Weed management in landscaped areas can be challenging. Weeds may need to be controlled for public safety, fire reduction, aesthetics, and elimination of harborage for other pests. While many non-chemical options for controlling weeds exist—such as physical removal with tools, steam, flame or steam devices, grazing animals, and others—there are some situations that may require the application of herbicides.

For decades, glyphosate has been a common active ingredient used to control weeds in both agricultural and nonagricultural settings. However, there has been significant public concern about the use of glyphosate and other herbicides due to their potential effect on water quality, public health, and non-target species. Because of this ongoing issue, many practitioners have been considering organically-acceptable herbicides as alternative solutions. While some information exists on how organic herbicides work, there is little research on their efficacy in urban landscapes.

 

Glyphosate vs. organic herbicides

Concerns about the potential risks of glyphosate have led to increased use restrictions, including outright professional or municipal use bans in some California cities, counties, school districts, and other sites. Professional landscape managers and other pest management practitioners who aim to reduce or eliminate glyphosate from their IPM programs are therefore seeking alternative products to control weeds.

Organic and alternative herbicides seem like simple substitutes since treatments may not require new application equipment or knowledge. However, knowing the differences in modes of action among glyphosate, organic herbicides, and other alternatives is important to ensure weed management goals are reached.

Organic herbicides may not have the same qualities and performance practitioners have become accustomed to seeing with glyphosate and other conventional herbicide products. For instance, organic herbicides work on contact as opposed to glyphosate, which moves through the entire plant. These organic contact herbicides are most effective at higher temperatures (80°F and higher) and in full sun. Since they work on contact, they are applied after emergence and work best on small annual weeds. For larger or perennial weeds, organic herbicides generally will only damage or burn the top growth of the weed and, after a couple of weeks, the weeds regrow. From the data presented below, regular repeated applications of these products may still be useful tools within an overall IPM program.

The research presented here was designed to address the need for glyphosate alternatives by providing information about organic herbicide efficacy. These trials build on previous work by other researchers examining organic and alternative herbicides in non-agricultural settings (see references).

Trials included mostly organically acceptable materials as well as selected non-organic but naturally-derived products. Experiments were performed on the campus of the California State University, Sacramento (CSUS) in summer months of 2019 and 2021. The research site received little foot traffic, was regularly irrigated, mowed, and largely shaded underneath trees for most of the day. Weeds present at the site were a mixture of broadleaves, grasses, and sedge with predominant species being broadleaf plantain (Plantago major), dandelion (Taraxacum officinale), wild strawberry (Fragaria vesca), bermudagrass (Cynodon dactylon), and clovers (Trifolium spp.)

Slightly different products were used between the 2 research years. There were 10 or 11 herbicide treatments along with an untreated control (Table 1). All organic products in the experiment are post-emergent, nonselective, contact herbicides except for the iron HEDTA product (Fiesta), which is selective for broadleaves only. Weed damage was rated by visual inspection using an index (scale) from 0 (no observable plant injury) to 10 (complete plant injury above ground). This damage is referred to as burndown (Figure 1).

Preliminary results

Many products showed rapid plant damage on both grasses and broadleaves on the first day after treatment (DAT). Figure 2 shows results from the 2021 trial, which included results similar to those observed in 2019 and other trials. It was observed that by 3 DAT, ammoniated soap of fatty acids, pelargonic acid + fatty acids, ammonium nonanoate, and caprylic acid + capric acid showed the best control of both grasses (A) and broadleaf (B) weeds in the plots. Products containing citric acid + clove oil, d-limonene, and clove oil + cinnamon oil did not perform well in this trial even after a second treatment.

The iron HEDTA product targets broadleaf weeds only, so it is not included in the chart illustrating grass weed control. Acetic acid (Danger signal word) was not included in the 2021 experiments due to the risk of application to bystanders at CSUS. One product containing acetic acid is included in Table 1 for cost comparison of various alternative herbicide products.

In general, most weeds began to regrow or recover about 2 weeks after treatment. Multiple successive treatments were made after regrowth was observed (around 3 weeks). Efficacy of most products had declined and weeds once again showed regrowth 17 days after the second treatment (Figure 2).

