- Author: Karey Windbiel-Rojas
- Posted by: Gale Perez
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.
[Originally featured in the Winter 2022 Issue of UC IPM's Green Bulletin Newsletter]
Weeds can be a problem in any landscaped areas including around trees, shrubs, flower beds, or lawns and turf. As we move from cool weather to warmer temperatures, you will see winter weeds grow and become a problem in established landscape plantings. Effective control of weeds include hand-weeding, hoeing, mulching, and herbicide applications. Good management depends on early attention to where weeds are establishing and adjusting the conditions that allow them to thrive.
Managing weeds in landscape plantings
Each type of planting bed will have specific techniques that work best. In general, dense plantings will shade out most weeds. Regardless of the type of landscape bed, it's always best to control perennial weeds before planting. Herbicides are effective in many types of landscape plantings. They are most effective when integrated with cultural practices. Many of the herbicide active ingredients available for weed control in landscape plantings are only for use by pest management professionals.
Tree and shrub beds
Landscaped areas made up of trees and woody shrubs don't need as much preplant weed control as other types of beds. Control perennial weeds after planting using methods like mulching, hand pulling, and herbicide treatments. Suppress weed growth by laying down landscape fabric, then adding an inch of mulch on top to thoroughly cover the fabric. If needed, use a preemergence herbicide. Supplement with spot treatments of postemergence herbicides and hand-weeding.
Ground cover beds
Since ground cover is expected to fill the entire bed, landscape fabric is not suitable for weed suppression. Perennial weeds should be controlled before planting. If perennial grasses are encroaching, they can be controlled with selective herbicides like fluazifop, clethodim, or sethoxydim. Spot applications of glyphosate or glufosinate can be used on perennial weeds. Mulch the bed to control annual weeds until the ground cover fills the area. Some hand weeding might be needed.
Annual flower beds
As with other landscaped areas, a dense planting will shade out weeds. Annual weeds can be managed with mulches, frequent cultivation, and hand-weeding. Periodic cultivation (every 3 to 4 weeks) will suppress many weeds. Since nonselective herbicides can't be used after planting annual beds, it's easier to manage perennial weeds beforehand. If cultural methods aren't working to control perennial grasses, you can use grass-selective herbicides with clethodim or fluazifop. Check the product label to be sure that it won't harm the annual flowers in the bed.
Herbaceous perennial beds
Manage weeds in herbaceous perennial beds as you would an annual flower bed. Be sure to get rid of perennial weeds before planting since the bed will be growing for more than one season. Use landscape fabric where possible along with mulches. You might need to supplement with hand-pulling followed by preemergence herbicides. Be aware that fewer perennial plants are included as sites on herbicide labels.
Mixed plantings
A planting bed of a mix of woody and herbaceous plants is a more complex situation. Different areas of the bed might need different treatments. Post-plant herbicide choices are limited so site preparation is critical in this type of bed. Plant woody species first and control the perennial weeds. After the first two growing seasons, add the herbaceous plants. Shade the soil with close planting. Group plants within the bed based on their weed management needs.
Cool weather weeds in landscapes
Some of the most troublesome weeds in planting beds during late winter and early spring are common groundsel, oxalis, mallows, and nutsedges.
Common groundsel
Common groundsel is most prolific in cool weather, germinating from seeds this time of year. This weed produces many seeds and can rapidly infest landscape beds. It is best controlled before it flowers. Mulch is highly effective at controlling common groundsel. Young plants can be hoed out. Diquat or glyphosate-based herbicides will control common groundsel in landscape beds.
Mallows
Mallows are annual weeds that begin growing with the first rains so you may already be seeing these sprouting up in landscape beds. This plant develops a long taproot so it should be pulled when it has four or fewer true leaves. At least three inches of mulch is needed to suppress mallow. Young mallow plants might be managed with 2,4-D products, but this herbicide will injure broadleaf plants growing nearby.
