Weed size and post-emergence herbicide efficacy

Weed management is essential during the establishment phase of orchards to improve the survival of newly transplanted nursery stock. In commercially bearing systems, weeds must be controlled in order to increase irrigation efficiency, provide equipment access, and ensure that fruits can be harvested effectively and economically. Furthermore, non-managed weeds may support populations of insect, vertebrate, and pathogenic pests that can significantly reduce tree health over time.

Weed control strategies may not always be 100% effective and escapes can occur for numerous reasons. Situations leading to herbicide failure include: improper herbicide selection or inappropriate timing of chemical applications, unfavorable weather conditions at the time of treatment, reduced herbicide activity due to poor water quality, and the development of herbicide resistance in the target weed population, among others. Plant size can also affect weed control; the efficacy of post-emergence herbicides is often diminished when products are applied to large/mature plants (Figure 1).

Figure 1. Ineffective management of large weeds in a plum orchard with a contact herbicide. Reduced control is attributed to poor herbicide coverage and subsequent plant regrowth.

 

As an example: weed scientists at the University of California, Davis, recently undertook studies to describe how the control of hairy fleabane (Conyza bonariensis), a close relative of marestail/horseweed (Conyza canadensis), differed with respect to plant size at the time of herbicide application. Treatments included: glyphosate (Roundup WeatherMax at 3.5pt/A), glufosinate (Rely 280 at 3 pt/A), paraquat (Gramoxone Inteon at 3pt/A), and saflufenacil (Treevix at 1 oz/A) applied at one of three different growth stages: small rosette (4- to 5-leaf, < 1 inch height), large rosette (15- to 20-leaf, < 7 inches in height), and bolting (> 20 leaves, 7 inches in height).

Greater than 90% control of hairy fleabane was achieved when plants were treated with glyphosate, regardless of size at the time of application (Table 1). Glyphosate is a systemic herbicide that it is translocated within treated plants; it eventually accumulates at meristems and inhibits the production of aromatic amino acids (tryptophan, tyrosine, and phenylalanine) that are needed for protein synthesis. Conversely, the efficacy of glufosinate, paraquat, and saflufenacil decreased when hairy fleabane plants began to bolt (Table 1). Unlike glyphosate, these active ingredients exhibit no or limited mobility in treated plants; as a consequence, herbicide-induced injury is limited to the tissues that come into contact with the spray solution. In the case of larger plants, the outermost portions of the canopy can act as a shield for the innermost leaves, stems, and buds, which may continue growing. Additionally, contact herbicides will not impact underground tissues (roots, rhizomes, bulbs, etc.), which also support plant regrowth.

Table 1. Hairy fleabane control with glyphosate (Roundup WeatherMax at 3.5pt/A), glufosinate (Rely 280 at 3 pt/A), paraquat (Gramoxone Inteon at 3pt/A), and saflufenacil (Treevix at 1 oz/A) as affected by plant size at the time of application.

 

Still, it isn’t sufficient to focus on in-season weed control as the sole metric of efficacy. Plants that escape treatment may reach reproductive maturity and produce viable seeds. These seeds are the foundation for weed problems in future years. As a consequence, weed escapes necessitate that growers engage in additional management practices that may have been unplanned and that add to the cost of crop production. Increased seedbank/in-field weed densities could also facilitate the development of herbicide resistance. When weed infestations are heavy, the probability of selecting for resistance can be high, even if the mutation rate that leads to the development of resistance is low.

There are several steps that growers can take to maximize weed control with post-emergence herbicides, including timing applications to treat weeds while they are small and tender. Additional strategies include: selecting the appropriate herbicides to control target weed species and applying them at appropriate rates, minimizing off-target movement due to rain and wind, calibrating spray equipment, properly, and using adjuvants, effectively, to ensure coverage and penetration. To minimize the potential for herbicide resistance, growers should diversify their chemical, cultural, and physical weed control strategies as much as is environmentally and economically possible.