Some of the organic herbicides tested exhibited quick results, with immediate burndown of contacted weeds observed within an hour or two. the majority of plant damage was observed between 1 DAT and 7 DAT. However, most weeds also completely regrew from the base or roots 2 to 3 weeks after each application.

 

Considerations when using organic herbicides

Urban landscape professionals need to consider the differences among conventional herbicides, organic herbicides, and other alternative herbicides (Table 2). Switching from glyphosate-containing products to organic herbicides will require a reallocation of resources to accommodate for more frequent applications, lower dilutions, and higher application volumes.

Resource shifts may include increased labor costs due to more frequent applications, possible increased supplies costs due to additional personal protective equipment (PPE) required, increased training required for handling of more acutely toxic products (those with Signal Words other than Caution), and higher herbicide product acquisition costs (Table 2).

What's next?

We know from pesticide use reports gathered from the California Department of Pesticide Regulation that herbicides are applied year-round under various temperatures and conditions. Therefore, practitioners need information about how well these products work in different conditions; such as across a range of temperatures, with varied weed species, in the presence of clouds or a canopy cover, and other factors. UC Cooperative Extension will continue to investigate these variables and will share findings via articles, workshops, seminars, and other extension methods.

 

References

Reiter, M and K Windbiel-Rojas. 2020. Organic herbicides and glyphosate for weed control: results of coordinated experiments in urban landscapes. CAPCA Advisor Magazine. February 2020. Pp 24-30 https://capca.com/wp-content/uploads/2020/01/202002-CAPCA_ADV_Feb2020_UCIPM_M-Reiter.pdf

Wilen CA. 2018. UC IPM Pest Notes: Weed Management in Landscapes. UC ANR Publication 7441. Oakland, CA. http://ipm.ucanr.edu/PMG/PESTNOTES/pn7441.html

Neal J, Senesac A. 2018. Are there alternatives to glyphosate for weed control in landscapes? North Carolina State University Publications. https://content.ces.ncsu.edu/are-there-alternatives-to-glyphosate-for-weed-control-in-landscapes

 

Acknowledgments

The author would like to thank the California State University, Sacramento for the use of their property for these trials.

 

Karey Windbiel-Rojas is an Associate Director for the UC Statewide IPM Program and Urban Area IPM Advisor for UC Cooperative Extension serving Sacramento, Yolo and Solano counties.

Branched broomrape is a parasitic plant that is an “A-listed” noxious weed in California, requiring crop destruction and a hold order to be placed on reported fields. It has been reported in several commercial tomato fields in Yolo County in recent years and is of growing concern to the California tomato industry. Until recently, there have been no registered chemistries to control branched or Egyptian broomrape in California.

Rimsulfuron, marketed as Matrix SG by Corteva, is registered on tomatoes in California is widely used both PRE and POST for nightshade and other broadleaf weed control. Based on research conducted in Italy that showed promising broomrape control with rimsulfuron applied in the irrigation water through surface drip lines (Conversa et al.), we did pilot studies in 2021 and 2022 in California.

With initially promising results, the California Tomato Research Institute submitted a 24c label (“Special Local Need”) request to California Department of Pesticide Regulation. The request was to add a chemigation application technique specifically for management of broomrape. The 24c label was approved in September 2022 and is available.

A breakdown of the 24c describing the approved chemigation application process is below.

