Subtropical Fruit Crops Research & Education
University of California
Subtropical Fruit Crops Research & Education

Posts Tagged: herbicide

Why Would You Want to Kill a Fig Tree?

This is the adventure of a livestock advisor trying to find a way to kill a fig tree.  Normally we are doing our best to make fruit trees grow.  In this case, Rebecca Ozeran had to find the best way to kill figs that were fouling the range.  Her solution is not applicable to fruit tree growers in California.  Triclopyr is not registered for edible fruit tree crops in CA.  So, don't try this at home.  There are other ways of doing it.

Mention of any pesticide in this message is not a recommendation.


Why a Livestock Advisor learned about fig trees
by Rebecca Ozeran

Livestock and Natural Resources Advisor

UCCE Fresno County


As a livestock and natural resources advisor, I don't expect to get questions about fruit trees such as figs. As it turns out, I recently needed to know just enough about figs to provide information on how to kill them - a west Fresno County landowner contacted me to ask how he could get rid of some fig trees that were threatening his belowground water pipelines.

Before writing this post, I did my due diligence and confirmed that no one has already discussed fig trees as the target of weed control on this blog. The only mentions of fig trees in the UC Weed Science blog history are a listing of approved herbicides to control weeds in California subtropical crops as of 2 years ago1 and a mention that fig was one of several crops undergoing residue trials last summer2. Triclopyr is, interestingly, not on the list of approved herbicides. (This will be important later.)

Now that we've established this, we have to ask: Why and how are we getting figs on rangelands in Fresno County? And why does it matter?



Figure 1. Mature fig trees in an orchard.

For starters, Fresno County used to have several thousand acres of cultivated figs (nearly 13,000 ac in 19663, but less than 7,500 ac as of 20164), so there are many places in the county where volunteer figs can be seen sprouting, including in newer subdivisions where fig orchards used to be, e.g. the "Fig Garden" region of the city of Fresno. Landowners also may have decided to plant figs on their rural properties as a source of fresh fruit and/or shade, and once established, the figs were able to reproduce and spread. Although fig populations seem to grow slowly in new areas, figs have invaded riparian and other natural areas throughout California's Central Valley5 and can be tough to control once established.

Of course, if fig didn't cause any problems where it grew in these natural areas, we wouldn't be talking about it today. Unfortunately, fig is capable of displacing native plants and forming thick clusters of fig where nothing else can grow5, often in riparian areas. This is problematic for livestock owners, since grazing animals don't find fully grown fig trees appetizing. This can also be bad news for the biodiversity found in riparian areas, as figs become a monoculture. In addition, fig tree clusters decrease ground cover from litter (fallen leaves, grass stems, etc.) which means the fig-dominated areas have more bare soil than grassy or shrubby areas.

Figure 2. Weedy figs create dense stands that exclude other vegetation.

Trees also tend to evapotranspire more water than herbaceous plants like grasses, so they can actually take more water from stream systems and lower stream water levels6. The combination of lower water levels and higher potential for soil erosion can then cause trouble for water quality, and especially for aquatic animals who may require certain water temperatures, clarity, or depth to survive. Lastly, there is the threat fig roots pose to water infrastructure (pipelines) as seen by the landowner who contacted me in the first place.

Hopefully at least one of those consequences helped to illustrate why someone might consider fig a weed. Fortunately for me, several Weed Workgroup members provided great information when I reached out for advice. Mechanical treatments are impractical, as fig can create new sprouts from cut stumps, stems, and roots, and repeated treatments are not always feasible. Applying glyphosate alone to the leaves is ineffective; more effective are cut stump or basal bark applications of various herbicides, including combinations listed in the informative Weed Control in Natural Areas7. Based on the book, very little is known about the efficacy of many herbicide treatments on fig itself, and treatment recommendations are based on their use in other species. Only triclopyr (told you it would be important) has been tested explicitly on fig. Triclopyr is particularly effective when applied as a basal bark treatment. A great resource explaining basal bark treatment can be found below8.

Figure 3. Tree receiving basal bark herbicide treatment.

A non-UC member of the workgroup, Dr. Kerri Steenworth of USDA-ARS, referred me to Dr. Katherine Holmes, a restoration ecologist who is currently Assistant Executive Director of Solano County RCD and Chair of Solano County Weed Management Area. Dr. Holmes has investigated riparian and rangeland restoration connected to fig tree invasion in California's Central Valley5,9,10. When I spoke with Dr. Holmes, she confirmed that triclopyr basal bark treatments have been the most effective in her experience. She has never attempted stem injection or cut stump application on figs but hypothesizes that the strong sap flow would likely reduce the effectiveness of injected herbicide, and that the root system of cut and treated stumps may still be able to create new stems. Dr. Holmes suggested coating the basal 6 to 8 inches of the fig trunk with a mixture of 75% Hasten (surfactant) and 25% Garlon 4 (triclopyr), as long as the tree wasn't in or near water. Basal bark treatments require that the tree is still alive for enough time for the herbicide to be distributed through the vascular system.