Any mention of herbicides in this blog does not constitute a professional recommendation by the author or her employers.

What’s with those different herbicide names?!?

Ever wonder why sometimes a weed scientist will say ‘glyphosate’ and another time they will say ‘Roundup’? There’s actually a good reason for this – they are either talking generally about an active ingredient or else specifically about a formulated product.

Let’s look at ‘glyphosate’ in more detail. There are three different ways that we can refer to it:

Firstly, by its chemical name

This is the name that describes the chemical composition of an active ingredient (probably the most rarely encountered term).

i.e. N-(phosphonomethyl) glycine

 

Secondly, by its common name

A unique name given to each active ingredient.

i.e. glyphosate

 

Trade name

The name an individual manufacturer gives a formulated product (or combination of chemicals) that make up a formulated product (a.k.a. the name the herbicide is marketed under).

i.e. Roundup, Accord, Rodeo, Touchdown, Acquire, Glyphomax, and approximately 700+ others!

And now you know.

Any mention of herbicides in this blog does not constitute a professional recommendation by the author or her employers.

Herbicide-related definitions

When I first started my Ph.D. in Weed Science, I encountered a strange, new language that appeared to be composed, almost entirely, of acronyms. PRE. POST. PD. Layby. PPI. AI. Etc…

You see, I didn’t grow up in, or even around, agriculture; I was born and raised in the “Coal Region” of Pennsylvania and I was infinitely more likely to see anthracite sliding down the chutes of coal trucks (into my neighbors’ basements) than I was a John Deere tractor.

So I had to learn. This blog is meant to be a primer for the (similarly) uninitiated.

Firstly, a definition to get us started.

Herbicide: To be honest, there are lots of definitions out there, and they all say just about the same thing: Herbicides are materials that are used to control or kill plants.

Herbicide label: A legal document (recognized by courts of law) describing the brand name or trade name of the product; the name and address of the manufacturer; the amount of active and inert ingredients in the container; the net contents of the container; the EPA registration and establishment numbers; whether the product is for general or restricted use; directions for use, storage and disposal; re-entry, replanting, harvesting and grazing restrictions; environmental hazards and first aid treatments.

Herbicides can be further defined in several ways based on their general mode of action (contact vs systemic), their selectivity (selective vs non selective), their timing (e.g. pre-emergence vs post-emergence) and their application strategy or placement (e.g. broadcast vs banded, soil vs foliar), in addition to other characteristics. This next section will attempt to address the multiple classifications that people may expect to encounter.

Definitions describing the placement of applications.

Soil applied: Herbicides applied to the soil that come into contact with germinating or emerging weeds or into contact with the roots of emerged weeds.

Foliar applied: Herbicides that are applied directly to the plants.

Broadcast: The application of herbicides evenly across an entire area.

Banded: The application of herbicides over a portion of the total treatable area (for example, in strips on top of a seeded row).

Directed: The application of herbicides that are targeted at a very specific area (for example, at the base of a crop plant). In certain situations, this might be referred to as a lay-by application.

Definitions describing the timing of herbicide applications.

Pre-plant (PP): Herbicides applied prior to planting. Often, this may refer to herbicides that are applied well in advance of crop planting in order to treat existing vegetation.

Pre-plant incorporated (PPI): Herbicides that are applied prior to planting and that are incorporated into the soil.

Pre-emergence (PRE): Herbicides that are applied prior to crop and/or weed emergence. The herbicides that are considered PRE may also be referred to as ‘residual’ herbicides meaning that they are applied to the soil where they provide ‘extended’ control of germinating or emerged weeds.

Post-emergence (POST): May also be referred to as ‘topical’ or ‘over-the-top’ herbicides. Herbicides that are applied after crop and weed emergence.

Definitions related to herbicide selectivity.

