1 - Weevils that attack only puncture vine (goathead) plants are one tool to use with others in managing puncture vines in some New Mexico gardens.

Yard and Garden March 4, 2017

Q.

I am working on an emerging goat head problem on my property. I have read about the use of puncture vine weevils as part of the solution. This sounds like a good additional armament in the fight. I am concerned however about the possibility of the weevil affecting other plants unintentionally. Nowhere have I seen anything in print that echoes this concern. Is this an issue?

- Bill P.-S.

A.

Puncture vine (goathead weevils) have been considered as one potential tool for managing these weeds for several years without any mention of unintended consequences, but to be sure I checked with NMSU Extension Weed Specialist, Leslie Beck. She has once again provided a wealth of useful information that I will quote below:

Biological control options for any weed will help to injure the plant and is a great tool in your weed management strategy. Though puncturevine stem and root weevils do not feed on plants other than puncturevine, it is not smart for these weed-feeders to kill their host...once they do the food source is gone. Therefore, biological control by itself will rarely control the weed completely on its own. However, any injury prior to implementing other management options (removal, herbicides, etc.) can help you increase injury and improve your control of the target weed.

Puncturevine (Tribulus terrestris) is a summer annual plant, meaning that the plants that you see above the ground will only survive for one growing season. The job of these plants is to germinate in the spring, grow very quickly, produce as much viable seed as possible, then die off when the temperatures become less ideal to sustain growth (in this case colder temperatures). Despite the onset of warmer temperatures these past few weeks, I really have not observed any puncturevine that has started germinating quite yet. However, temperatures are certainly warm enough (soil temps of 60-65F) for long enough periods of time that germination could occur at any moment. This plant contributes as much viable seed as possible to the soil in the form of chambered burrs, also known as goatheads, which disperse by adhering to shoes, clothing, tires, and the fur/feet of animals. Once introduced to the soil, the seed can lay dormant for roughly 5 - 7 years prior to germination.

Because puncturevine is an annual plant, there are products called preemergent (PRE) herbicides that have the ability to control certain weeds as they germinate and emerge from the soil. It is very important to make applications of these products before puncturevine starts to germinate (mid-March to early April depending on temps.) because once the plant has emerged from the soil and can be seen on the surface these herbicides will be completely ineffective. Active ingredients that have been reported to help control germinating puncturevine include oryzalin, benefin, and trifluralin. An available organic PRE herbicide option is the active ingredient corn gluten meal; however, it does not seem to be very effective specifically on puncturevine.

Once the plants emerge from the soil, the only herbicide options available are going to be a postemergent (POST). Products that contain active ingredients in a typical 2- or 3-way herbicide (2, 4-D, dicamba, MCPP) can be effective against broadleaf weeds like puncturevine with little damage to the turf (read the label). Some organic active ingredient options could include herbicide or enhanced vinegars (acetic acid), citric acids and oils, and herbicide soaps. These products have a label, which contains application instructions that you must follow, and are intended for use as an herbicide in the environment. Your household distilled and apple cider vinegars are NOT labeled for use as an herbicide and thus, can not be used as an herbicide. The active ingredients of both the synthetic and organic herbicide options mentioned above cannot choose from one broadleaf to another, or between broadleaves and grasses (non-selective) and may have the potential to drift of leach offsite. Therefore great care must be taken to avoid injury to other desirable plants. Always ALWAYS read and follow the label directions and restrictions for any herbicide product prior to every application.

Even though puncturevine creeps outward along the ground as it grows, the stems do not have the ability to root into the ground as they spread, thus it is easy to trace the origins of the growth of this plant back to its central taproot to remove. Again, it is important to try and eradicate these plants prior to their production of viable seeds and remove as much of the root as possible to discourage regrowth. With puncturevine, you want to try and manage them prior to the production of the yellow flowers...those flowers eventually transform into the goatheads. Puncturevine seeds require sunlight in order to germinate; therefore a 2 - 4 inch thick layer of mulch or an opaque plastic covering can also help to discourage weed seed germination.

As with any annual weed, the key is to try and treat them or remove them when they are young and newly germinated. Not only will the plant respond more to any herbicide application, it has not yet had the ability to produce and drop viable seed which contributes to the dormant seed populations in the soil. Similarly, no one management practice by itself (i.e. herbicides, biological control) will control weeds for good. Combine as many of the management practices mentioned above together for an overall integrated pest management strategy and you will increase the success of your weed control in a more environmentally friendly and sustainable way.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

Links:

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