In sum, fig trees are a weed issue I never imagined, but fortunately there seems to be an effective solution. More research on treating this species as a weed could be valuable, as fig production is in decline in Fresno County and fig invasion may continue in natural areas. For now, we at least have one blog post about controlling weedy fig trees.



  1. Weed Control in Subtropical Tree Crops. //
  2. Treevix Labeled for Use in California Pomegranates. //
  3. 1966 Fresno County Crop Report. Available at
  4. 2016 Fresno County Crop Report. Available at
  5. Holmes, K. 2008. Invasive fig trees (Ficus carica) in      the riparian forests of California's Central Valley: population growth,      community impacts, and eradication efforts [dissertation]. Available at
  6. Hibbert, A. R. 1983. Water yield improvement potential      by vegetation management on western rangelands. Water Resourves Bulletin      19: 375-381.
  7. DiTomaso, J.M., G. B. Kyser, S. R. Oneto, R. G. Wilson,      S. B. Orloff, L. W. Anderson, S. D. Wright, J. A. Roncoroni, T. L. Miller,      T. S. Prather, and C. Ransom. 2013. Weed Control in Natural Areas in the      Western United States.
  8. Enloe, S., N. Loewenstein, W. Kelley, and A. Brodbeck.      2010. Basal bark herbicide treatment for invasive plants in pastures,      natural areas, and forests. Alabama Cooperative Extension System, Agronomy      and Soils Series.
  9. Holmes, K. A. and A. M. Berry. 2009. Evaluation of      off-target effects due to basal bark treatment for control of invasive fig      trees (Ficus carica). Invasive Plant Science and Management      2:345-351.
  10. Holmes, K. A., S. E. Greco, and A. M. Berry. 2014.      Pattern and process of fig (Ficus carica) invasion in a California      riparian forest. Invasive Plant Science and Management 7:46-58.


  1. Retrieved from
  2. Retrieved from
  3. (C) David R. Jackson and Penn State Extension.      Retrieved from


fig fruit
fig fruit

Posted on Monday, November 6, 2017 at 5:30 AM
  • Author: Rebecca K. Ozeran
Tags: basal application (1), fig (5), herbicide (11), perennial (1), triclopyr (1)

Horseweed Resistance

Researchers have now confirmed that six glyphosate-resistant weed species have been identified in California. Four have been known to exist for some time; they are horseweed (marestail, Conyza sp.), hairy fleabane, rigid ryegrass and annual ryegrass. To that list, junglerice and Palmer amaranth in the Central Valley have been recently added to the list. Additional weeds that have become more of a challenge to control and are on the suspect list are goosegrass and, in the central San Joaquin Valley, the summer grasses sprangletop and witchgrass.

There have never been a lot of herbicides registered for avocado, largely because once they are mature, they self-mulch with their leaves. It's really young orchards that have a weed problem. Growers for the most part have not used pre-emergent herbicides and relied on the many glyphosate products that originally started with Roundup. Sethoxydim (Poast) is registered for non-bearing orchards, but then what can be used as post-emergence herbicides when there is still large open spaces between trees when they are young and bearing? There are still some formulations of paraguat and diquat available for use on avocado, but horseweed is also showing resistance to these materials. Paraquat is also a restricted use material so limits who can apply it. That leaves some of the pre-emergents, such as norfluazon (Solicam), oxyfluorfen (Goal), oryzalen (Surgflan) and simazine (Pricep). These all require water to activate them, and in years with low rainfall efficacy can be erratic. 

From a postemergence standpoint for glyphosate-resistant horseweed the options are pretty slim in avocado. 

  • Paraquat is registered and likely to be pretty effective but has regulatory and safety baggage.

  • Diquat is registered in nonbearing orchards and likely to be effective.

  • Carfentrazone is registered but not very effective on Conyza.

Oxyfluorfen will help glyphosate performance to some extent on Conzya, but not likely to be fully satisfactory. 

From a preemergence standpoint:

  • Simazine should be really effective on Conzya.  It would have groundwater and runoff concerns in some areas.

  • A combination of oxyfluorfen plus pendimethalin or oryzalin would probably be the best available bet for broad spectrum PRE control with decent crop safety.

  • Isoxaben is registered on non-bearing avocado with reasonable Conzya control and good crop safety

Flumioxazin has a non-bearing label and should be pretty good on Conzya.