Non-selective: Synonymous with ‘broad-spectrum’; a herbicide that controls many different types of plant species.

Selective: A herbicide that is effective at controlling some species but not others (for example, mostly broadleaves or mostly grasses).

Definitions related to activity.

Contact: Herbicides that affect only the plant tissues that they come into contact with.

Systemic: Herbicides that are translocated, or moved, throughout a plant.

Site of action: The specific, biochemical site within a plant with which a herbicide directly interests. Often confused with ‘mode of action’, which references the entire sequence of events that results in plant death and injury. For more information about herbicide sites of action, see the WSSA website.

The majority of this information was gleaned from training materials developed by the WSSA. The information was kept relatively simple on purpose.

This post was originally published on the Weed Science Blog at the UC ANR website.

Post-emergence herbicides for field bindweed management: examples from processing tomato systems in California

Field bindweed (Convolvulus arvensis L.), a deep-rooted and drought tolerant perennial,  is a significant concern of the processing tomato industry in California. If allowed to compete with tomatoes during canopy establishment, field bindweed can significantly reduce both fruit number and quality.  Furthermore, field bindweed vines can become physically entwined with tomato plants, which, in turn, can reduce harvest efficiency.

Although bindweed seedlings are relatively easy to manage using physical and chemical control strategies, established plants with extensive root systems are relatively tolerant to most management practices. For example, perennial bindweed control with tillage and cultivation is made more difficult by the weed’s significant below-ground nutrient reserves and regenerative capacity. Infrequent mechanical cultivation may also facilitate plant spread by dispersing root fragments as opposed to exhausting stored energy. Suppression of established bindweed using chemical tools may be equally challenging, especially in crops like processing tomato, where effective herbicide options are limited.

Post-emergence herbicides (applied as a pre-plant burn down, for post-harvest field cleanup, or used in-crop) can be important tools for managing field bindweed infestations. In 2013 and 2014, we conducted two trials at the University of California, Davis, research farm to evaluate the efficacy of glyphosate (as Roundup Powermax), rimsulfuron (as Matrix), and carfentrazone (as Shark) for the control of vigorously growing field bindweed vines. All herbicides were applied post-emergence (POST) using a CO₂-pressurized backpack sprayer equipped with three 8002VS flat-fan nozzles (TeeJet Technologies, Wheaton, IL) spaced 16-20 in apart and calibrated to deliver 30 GPA. Adjuvants were used according to label recommendations. Control of field bindweed was rated for 3-5 weeks after application.

Figure 1. Bindweed control (%) in response to post-emergence applied herbicides (in field, 2013) for up to 35 days (or 5 weeks) after herbicide application (DAA). Herbicides: Roundup Powermax = glyphosate, Shark = carfentrazone, Matrix = rimsulfuron.

Figure 2. Bindweed control (%) in response to post-emergence applied herbicides (in field, 2014) at application and at 1, 2, and 3 weeks after treatment (WAT). Herbicides: Roundup Powermax = glyphosate, Shark = carfentrazone, Matrix = rimsulfuron.

Results presented in Figures 1 and 2 show that POST herbicide applications of rimsulfuron and carfentrazone were largely ineffective at controlling field bindweed. In the 2013 trial, Field bindweed control with rimsulfuron did not exceed 50%; in 2014, rimsulfuron was unable to provide more than 20% control at any observation date. Carfentrazone will burn down aboveground bindweed vines, giving the appearance of effective management, although regrowth can rapidly occur. In our research trials, carfentrazone controlled field bindweed 60 and 95% at 1 WAT; however, within 3-5 WAT, control fell to 5 and 40%. Glyphosate is slower to demonstrate activity, although its suppressive ability may persist for a longer period of time.