Nutsedges
Purple and yellow nutsedge are perennial plants that sprout in spring from tubers. Remove these weeds as soon as possible to prevent tuber production. Tubers (sometimes referred to as “nuts” or “nutlets”) are key to nutsedge survival. Once established, nutsedge plants are difficult to control. They don't grow well in shade so dense plantings of ground cover or shrubs will suppress nutsedges. Few herbicides are effective at controlling nutsedge.
Oxalis (creeping woodsorrel and Bermuda buttercup)
While Oxalis (creeping woodsorrel) can bloom almost any time during the year, spring is a time of heavy flowering and seed formation. Buttercup oxalis sprouts in fall and is a major weed in ornamental plantings. Hand pulling can control these weeds but be aware that mowing can spread creeping woodsorrel. Landscape fabric with two to three inches of an organic mulch on top can control oxalis. There are no selective postemergence herbicides for creeping woodsorrel in ornamental plantings.
Herbicide injury
Desired plants could be injured when herbicides are used in established landscape beds. Herbicide damage symptoms vary depending on the herbicide and the plant. Symptoms can include yellowing, bleaching, distorted growth, and death of leaves. Avoid herbicide injury by following the label about the site, plant, and application rate. Granular formulations are less likely to damage plants than sprays. When using a nonselective liquid herbicide, apply on a calm day using low pressure and large droplets. Use a shielded sprayer to avoid contact with nontarget plants. If plants are injured from soil-applied herbicides, the damage is often temporary but can cause growth inhibition. Adding organic amendments and keeping the soil moist will help the herbicides to break down faster.
For more details and for information about weed management before planting a landscape bed, see Pest Notes: Weed Management in Landscapes.
- Author: Saoimanu Sope
UC climate-ready landscape trials identify low-water yet attractive plants
Good news: roses can be a part of your water-efficient landscape. Lorence Oki, UC Cooperative Extension environmental horticulture specialist in the UC Davis Department of Plant Sciences, identified rose cultivars that remain aesthetically pleasing with little water.
Oki is the principal investigator of the Climate-Ready Landscape Plants project, which may be the largest irrigation trial in the western U.S., and the UC Plant Landscape Irrigation Trials (UCLPIT), the California component of that project. These projects evaluate landscape plants under varying irrigation levels to determine their optimal performance in regions requiring supplemental summer water.
“There are some assumptions that pretty plants use a lot of water, like roses,” Oki said. “Everyone thinks they need a lot of water, but we've found some that don't, and they still look great. A water-efficient landscape doesn't need to look like a Central Valley oak-grassland in the summer. It can look really attractive.”
In 2021, Oki's team at UC Davis identified Lomandra confertifolia ssp. pallida "Pom Pom" Shorty and Rosa "Sprogreatpink" Brick House® Pink as two of the best low-water plants in the trial. 
“The useful tip or information that is shared at the end of each trial is the selection and designation of plants as Blue Ribbon winners. These are the plants that looked good with an overall rating of 4 or higher throughout and were on the low (20%) water treatment,” said Natalie Levy, associate specialist for water resources, who manages the project at the UC ANR South Coast Research and Extension Center.
How plants earn a blue ribbon
Each trial year, the selection of new plants is based on research recommendations and donated submissions from the nursery industry. The landscape plants are trialed in full sun or 50% shade cover.
Irrigation treatments are based on the rate of evaporation and plant transpiration (evapotranspiration) measured through a local California Irrigation Management Information System (CIMIS) weather station that provides a reference evapotranspiration (ETo) rate.
Three levels of irrigation are provided to the plants equal to 20%, 50%, and 80% of ETo. The volume of water applied is the same at each irrigation based on soil characteristics, but the interval between applications varies with weather and the treatment. Using this method, irrigations for the 20% treatment are less frequent than the 80% treatment.
“The 20% treatment during the 2022 trial was irrigated an average of once per month while the 80% treatment was irrigated weekly,” explained Levy.