There has always been concern about the effect on shallow rooted avocado using pre-emergents. This has not been documented, but based on grower experience.

There have never been a lot of herbicides registered for avocado, largely because once they are mature, they self-mulch with their leaves.  It's really young orchards that have a weed problem.  Growers for the most part have not used pre-emergent herbicides and relied on the many glyphosate products that originally started with Roundup.  Sethoxydim (Poast) is registered for non-bearing orchards, but then what can be used as post-emergence herbicides when there is still large open spaces between trees when they are young and bearing?  There are still some formulations of paraguat and diquat available for use on avocado, but horseweed is also showing resistance to these materials.  Paraquat is also a restricted use material so limits who can apply it.  That leaves some of the pre-emergents, such as norfluazon (Solicam), oxyfluorfen (Goal), oryzalen (Surgflan) and simazine (Pricep).  These all require water to activate them, and in years with low rainfall efficacy can be erratic.

We are currently working on getting registration the post-emergence herbicide glufosinate (Rely) registered for avocado.  It's used extensively in other tree crops with success to control many weeds including the Conyza weeds. The registration is being sought through the IR-4 Project (Interregional Research Project Number 4) since 1963 has been the major resource for supplying pest management tools for specialty crop growers by developing research data to support new EPA tolerances and labeled product uses.


Photos of young and maturing Conyza


horseweed mature
horseweed mature

Posted on Wednesday, June 15, 2016 at 1:14 PM
Tags: Conyza (2), glyphosate (6), hairy fleabane (3), herbicide (11), horseweed (6), mare''s tail (2), marestail (4), marestail (4), resistance (13)

An Organic Herbicide that Seems to Work

When Richard Smith tells you that he is impressed with efficacy of an organic herbicide you better take notice. Richard shared his results on this blog site last year and showed good weed control with 'Suppress' from Westbridge. This OMRI approved herbicide is a mix of caprylic and capric acids and is a contact material that interferes with plant cells membranes causing leakage and desiccation.

It looked like a good fit for organic plasticulture systems such as strawberry that have wet weedy furrows which are difficult to access with mechanical tools because of proximity to plastic. We placed a trial in a very weedy field that also had one of the SoCal classics–yellow nutsedge. 'Suppress' at 6 and 9% by volume was applied to well-established weeds just before strawberry planting.

The effects of application were noticeable in minutes. We learned that:

  • Both 6 and 9% rates provided nearly 100% control of common lambsquarter (predominant species) and other occasional broadleaved weeds.

  • About 10-15% of common purslane plants survived and yellow nutsedge seemed unaffected by application. However, biomass of both purslane and nutsedge were significantly reduced, suggesting that production of seed and tubers for the two weed species, respectively, may be delayed.

  • Good coverage was important and some of the horizontally–inclined leaves of broadleaf weeds likely intercepted the herbicide deposition to vertically inclined nutsedge shoots.

  • When we simulated drift by over-spraying strawberry, it responded just like any broadleaf perennial plant—the canopy wilted and dried but in 3 weeks the new leaves developed from the crown. This was also true for the neighboring bindweed that lost above ground canopy but had new growth within a month after 'Suppress' application.

  • Since perennial weeds or those in soil seedbank are not controlled, repeated applications are needed with obvious caution of avoiding the spray drift to the crop.


Posted on Monday, February 22, 2016 at 8:23 AM
Tags: avocado (283), citrus (328), herbicide (11), organic (12), weeds (32)

New UC IPM photo repository shows plant damage from herbicides

UC Statewide IPM Program

Identifying nontarget crop and ornamental plant damage from herbicides has become much easier with the launch of a new online photo repository by the Statewide IPM Program, University of California Division of Agriculture and Natural Resources.

Herbicides applied to manage weeds may move from the site where it was applied in the air or by attaching to soil particles and traveling as herbicide-contaminated soil.  When an herbicide contacts a nontarget plant, a plant it was not intended to contact, it can cause slight to serious injury.  Herbicide injury also occurs when the sprayer is not properly cleaned after a previous herbicide application.  Herbicide residue can be found in the spray tank, spray lines, pumps, filters and nozzles so a sprayer must be thoroughly cleaned after an application.  Dry herbicide particles can be redissolved months later and cause herbicide damage to plants.  Economic damage includes reduced yield, poor fruit quality, distorted ornamental or nursery plants, and occasionally plant death. 

Accurately diagnosing plants that may have herbicide injuries is difficult.  In many cases, herbicide symptoms look very similar to symptoms caused by diseases, nutrient deficiencies, environmental stress and soil compaction.  Plant disease symptoms such as mottled foliage, brown spots or stem death and plant pests such as insects or nematodes cause foliage to yellow and reduce plant growth similar to herbicide injury. 