A similar study was conducted in 2016 (Table 1), except that estimates of bindweed cover, plant vigor (on a scale of 1-5, where 1 = poor and 5 = excellent) and the percentage (%) of vines that were producing flowers were determined instead of control. The untreated check plots produced more cover (40-73%) at 1, 3, and 5 WAT as compared to the glyphosate (23-50%) and rimsulfuron (32-43%) treated plots; the higher rate of glyphosate was more effective at suppressing bindweed (25-25% cover) than the lower rate (23-50%). Bindweed vigor in the untreated plots ranged between 3 and 3.5 at every observation date. The vigor ratings for the glyphosate and rimsulfuron treatments ranged from 3-4 at 1 WAT to 1-2.2 at 5 WAT. Reductions in vigor were associated with chlorosis and necrosis of leaf and stem tissue. Field bindweed flowering was also affected by POST herbicide treatments.  Few vines exhibited any flowers, regardless of treatment on 18 May. On 1 June and 14 June, 42 and 50% of the vines untreated check plots were flowering; conversely, less than 7% of the vines in the glyphosate or rimsulfuron treatments were flowering at the same observation periods.

Table 1. Field bindweed cover (percent (%) of plot area covered with vines), plant vigor, and percent (%) of vines with flowers in response to post-emergence (POST) herbicide applications at approximately 1 (18 May, 3 (1 June), and 5 (14 June) weeks after treatment (WAT). Roundup Powermax = glyphosate, Matrix = rimsulfuron, UTC = untreated.

A comparable trial was also conducted in the greenhouse (Tables 2, 3, 4). Results show that bindweed injury, growth, and biomass accumulation were more affected by glyphosate than by rimsulfuron and carfentrazone. The injury observed in the rimsulfuron and carfentrazone treatments was more severe than what had been witnessed, previously in field trials. This is likely due to the fact that the field bindweed plants used in the greenhouse had been grown from exhumed rhizomes and did not possess the ample storage reserves that large, field-grown patches are expected to have. As has been described, previously, the highest rate of glyphosate was the most effective treatment for injuring field bindweed and suppressing plant growth.

Table 2. Number of field bindweed vines greater than 4 inches in length and length in cm of the longest vines at the time of application and at 28 days after treatment (DAT) in response to post-emergence (POST) herbicide applications. Roundup Powermax = glyphosate, Shark = carfentrazone, Matrix = rimsulfuron, UTC = untreated.

Table 3. Field bindweed injury at 3, 7, 14, 21 and 28 days after treatment (DAT) in response to post-emergence (POST) herbicide applications. Roundup Powermax = glyphosate, Shark = carfentrazone, Matrix = rimsulfuron, UTC = untreated.

Table 4. Above and below ground field bindweed biomass at 28 days after treatment (DAT) in response to post-emergence (POST) herbicide applications. Roundup Powermax = glyphosate, Shark = carfentrazone, Matrix = rimsulfuron, UTC = untreated.

It is important to recognize that the timing of herbicide applications can also significantly affect weed control performance. For example, numerous growers, commercial applicators, and university personnel have reported that the performance of several herbicides (i.e. glyphosate, glufosinate, paraquat) may fluctuate with respect to application time of day (diurnally). Possible factors influencing herbicide performance include: circadian changes in leaf angle that affect herbicide interception; differences in humidity and temperature that may affect herbicide absorption and translocation; the presence of dew, which can reduce herbicide retention; and physiological processes that may be affected by lack of sunlight.

In 2016, we conducted trial to evaluate if the herbicidal efficacy glyphosate (as Roundup Powermax at 1 and 2 qt/A), rimsulfuron (as Matrix at 2 oz/A), carfentrazone (as Shark at 2 oz/A), and paraquat (as Gramoxone Inteon at 3 pt/A) varied with the time of day the herbicides were applied. Herbicides were applied to vigorously growing bindweed vines on 29 June, 2016, at five different times during the day: sunrise, 2 hours after sunrise, mid-day, 2 hours before sunset, and at sunset. All herbicides were applied using a CO₂-pressurized backpack sprayer equipped with three 8002VS flat-fan nozzles (TeeJet Technologies, Wheaton, IL) spaced 16-20 in apart and calibrated to deliver 30 GPA. Field bindweed cover (percent of the plot area covered with field bindweed vines) was rated for 5 weeks after the treatments were applied (WAT).

