1. Overview

2. ID Guide

What does it look like?

Scentless chamomile is a short, annual (sometimes biennial) herb with small daisy-like flowers having white petals and a yellow center. The Asteraceae (sunflower) family is notoriously large, with many species possessing similar foliage and flowers to scentless chamomile. Most of these similar plants can be distinguished by their flowers or crushed foliage, which have either a pleasant or bitter smell, while scentless chamomile, as the name implies, is scentless. [3,6]

Having trouble identifying a weed? Contact your local conservation district or county extension agent.

Key Features

• Plant: The stems typically exhibit an erect growth pattern, with ascending branches that may display sparse hairiness during their early stages of development; however, they can also appear prostrate. The plant will typically reach heights of 12 to 32 inches. [1,3]
• Roots: This plant grows from an extensive, fibrous root system, although it is quite shallow. New roots will sprout from the bottom of older stems. [4,6]
• Leaves: The leaf blades measure approximately 1 to 2 inches long and occur in an alternating pattern. Leaves are deeply divided into many thread-like segments, forming a lacy pattern distinctive of chamomile species. They clasp the stem and will become shorter nearer the stem tips. [3,5,6]
• Flowers: Multiple inflorescences branch from the top of the stem. Flowers are small (approximately 1.5 inches) and daisy-like, featuring white petals and yellow centers. Each flower has about 10-24 florets surrounding a center, which is made up of several hundred yellow disc florets. Where the flowers attach to the stem, green bracts appear, their edges dry and brown. [1,6]
• Seeds: One seed will be produced by each flower petal (floret). When mature, these small seeds are elongated and brown with no pappus. Three distinct ridges appear along the length of the seed. Healthy populations of scentless chamomile can produce 1.8 million seeds per square foot. [6]

3. Infestation Basics

Where and how does it grow, and how does it spread?

Scentless chamomile is a plant that can grow in many parts of the world, especially in places that have been disturbed by human activity. It originally comes from Europe and Western Asia, spreading from countries like Spain and Italy all the way up to Scandinavia and extending east to Russia and parts of Central Asia. You can also find it growing naturally in Iceland, and it has become established in places such as New Zealand and several regions in Australia, including New South Wales, Victoria, and Tasmania. In South America, it has taken root in southern Chile and Argentina.​ [1]

In Japan, scentless chamomile is seen as a problematic invasive species. In Canada, it first appeared in a herbarium record in 1876 in New Brunswick and was collected in Saskatchewan in 1928. Since then, it has become a troublesome weed across the country, especially in the eastern provinces and the prairies. Due to its invasive nature, scentless chamomile is considered a harmful weed in provinces such as Alberta, Saskatchewan, British Columbia, Quebec, and Manitoba. Despite its issues, it is still grown in Europe, and some specialty seed suppliers sell it there under the name common chamomile. [1]

This weed is capable of germinating over a broader time period than most weeds. Seedlings that emerge before July will typically behave as annuals, as the longer growing season will enable the plant to bolt and complete its cycle. Late bloomers will spend the winter developing vegetative structure and root reserves, bolting the following season. [6]

The plant reproduces entirely by seeds, which lack a pappus to facilitate wind dispersal. As a result, most new plants will emerge near the parent plant unless dispersed by flooding or mechanical means. [6]

Scentless chamomile will grow in a wide variety of soil types, including those with periodic flooding. It can commonly be found in or near pastures, roadsides, construction sites, or ditches. In climates with more rainfall, the plant seems to prefer light, well-drained soils. It is cold and drought-tolerant and can germinate year-round in areas with mild winters. [5,6]

Key takeaway: Successful control of scentless chamomile requires suppressing seed production. This can be difficult as the plant has a relatively long flowering season, particularly in mild climates.

Why is it so invasive?

This weed is established aggressively through prolific seed production. It is also resistant to many herbicides and possesses an extensive and resilient seed bank.