From Corteva Matrix Label (https://ucanr.edu/blogs/UCDWeedScience/blogfiles/97715.pdf):

• For use on processing tomato for management of broomrape (Phelipanche ramosa, aka Orobanche ramosa) and Egyptian broomrape (Phelipache aegyptiaca). For management of broomrape, apply Matrix SG through buried- or surface-drip irrigation tubing to transplanted tomato. Apply at an application rate of 1.33 oz/A for up to 3 applications per season at approximately 30, 50 and 70 days after transplanting.
• Refer to product label for Matrix SG for Use Precautions, Mixing and Application directions.
• Surface or buried drip irrigation applications simulate banded applications.
• The amount of water and injection time may vary depending on soil type and irrigation system used. Introduce Matrix SG into the irrigation system at approximately the midpoint of the irrigation set to limit movement of the herbicide beyond the tomato root zone, where broomrape germination and attachment occurs, which may improve broomrape control. Factors such as soil type, irrigation system, injection timing and length, drip tape placement, etc. may affect weed control when Matrix SG is used through the drip irrigation system.
• After Matrix SG has been evenly applied across the field, flush the irrigation system prior to ending the irrigation.
• Do not apply more than a total of 4.0 oz/A Matrix SG (0.0625 lb ai/A rimsulfuron) on tomato during the same year.
• Do not make more than 3 applications of Matrix SG per year.
• Preharvest Interval: Do not apply within 45 days of harvest.  
• This label must be in the possession of the user at the time of application. Follow all recommendations and restrictions on the Matrix SG Section 3 labeling.

It is important to note that when using rimsulfuron as a chemigated material following the label, you are using the yearly maximum rate of 4 oz/acre. For growers that do not use rimsulfuron as a PRE or POST product, this will not change current weed control programs. However, if growers currently utilize rimsulfuron as a PRE or POST product in their current programs, it will need to be excluded to avoid going over the annual 4 oz/acre maximum. This may affect general weed control, as rimsulfuron is often used to target nightshade species. Research into alternatives to rimsulfuron for general weed control in processing tomatoes will begin in 2023.

As with any pesticide, it is important to read and follow the most current label instructions for application.

We would like to thank the California Tomato Research Institute for funding ongoing broomrape management research and coordinating the 24c request, Corteva Agrisciences for providing product for the research and internal data from the US and Europe to CDPR, and our cooperators who hosted herbicide research trials that support the entire processing tomato industry.

 

Matt Fatino is a UC Davis Ph.D. student with Brad Hanson's Lab.

 

The heavy rains have stopped a lot of growers from getting into their wheat fields, and growers may be wondering what the best move is given the break in the weather. Our forecast for rain in the next 10 days looks spotty, and that has some implications for both nitrogen and herbicide management in small grains.   

Nitrogen (N) management.

Studies and experience indicate that in-season applications of N provide a much better response to fertilizer than preseason applications. In years such as this, that is more likely the case because heavy early-season rainfall reduces the amount of plant available N in the root zone. Growers may be wanting to apply nitrogen (N) soon, but there a couple things to consider:

1. Granular fertilizers such as urea or ammonium sulfate need some rain to fall after they are applied in order to be successfully integrated into the soil. A general rule of thumb is that 0.25 inches or greater in the same storm will be sufficient to dissolve the fertilizer and get it into the soil profile. Timing applications ahead of a storm or irrigation event is advisable. If the weather dries up and no new storms materialize, N applied now may not become available to the plant in the near-term. N needs water to be utilized by the plant.

2. Much of the wheat in the southern Sacramento valley is still in the early tillering phase (Feekes scale: 1-3). This means that nitrogen uptake has been pretty limited so far this season (around 1-2% of the total for the season). In other words, there's a long way to go.

With these things in mind, growers should continue monitoring the weather closely. Waiting a few more days is a given in most cases anyway as a lot of the acreage is still too wet to drive on. Now would be a good time for some soil nitrate quick tests, which can give growers and idea of the current N status in their soil. If we see another large storm system coming through in early to mid-February, growers will want to consider a top dress, but for now, depending on your crop's maturity, it might be prudent to watch the weather, monitor the crop, and wait until we know more.

The Nitrogen Fertilizer Management Webtool for California Wheat can help growers track their crop growth status over time and will chart N-uptake curves. If you have an N-rich reference zone, small seedlings may not yet be old enough for the model to produce a recommendation because the amount of N taken up by the crop at this stage is so minimal relative to the seasonal demand (Feekes 2-3). If crops are slightly more mature (Feekes 4-5), the N-rich reference zones may provide additional information to inform grower decision-making. By early- to mid-February, most of the acreage in the southern Sacramento Valley should be mature enough to provide data that can be used in the webtool.