Table 5. Field bindweed cover for up to 5 weeks after treatment (WAT), which occurred on 29 June, 2016, in response to post-emergence (POST) herbicide applications made at sunrise, 2 hours after sunrise, midday, 2 hours before sunset, and sunset. Herbicides: Roundup Powermax = glyphosate, Shark = carfentrazone, Matrix = rimsulfuron, Gramoxone Inteon = paraquat, UTC = untreated.

Results from the diurnal timing trial (Table 5) indicated that herbicide, alone, had a significant effect on bindweed cover; the timing of herbicide applications and the interaction between herbicide and the time of day when the herbicides were applied were not significant. Although the carfentrazone and the paraquat treatments worked rapidly (field bindweed cover was reduced by more than 90% at 1 WAT), the vines regrew vigorously; cover at 5 WAT was 45 and 42%, which was similar to what was observed for the untreated check (37%). Glyphosate, while slower acting due to its systemic nature, was significantly better at reducing field bindweed cover at 5 WAT (2 and 5%) relative to the untreated check. In this trial, rimsulfuron did not reduce field bindweed cover, relative to the control, at any point in time. Bindweed cover in the untreated check and the rimsulfuron treatment decreased over time due to the development of powdery mildew, which resulted in leaf loss. Interestingly, the bindweed regrowth in the carfentrazone and paraquat treatments was robust and did not show signs of infection during the course of this study.

Rimsulfuron is labeled for POST use in California processing tomatoes (http://ipm.ucanr.edu/PMG/r783700311.html) although its efficacy as a POST herbicide, with respect to field bindweed control, is poor. Paraquat and carfentrazone are both labeled for the control of emerged weeds prior to transplanting, although perennial weed control will not be long lasting as both products are contact herbicides and will do little more than burn off any above ground foliage. Glyphosate, was the most effective POST product for suppressing field bindweed growth across all trials; it is labeled for use as a pre-plant burn down. One of the tenets of integrated pest management is to start off clean and remain clean to prevent weed interference in the current and future crops. Growers with significant bindweed problems should strive to ensure effective burn down of existing vines prior to crop planting and following harvest; the management of bindweed in rotation crops and following rotation crop harvest is also encouraged.

The mention of herbicides in this post does not constitute a professional recommendation by the author or her employers.

The efficacy of homeowner, ready-to-use herbicides on field bindweed

Do you have a problem with field bindweed? Let’s face it, if you have it, you have a problem. The species is problematic in many agricultural and horticultural crops; it is also a concern of homeowners, many of whom aren’t familiar the biology and ecology of bindweed as well as the tools available to manage the species.

So, first, some basic information…

SPECIES NOMENCLATURE:

Field bindweed was first named by Linnaeus in 1753; its Latin binomial (Convolvulus arvensis) is derived from convolvere (“to roll together”) and arvense (“in the field”). Which is pretty appropriate, if you ask me.

BINDWEED BIOLOGY

Field bindweed is a persistent perennial in temperate climates that can colonize a multitude of habitats including: roadsides, railways, pastures, cultivated fields, orchards and vineyards, lawns, and home gardens. Emerging vines may arise from germinated seeds (as evidenced by the presence of cotyledons, which are almost square but for rounded edges) or from underground stems called rhizomes (no seed leaves are present) (Figure 1). Field bindweed leaves average between 1/2 and 2 inches in length, are alternately arranged along the vine, and are arrowhead-shaped. The species displays some plasticity with respect to the growing environment; under ideal conditions, the leaves (and vines) will be larger and more robust than they might appear during times of drought. The vines are typically prostrate, although they will grow and twine up through flowers, crops, and shrubs.