In a stand of scentless chamomile, each square meter can produce 1.8 million seeds, which can remain viable in the soil for up to fifteen years. It does not grow well in the presence of competition, and healthy soils with established plants are not conducive to the weed’s growth. Seeds are quite resilient, and up to 26% of seeds remain viable after passing through an animal’s digestive tract. [3]

The biggest threat Scentless chamomile poses is to cereal grains such as wheat and oats. It is also problematic in seed and hay crops and can cause harm to animals when present in pastures. It is estimated that 67% of the United States is susceptible to infestation by scentless chamomile. [1,6]

Key takeaway: This is a highly invasive weed that spreads through massive seed production, is resistant to many herbicides, and poses a threat to cereal and forage crops. It struggles to establish in healthy, competitive plant communities.

Common risk factors for invasion

• Organic Loam or Clay Soils: The roots of scentless chamomile thrive in non-calcareous soils with a pH of 5.5 to 7.9. Although there is some evidence that the weed will do just as well in high pH soils. Scentless chamomile can thrive in sandy soil with low nitrogen levels (around 14 ppm) and low phosphorus levels (approximately 0.31 ppm). It is capable of effectively competing with barley in these low-nitrogen conditions. [3,4,5]
• Soil disturbance: Scentless chamomile tends to thrive in areas where the soil has been disturbed, such as construction sites, tilled fields, or overgrazed pastures. This disruption weakens the existing vegetation, creating opportunities for the weed to establish itself. To reduce the risk of invasion, it’s important to minimize unnecessary soil disturbance. This weed is often found in neglected or abandoned locations, such as roadsides, railways, and vacant lots. These undisturbed environments provide the time and space for it to spread without facing competition or control. Regular monitoring and effective weed management in these areas can help mitigate this risk. [2]

4. Management Strategies

Scentless chamomile seeds can remain viable for more than 10 years, meaning if the plant is allowed to set seed for the first time, eradication becomes extremely difficult. If the plant has already become established, control is a more feasible goal than eradication, specifically with the use of herbicides. [5]

Mowing, tilling, and hand-weeding used in combination can help keep the plant at bay. The most successful treatment option is competition. The successful reintroduction of desirable plant species, especially those that rebuild depleted soils, can effectively crowd out scentless chamomile, as the weed can only persist in frequently disturbed soils. [3]

DON’Ts

• Don’t compost flowering plants or seeds. Composting may not kill all seeds. Bag and dispose of flowering or seed-bearing plants properly.
• Mow only once or too late. One mowing won’t stop scentless chamomile as it can re-flower below the cut line. If mowing, start high and make successive cuts lower before flowering occurs.
• It is advised not to engage in tilling activities within moist, cool soil conditions. Such practices may jeopardize the integrity of live plants and facilitate the dissemination of root fragments, particularly those comprising the fibrous root mass.

6. References

Citations

[1] PPQ. 2018. Weed risk assessment for Tripleurospermum inodorum (L.) Sch. Bip (Asteraceae) –Scentless mayweed. United States Department of Agriculture, Animal and Plant Health Inspection Service, Plant Protection and Quarantine (PPQ), Raleigh, NC. 23 pp. 

[2] MSU. (2018). Scentless chamomile-March_2018 – MSU Extension Invasive Plants | Montana State University. Montana.edu. https://www.montana.edu/extension/invasiveplants/extension/monthly-weed-posts/2018_march.html

[3] Klein, H. (2008). Scentless chamomile. In Alaska.edu. Alaska Natural Heritage Program. https://accs.uaa.alaska.edu/wp-content/uploads/Tripleurospermum_inodorum_BIO_TRIN11.pdf

[4] Kay, Q. O. N. (n.d.). Tripleurospermum Inodorum (L.) Schultz Bip. Journal of Ecology, 82(3), 681–697. https://doi.org/doi.org/10.2307/2261275.

[5] UNR. (2020). Scentless Chamomile: Taxonomy, Ecology, and Control | Extension | University of Nevada, Reno. Extension | University of Nevada, Reno. https://extension.unr.edu/publication.aspx?PubID=2422

[6] McClay, A.S. 2022. Scentless Chamomile (Tripleurospermum inodorum): History and Ecology in North America. In: R.L. Winston, Ed. Biological Control of Weeds in North America. North American Invasive Species Management Association, Milwaukee, WI. NAISMA-BCW-2022-35-SCENTLESS CHAMOMILE-P.