94

22 spiny 100gpa 5% 8 47 54 84

23 spiny 100gpa 10% 17 36 67 93

24 spiny 100gpa 15% 19 40 73 100

25 untreated 11 34 53 81