  Figure 1. Field bindweed seedling emerging (left) as compared to a vine emerging from over-wintering rhizomes.

Flowers are trumpet-shaped (resulting from the fusion of five petals) and are about 1 inch in diameter. Flowers are white to pink in coloration and last for a single day. Fun fact: a related species, Convolvulus tricolor, was included in Linneaus’ flower clock. To find out more, do an Internet search for ‘flower clock’, ‘circadian rhythm’, and/or ‘chronobiology’. Field bindweed seeds are produced in papery capsules with an average plant producing between 500 to 600 seeds (Figure 2). Individual seeds are 3-sided, brown to grey in color, and have hard, impermeable coats. This mechanically enforced dormancy ensures that seeds can survive for many years in soil.

Figure 2. Rounded capsules containing between 1 and 4 field bindweed seeds, each, along the length of a vine. 

Field bindweed is less affected (than many other species) by drought conditions due to its deep and extensive root system. Although the majority of the species’ lateral roots and rhizomes are found in the upper two feet of the soil surface, some vertical roots can extend to depths of 10 to 20 feet, or more.

Field bindweed can be easily mistaken for other closely related species, including: hedge bindweed (Calystegia sepium), wild buckwheat (Polygonum convolvulus) and numerous morningglory species (Ipomoea spp.). For more information regarding species identification/discrimination, please see the following websites:

King County, WA: http://www.kingcounty.gov/environment/animalsAndPlants/noxious-weeds/weed-identification/field-bindweed.aspx

Oregon State University: http://oregonstate.edu/dept/nursery-weeds/feature_articles/vines/vine_weeds.html

UC IPM: http://www.ipm.ucdavis.edu/PMG/WEEDS/morningglories.html

Next up, a look at some control options…

BINDWEED MANAGEMENT

Anyone who has attempted to manage field bindweed knows that the efficacy of any strategy (chemical, cultural, and physical) is dependent upon the size of your infestation, as well as the magnitude of your patience. The UC IPM webpage provides advice regarding field bindweed control. It is well worth a review.

In August/September 2015 and July 2016, I conducted two small studies to look at the efficacy of ‘homeowner’ or ‘off the shelf’ or ‘ready-to-use’ herbicides (purchased from local hardware stores) on field bindweed suppression.

Study 1 was conducted in August, 2015. Each herbicide was applied to three 1m by 1m replicate plots. The active ingredients evaluated included:

• citric acid, plant oils (cinnamon, clove, soybean, rosemary, sesame, thyme) (9.5% of total formulated product volume)
• commercial soaps of fatty acids (3.68%)
• glyphosate (25%)
• 2,4-D (9.74%), dicamba (0.84%), mecoprop (1.83%)

Study 2 was also conducted in August, 2015. Each herbicide was applied to four 1m by 1m replicate plots. The active ingredients evaluated included:

• citric acid, plant oils (cinnamon, clove, soybean, rosemary, sesame, thyme) (9.5%)
• commercial soaps of fatty acids (3.68%)
• glyphosate (2%), pelargonic acid (2%)
• 2,4-D (0.31%), quinclorac (0.1%), dicamba (0.03%)

Study 3was conducted in July, 2016. Each herbicide was applied to three 1m by 1m replicate plots. The active ingredients evaluated included:

• citric acid, plant oils (cinnamon, clove, soybean, rosemary, sesame, thyme) (9.5% of total formulated product volume)
• commercial soaps of fatty acids (3.68%)
• glyphosate (1.12%)
• MCPA (0.33%), mecoprop (0.07%), dicamba (0.02%), carfentrazone (0.002%)

The glyphosate and 2,4-D/dicamba/MCPP products in Study 1 were diluted to 1 oz and 4.5 oz product/gallon, respectively. All of the products were applied to emerged bindweed vines (which were flowering at the time of application). Plants were treated ‘until wet’ (as suggested on the product labels), meaning that the herbicide solutions were beginning to run off of the leaves; none of the products provided soil-based residual/extended weed control.