During the deficit irrigation trial, monthly height and width measurements are taken to determine the plant growth index. Monthly qualitative aesthetic ratings on a scale of 1 to 5 are determined for foliage appearance, flowering abundance, pest tolerance, disease resistance, vigor and overall appearance.
A second round of flowering abundance and overall appearance measurements are also taken to capture more of the blooming period. For example, UCLPIT identified in the 2020 trial at South Coast REC that the "Apricot Drift" rose had a mean overall appearance score of 3.5 out of 5, deeming it “acceptable to very nice” and a low water use plant within the Water Use Classification of Landscape Species or WUCOLS guide.
Project expands options for landscape planting
“(WUCOLS) only has 3,500 plants in it. There are guesses that there are close to 10,000 cultivars in urban landscapes in California, if not more,” said Oki. “WUCOLS also didn't have numerical ratings. Instead, you'll see verbal ratings like ‘low water use' or ‘high water use.'”
The UCLPIT project has not only developed numerical recommendations for irrigation, but it has also added new landscape plants that are compliant with California's Model Water Efficient Landscape Ordinance. In fact, UCLPIT's data is one of the few sources that can be used to supplement WUCOLS.
Geographic diversity of trial sites adds to knowledge base
In addition to UC Davis and South Coast REC in Irvine, the trials have expanded beyond California as the Climate-Ready Landscape Plants project and is in progress at Oregon State University, University of Washington, University of Arizona and Utah State University thanks to a USDA/CDFA grant awarded in 2020.
Lloyd Nackley, associate professor of nursery production and greenhouse management at Oregon State University, is the principal investigator of the trial in the Portland metro area, which is entering its third year.
“People know that there are drought tolerant plants, but there are many. We're trying to highlight lesser known or newer varieties. And even though the trial is three years, most gardeners would hope that their garden lasts longer than that,” said Nackley.
One of the observations that Nackley recalls is of the Hibiscus Purple Pillar plant. Unlike the trial at South Coast, the Purple Pillar did not perform well in Oregon in the spring.
“It wasn't until August that we saw the plant bloom and begin to look like what we saw from South Coast in April,” Nackley said.
Ursula Schuch, horticulture professor and principal investigator of the trial taking place at the University of Arizona, was also surprised at the range of performance among different plant types and the effects of irrigation, heat and temperature.
“This research will reassure green industry professionals that they can stretch their water budget to successfully cultivate more plants, watering them according to their needs instead of irrigating every plant according to the highest water-using plants,” said Schuch.
Although research is only conducted in the West, the hope is that there will be trials in other regions of U.S.
Doing so would yield comprehensive information about the plants and their performance in different climates. As extreme weather events persist in the U.S., disease pressure and risks do too. Trials throughout the country would provide location-specific data regarding disease susceptibility.
To learn more about the UCLPIT research project, visit https://ucanr.edu/sites/UCLPIT/
- Author: Lauren Fordyce
An immense amount of rain has fallen across California in recent weeks. While rain is incredibly beneficial, in excess it can cause serious problems. The continued wet conditions opens the door for many pests, so see the list of dos and don'ts below for common wet weather pest prevention tips.
DO
- Check for snails and slugs. These critters thrive in moist environments and can often be seen on sidewalks and driveways after rain. Their feeding causes irregular holes on leaves and flowers, and they'll leave behind a slimy trail. Pesticide sprays and dusts will not be effective under such wet conditions and therefore, should not be used. Instead, hand pick them or trap them using a wooden board trap.
- Remove weeds and unwanted plants. When the ground is wet it is much easier to pull weeds and remove unwanted plants from your garden or landscape. Use this time to your advantage by catching up with any winter annual weeds, like oxalis, nutsedge, and groundsel. Be on the lookout for more in the coming weeks.
- Dump standing water. As the weather warms, any areas left with standing water will be a breeding zone for mosquitoes. This wet winter is already favoring a big mosquito season, so do your part to reduce habitat around your home. This can include dumping flowerpots and saucers, wheelbarrows or buckets, and cleaning clogged storm drains or gutters.