Herbicide Considerations:

Italian ryegrass / weeds in general: So far I've seen chickweed in the valleys and Italian ryegrass in the hills, mostly at an appropriate stage for herbicide treatment depending on whether or not your wheat is mature enough. With good soil moisture, warming conditions, and lengthening days, weed growth is accelerating.  Pay attention to the labels, but most of the ALS inhibitors (Osprey and Simplicity) and ACCase inhibitors (Axial) labels suggest spraying when weeds are between 2 leaves and two tillers. Some of those herbicide programs have slightly wider windows, but experience from a look-on-the-bright-side herbicide trial last year highlighted three lessons:

1. Spraying herbicides on drought-stressed weeds reduces efficacy, sometimes catastrophically. The same is true of weeds that have matured beyond the ideal growth stage.

2. Treatment windows on the labels are relatively optimistic about efficacy: yes, you CAN spray Italian ryegrass at three tillers in some cases, but herbicide performance on the early or late end of that window may be less dependable, especially under less-than-ideal conditions.

3. Weeds will out-compete your small grains if given the chance: every day that weeds are in there is a day that the crop is going to get edged out. Weeds grow faster and are more competitive and have the ability to completely overtake the crop in the course of a few warm weeks. Even if you manage to wipe out Italian ryegrass, if it's too late in the season, your crop's yield potential will have dropped substantially because so much of its key growth stages were spent competing with weeds for resources.

 

What does all this mean? Once you can get into your field, this might be a good time to consider treating weeds while they are in the seedling stage. 

In terms of Nitrogen management, however, now might be a time to watch and wait until we have a bit more certainty about the weather. Growers will ultimately need to make the call on how to respond but having more information from soil nitrate quick tests and knowing what growth stage your crop is in will help inform those decisions.  

From the Topics in Subtropics blog ¦ Oct. 21, 2022

 

Control of weeds has always been a major economic cost in subtropical fruit production because of favorable climate that allows for weed germination and year-round growth. The use of chemical weed control has increased dramatically due to labor costs, equipment costs, product costs and availability, the shift to more narrowly spaced tree rows, and installation of low volume irrigation systems that prohibit the operation of mowing or tillage equipment under the tree canopy area (Futch 2001).

However finding herbicides that are labeled for certain subtropical crops can be a bigger challenge. UC Weed Science has updated the Subtropical Crops herbicides usage chart for California growers (2020). It also seems that preventive programs are most frequently overlooked as a method of weed control. Preventive programs entail the use of such practices as sanitation, spot spraying, or hand labor to prevent the source of weed infestation (seed and/or vegetative) from widespread dissemination throughout a given area. By removing the undesirable weed species prior to seed development, dissemination by wind or mechanical transport on equipment can be effectively delayed.

Weed control programs will vary from location to location within the state and can even vary within a given site based upon specific conditions such as soil type, variety, method of herbicide application, and the presence of specific weed species.

Before herbicide application, growers should survey the grove and determine the stage of growth and type of weeds for that given location. Many products do not provide control of emerged species, thus requiring the application of more than one product to provide both preemergence and postemergence protection. Rotation of soil-applied herbicides should also be considered to prevent the buildup of resistant annual and perennial weeds. The resistant species may not be evident initially; however, if the same herbicide and cultural program is maintained, over time their populations may build up until they infest the entire grove and become the dominant weed species.

Herbicide damage to foliage and fruit has also been noted when herbicides were applied under windy conditions or use of improper equipment allowed the materials to contact areas other than the weeds or soil. Please make sure to follow the label's direction and use caution.

Attached is a chart listing of herbicides registered for avocado, citrus, date, kiwi, fig and pomegranate, along with the Big Trees' herbicides. Check out how many more are available on almond than avocado. Both start with an A.

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Sonia Rios is the UC Cooperative Extension Riverside and San Diego Counties Area Subtropical Horticulture Advisor.

Original source: Topics in Subtropics blog ¦ Oct. 21, 2022