Results are presented in Tables 1,  2 and 3.

Table 1. Field bindweed control (as a percentage from 0 [no injury observed] to 100 [completely dead]) in study 1 from 3 DAT to 28 DAT.

	Percent (%) Control
	3 DAT	7 DAT	14 DAT	21 DAT	28 DAT
citric acid, essential oils	30	51	40	23	20
fatty acid soap	99	96	53	33	30
2,4-D, MCPP, dicamba	10	41	99	99	99
glyphosate	6	46	99	99	99

 

Table 2. Field bindweed control (as a percentage from 0 [no injury observed] to 100 [completely dead]) in study 2 from 3 DAT to 28 DAT.

	Percent (%) Control
	3 DAT	7 DAT	14 DAT	21 DAT	28 DAT
citric acid, essential oils	27	32	12	12	12
fatty acid soap	98	96	63	38	36
2,4-D, quinclorac, dicamba	22	40	95	99	99
glyphosate, pelargonic acid	90	97	99	96	96

 

Table 3. Field bindweed control (as a percentage from 0 [no injury observed] to 100 [completely dead]) in study 3 from 3 DAT to 28 DAT.

	Percent (%) Control
	3 DAT	7 DAT	14 DAT	21 DAT	28 DAT
MCPA, mecoprop, dicamba, carfentrazone	50	97	98	96	93
glyphosate	0	69	87	87	86
citric acid, essential oils	85	67	24	12	3
fatty acid soap	95	86	64	38	17

In each of the studies, control of field bindweed with the the citric acid/essential oil and fatty acid products ranged from 85-99% at 3 days after treatment (DAT). Control decreased as with increased time (32-96% control at 7 DAT, 12-64% control at 14 DAT, 12-38% control at 21 DAT, 3-36% control at 28 DAT). Both the citric acid/essential oil and fatty acid herbicides are considered ‘contact’ products, meaning that they are only active against the plant tissue that they come into contact with. In essence, these products burned only the leaves and stems that they were applied to. While these contact herbicides are likely to be effective at managing bindweed seedlings, perennial vines can draw upon stored root reserves and re-sprout. Re-treatment on a regular basis will likely be necessary.

The glyphosate and auxinic herbicide-based products acted more slowly (because of their systemic nature), but provided 86-99% control of field bindweed at 28 DAT.Systemic herbicides are more likely to provide ‘long-term’ control of perennial weeds as these products will be moved away from the treated foliage and  dispersed throughout the  plants. For example, glyphosate is translocated to all growing points (i.e. both shoort and roots), where it interferes with amino acid synthesis; as a consequence, field bindweed treated with glyphosate will regenerate/recover more slowly. This doesn’t mean that re-treatment is not needed; bindweed is a tough perennial and a single herbicide spray (or cultivation event, or hand-weeding party) is unlikely to eradicate this pest.

HERBICIDE PERFORMANCE AND SAFETY

Before choosing a herbicide, be sure to understand how and for how long the products will work. For perennial weeds, systemic herbicides will likely provide greater/longer suppression than contact products, although it is reasonable to expect that a single application may be insufficient for complete control. This is an important point to remember: herbicides are not always 100% effective. Herbicides may fail for many reasons including: poor spray coverage on large weeds, applications to stressed weeds, herbicide wash off, use of an ineffective active ingredient against the target species, etc. Herbicides, if used improperly, can cause damage to desirable species; anyone using herbicides should be aware of their selectivity (i.e. what plants they control) and the conditions that can induce spray drift or volatility. And, as always, people using herbicides should be mindful to follow ALL instructions listed on the labels to ensure the safety of themselves, their family and pets, and the environment.

The mention of herbicides in this post does not constitute a professional recommendation by the author or her employers.