- Continue to conserve water. During the fall and winter months, adjust your irrigation schedule to reflect the increase in rainfall. Consider purchasing a rain sensor for your sprinkler system to avoid irrigating while it is raining. Overirrigating your plants during this time can lead to root rots and other water-borne pathogens so do what you can to reduce excess water.
- Be on the lookout for ants, cockroaches, and earwigs. These insect pests may invite themselves into your home when flooding or heavy rains make it unfavorable for them outside. Seal any cracks or openings in your home to prevent them from coming indoors. Use weatherstripping and door sweeps, and place sticky traps near entryways. Keep food sealed tightly and maintain a clutter-free environment to prevent these pests from establishing indoors.
- Remove mushrooms. Wet weather encourages the growth of above ground fruiting bodies of fungi. While not harmful to your garden or lawn, you may want to remove mushrooms to prevent children and pets from consuming them.
DON'T
- Fertilize your garden, lawn, or outdoor potted plants. Any fertilizer applied now will likely be washed off the ground or rapidly leached out of the soil and into our waterways. Wait until the winter storms have passed and there is a stretch to time between days with rain.
- Use pesticides (sprays, dusts, drenches). Similar to fertilizers, these products are more likely to just contaminate waterways than control any pests. Pesticides applied to foliage as sprays will be washed away quickly with daily rainfall and those applied to the soil as a systemic drench will likely not be taken up by the plant in waterlogged soils and instead will become runoff. Pesticide dusts need to remain dry to be effective, so now would not be the time to use those outdoors either.
- Prune plants. Unless necessary to remove damaged limbs or branches from the recent windstorms, avoid pruning plants under wet conditions. Pruning at this time can make plants vulnerable to pathogens and easily spread disease from one plant to another. Apricots, cherries, and olives should never be pruned during cool, wet conditions.
- Worry about tiny piles of soil. Earthworm activity is increased during rainy weather, so if you are seeing piles of soil on top of landscape beds or the lawn, don't worry—it's just the earthworms coming out and getting some air. As they do this, they help aerate the soil. Earthworms deposit castings when they ingest soil and leaf tissue and emerge from the soil surface to remove fecal matter. Castings are rich in nutrients and organic matter and can provide some benefits to turfgrass plants.
- Compact your soil. Avoid driving or excessive walking on soft ground and keep heavy machinery off wet soils. Compacted soils make lawns, trees, and shrubs more susceptible to diseases, drought, and insects as they restrict oxygen and water from plant roots.
For year-round actions to keep landscape plants healthy, reduce pest problems, and prevent future issues, see the Seasonal Landscape IPM Checklist to find monthly activities specific to your county or region.
Ripe, juicy, sweet blackberries: what's not to love? Blackberries are grown for us to eat and enjoy, but some species can be considered weeds when they take over home landscapes, roadsides and waterways, and other areas. The most problematic species are the introduced wild blackberries, cutleaf blackberry and Himalayan blackberry. Blackberries can be highly competitive, smothering existing plants with their dense stands. Accumulation of dead stems can create a dangerous fire hazard.
In urban landscapes, blackberry brambles can create habitat and food for wildlife and birds, but also for rats and other pests. When invasive wild blackberries take over a landscape with their thorny, fast-growing stems, the fruit may seem less loveable. However, since people enjoy the fruit, wild blackberry plants can be controlled to a desired level.
The newly revised Pest Notes: Wild Blackberries, authored by UC Cooperative Extension advisor Scott Oneto and emeritus UC Davis weed scientist Joe DiTomaso, includes detailed descriptions of the wild blackberry species found in California, as well as information on how to control blackberry populations using mechanical and updated chemical management strategies.
![Wild blackberries growing along a roadway and encroaching into the home landscape. [Credit: Scott Oneto]](https://ucanr.edu/blogs/blogcore/allPostsByCategory.cfm?getallposts=1&tagname=landscape/blogfiles/95850_original.jpg "Wild blackberries growing along a roadway and encroaching into the home landscape. [Credit: Scott Oneto]")
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