Dr. Kassim Al-Khatib, weed science professor at UC Davis and director of the UC Statewide Integrated Pest Management Program (UC IPM), has gathered nearly a thousand photos of herbicide-damaged plants, drawn from his own and others' research. The images are cataloged to show damage that can occur from 81 herbicides in more than 14 specific herbicide modes of action, applied in the field to demonstrate the symptoms or when known herbicide spray has drifted onto the plant.

Each image is characterized with the name of the plant, mode of action of the herbicide, and notes the specific symptoms of damage. Together these photos provide a comprehensive archive of damage to over 120 different crops and ornamental plants by known herbicides, which users can easily compare with what they see in the field.

Also included in the repository is information about the modes of action of various herbicides and an index of example herbicide trade names and active ingredients. Users can learn how unintended injury from herbicide occurs from misapplication and carryover from previous crops in addition to drift and herbicide-contaminated tanks. 

The repository can be found at Increased knowledge about what causes herbicide damage and how it occurs can lead to fewer cases of herbicide injury occurring through drift or herbicide-contaminated tanks.  Using the repository can increase the skill to correctly identify plant damage.  Correctly identifying damage as herbicide injury and not from a plant pest or nutrient deficiency can prevent unnecessary applications of pesticides or fertilizers.  Fewer applications can lessen the risk of harm of pesticides and fertilizers to people and the environment.

asphyxiated avocado
asphyxiated avocado

Posted on Thursday, March 5, 2015 at 8:45 AM
  • Author: Tunyalee Martin and Chris Laning
Tags: damage (24), drift (3), herbicide (11), symptoms (3), weeds (32)

Herbicide resistant weeds in your neighborhood

It is not always easy to kill weeds with herbicides for several reasons, but if you apply the right material at the right time to susceptible weeds you expect control. But you should never assume it, because resistant weeds rely on this assumption.

Repeated use of herbicides with the same mode of action (usually the same target site within plant) selects for naturally occurring resistance traits in weed population.  The few resistant weeds proliferate since there is no longer competition from susceptible types and if other control measures are not used.

Hairy fleabane (Conyza bonariensis) resistance to the two most commonly used herbicides – glyphosate (Roundup) and paraquat  -  is widely reported in California, including Ventura County. A close relative – horseweed or mare’s tail (Conyza canadensis) has wide-spread glyphosate resistance, as well.

These weeds are most frequently found at road sides, ditches and just about every site with infrequent disturbance. Of course, that is where glyphosate and paraquat are routinely applied.  What’s worse, they produce thousands of wind-dispersed seeds that travel up to 3 miles and carry the herbicide resistance in them to new places. In fact these and other wind-dispersed weed species are increasingly common in our agricultural and urban areas because we fail to control them before seed production. The seed germinate on moist soil surfaces without incorporation and rapidly grow and reproduce.

There are several strategies to manage these herbicide resistant weeds that can be combined:

  • Identify what species of weeds you have and select management options specific to your weeds and crop. You can look up susceptibility of weeds to herbicides for most crops at These susceptibility tables are based on results of University of California research trials in those crops and are updated periodically.
  • Always evaluate the efficacy of your herbicide application and look for weeds that escaped treatment. If one or two species survived treatment – they are likely resistant. If most weeds did - perhaps there was a general problem with herbicide application.
  • Control weeds when they are small. Large mature weeds are more difficult to kill with herbicides but even resistant weeds can be controlled with herbicides when small. These weeds only reproduce by seed, if you see them flowering it may be already too late.
  • To control escaped weeds use herbicide with a different mode of action appropriate to your crop/non-crop site.
  • Use mechanical methods of weed removal – there has not been reported resistance to a cultivator or hoe
  • Be a good neighbor – communicate with land-owners near you about the wind-dispersed weeds traveling between nearby properties and control them. Even if you manage weeds very well, wind dispersed species can travel to your site from surrounding areas, establish and compete at the time when neither herbicides nor labor for hand-weeding are available.

Herbicide resistance has nowadays been reported for most herbicidal modes of action. Because of its intensive, high-input cropping systems, the United States has more resistant bio-types (131) than any other country. Hopefully we’ll have fewer in Ventura County if we pay attention. California Weed Science Society has a priority focus on the issue and has more information at:


Young plants of hairy fleabane (top) and Horseweed/mare’s tail (below) should be controlled before flowering.

hairy fleabane
hairy fleabane


Posted on Friday, July 5, 2013 at 3:45 PM
Tags: hairy fleabane (3), herbicide (11), horseweed (6), mare''s tail (2), resistance (13), resistant (1), weeds (32)

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