BY CELESTINE DUNCAN
Scentless chamomile, also known as daisy or scentless false may-weed (Matricaria perforata or Tripleurospermum perforatum), is an annual, biennial, or rarely perennial forb. The plant is native to Europe and was likely introduced to North America in the 1930’s as an ornamental or crop-seed contaminate. The weed is widespread in Canada, Alaska, and much of the United States (Figure 1). It establishes well in moist, disturbed areas along streambanks, meadows, riparian areas, pastures, and hayfields.
Scentless chamomile reproduces only by seed, with individual plants capable of producing more than 300,000 seeds a year.
Seed can be widely dispersed on equipment, in contaminated crop seed and feed, and by water. Seedlings germinate primarily during spring and fall, forming a dense mat, especially on disturbed areas. Fall-germinated seedlings overwinter and are usually the first to flower in spring. The seed does not need a dormancy period to germinate and can remain viable for up to 15 years when buried in soil.
Flowering occurs from May through October, depending on moisture. Flowers have a yellow central disk surrounded by while petals, similar to oxeye daisy (Fig. 2 and Sidebar: What’s the Difference Between Scentless Chamomile and Oxeye Daisy). Flowering stalks are slender, branched, and from six inches to three feet in height. Leaves are alternate, fernlike, finely divided, and odorless when crushed (Fig. 3). This is not the chamomile used as a tea.(1)
Scentless Chamomile is not competitive in healthy perennial plant communities. However, it quickly colonizes disturbed sites and can reduce establishment of desirable native vegetation. Although most grazing animals avoid the plant, it has been observed to cause blistering on livestock muzzles and irritation to mucous membranes.
The key to effective management of scentless chamomile is prevention, early detection, and control. Eradication is difficult once the invasive plant has established and produced seed. Transportation networks, such as road and railway systems, serve as corridors for scentless chamomile spread. Maintaining weed-free road and railway right-of-ways, cleaning equipment, and tarping hay/grain trucks will reduce seed spread. Seeding desirable perennial vegetation on disturbed sites will reduce establishment of scentless chamomile.
Several selective broadleaf herbicides will effectively control scentless chamomile, including Transline® and Milestone® specialty herbicides. Scentless chamomile is reported to be resistant to 2,4-D, and applicators should alternate between different herbicides to reduce the possibility of developing herbicide resistance.
Field trials were conducted at more than 32 sites in North America to determine the effectiveness of Milestone on scentless chamomile. Plant growth stage at application ranged from rosette to late-bud. Results of studies showed that applying Milestone at 4 to 7 fluid ounces per acre (fl oz/A) from the rosette to the pre-bud growth stage provides good control of scentless chamomile. Optimum control is achieved when plants are 12 inches in height or less. Scentless chamomile plants treated at the flowering growth stage are still capable of producing seed. The addition of metsulfuron-methyl to Milestone does not significantly improve control over Milestone applied alone.
Areas with desirable herbaceous vegetation should not be treated with a non-selective herbicide, such as glyphosate. Vegetation growing on the site will be killed or injured with a glyphosate application thus eliminating desirable plants that compete with newly germinating scentless chamomile.
Mechanical and Manual Control
Mowing can reduce seed production in pastures, hay land, and non-cropland. The plants should be cut before the flowers are fully formed; however, plants will flower below the cutting height. Repeated mowing can also reduce the competitive ability of desirable vegetation, causing an increase in scentless chamomile seedling establishment.
Tillage effectively controls scentless chamomile seedlings on pasture or cropland. Frequent, shallow tillage will kill seedlings. Deep cultivation should not be used because it buries seed and extends the longevity of seed viability in soil.
Hand-pulling is effective on small infestations and can prevent the spread of scentless chamomile into new areas. Plants that are flowering should be bagged and removed from the site.
Two insects are established for the control of scentless chamomile in Canada: Omphalapion hookeri, a seed-head-feeding weevil from Germany was released in British Columbia in 1992 and reduces seed production in scentless chamomile; and Rhopalomyia tripleurospermi, a gall midge from eastern Europe, released in British Columbia in 1999 reduces scentless chamomile vigor and flowering ability. Both insects are listed as established and dispersing in Canada. In the United States, collection and release of these two agents has not been approved by Animal Health Inspection Service (Merenz pers. comm.).
Land management practices that improve desirable plant competition reduces establishment of scentless chamomile and increases effectiveness of management methods. Practices that increase the competitiveness of desirable plant species and communities—such as proper grazing management, critical area planting (e.g. following wildfire or disturbance), and seeding competitive forage on hay and pastureland—will make the environment less suitable for invasive plant establishment and spread. Use of selective broadleaf herbicides can effectively remove scentless chamomile from desirable grassland communities. With any management method, follow-up monitoring after treatment is important to control seedling populations germinating from the long-lived soil seed bank.
Alberta Department of Agriculture. 2007. Scentless Chamomile Biology and Control. Agri-Facts Agdex 640-6. ( www1.agric.gov.ab.ca/$department/deptdocs.nsf/all/agdex871 )
British Columbia Ministry of Forest, Lands and Natural Resources. Omphalapion hookeri (Kirby) and Rhopalomyia tripleurospermi (Skuhrava). Accessed: January 1, 2018. Available online
( www.for.gov.bc.ca/HRA/plants/biocontrol/index.htm )
Dow AgroSciences. Internal field trial data.
Merenz, Richard. 2017. Personal communication. Plant Protection and Quarantine Officer, USDA APHIS PPQ. Helena, MT.
USDA, NRCS. 2018. The PLANTS Database (plants.usda.gov, January 1, 2018). National Plant Data Team, Greensboro, NC 27401-4901 USA.
1. People living in North America are familiar with chamomile tea, an infusion used to calm upset stomachs or to help with sleep. Two types of chamomile are used for health: German chamomile (Matricaria recutita) and Roman chamomile (Chamaemelum nobile).
2. Metsulfuron-methyl rate ranged from 0.37 to 0.45 ounces product per acre.
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Milestone® specialty herbicide is not registered for sale or use in all states. Contact your state pesticide regulatory agency to determine if a product is registered for sale or use in your state. When treating areas in and around roadside or utility rights-of-way that are or will be grazed, hayed, or planted to forage, important label precautions apply regarding harvesting hay from treated sites, using manure from animals grazing on treated areas or rotating the treated area to sensitive crops. See the product label for details.
State restrictions on the sale and use of Milestone and Transline apply. Consult the label before purchase or use for full details. Always read and follow label directions.