A version of this post was originally published by Lynn M. Sosnoskie at the UC ANR Weed Sciences blog site: http://ucanr.edu/blogs/ucdweedscience/

Drought and Herbicide Efficacy: Some Thoughts

Much of the Western United States has been suffering drought conditions for several years; Under prolonged periods of dry weather, weed control is likely to suffer. This is especially troubling for growers as both crops and weeds will be competing to capture limited soil moisture, which could result in significant yield losses.

Although fewer weed seeds may germinate under dry conditions, weeds that do emerge and become established may be more difficult to manage with herbicides. Drought-stressed weeds are likely to have thicker cuticles (which is the waxy coating on the surface of the leaf), which can inhibit the absorption of post-emergence products. Additionally, plant architecture can be altered when it is hot and dry (e.g. fewer and drooping leaves) meaning that herbicide capture and retention may be reduced. When weeds aren’t actively growing, systemic herbicides (e.g. glyphosate) may not be effectively translocated to their target sites. Although contact herbicides (e.g. paraquat, carfentrazone, oxyfluorfen) are less likely to be affected by dry conditions, herbicide efficacy could be reduced if spray droplets dry rapidly (either in the air or on plant surfaces) and sufficient coverage isn’t achieved.

Species shifts may also occur; deep-rooted, drought-tolerant perennials (like field bindweed (Convolvulus arvensis) may become more dominant in dry conditions. With roots that can extend many feet (some reports suggest up to 10-20 feet deep!) underground, this species is capable of accessing moisture that is unavailable to other, shallowly-rooted weeds. It’s this extensive root system (and carbohydrate reserves) that also enables the species to tolerate weed control measures, such as herbicide applications and cultivation.

Without precipitation or irrigation, many soil-applied herbicides cannot be activated (which means the herbicide is moved into solution so that they can be taken up by emerging weed seedlings). Some herbicides can be mechanically incorporated, although product distribution may be uneven in dry soils. Furthermore, herbicides may become tightly bound (adsorbed) to soil particles, which could result in carryover injury on following crops; growers may need to be more cautious with respect to subsequent crop planting dates after a dry season. Additionally, the potential for photo-degradation or volatilization (in part due to poor incorporation and activation) can be increased under hot and dry conditions, resulting in reduced herbicide efficacy and/or off-target movement. Dry conditions can also affect spray droplets; desiccation of the droplet after it has left the nozzle, but before it contacts a plant or soil, may increase the chances of drift (i.e. droplet size is shrinking and is more prone to movement). Additionally, the rapid drying of droplets on a leaf surface may reduce the potential for the herbicide to be taken up by the plant.

Weed control will, undoubtedly, be made more difficult during a drought. The efficacy of both soil- and foliar-applied herbicides may be reduced. Poor performance may be the result of drought affects on the weeds (e.g. altered growth, development and physiological activity) or on the herbicides (e.g. reduced activation, adsorption, uneven distribution, volatilization). Mechanical weed control (e.g. shallow cultivation) may be an option to eliminate small, annual weeds, but could result in the loss of soil moisture. Needless to say, the timing of control applications will be critical; growers should attempt to make foliar applications when emerged weeds are still small and succulent or soil applications as close to a rainfall or irrigation event as possible in order to maximize performance. Seek the advice of consultants and extension personnel if you are unsure about the use of a specific product, whether it be an herbicide or an adjuvant, in a drought situation. ALWAYS follow instructions/recommendations on the product label and ensure that equipment is properly calibrated so that pesticides (and money) are not being wasted, the environment is being protected, the potential for injury to current and future crops is minimized, and illegal residue levels are prevented.

The mention of herbicides in this post does not constitute a professional recommendation by the author or her employers.

A version of this post was originally published by Lynn M. Sosnoskie at the UC ANR Weed Sciences blog site: http://ucanr.edu/blogs/ucdweedscience/