Scentless chamomile (Matricaria perforata Merat)
Growth: This fibrous rooted plant can act as either an annual, winter annual, biennial or short-lived perennial. When overwintering occurs, large bushy plants result. Plant heights reach 15 cm to 1 m (6 to 39 in.).
Leaves: The light green leaves are alternate in arrangement, stalkless, very finely divided and smooth.
Stems: Scentless chamomile plants have hairless stems with many ascending branches.
Flowers: Topped with daisy-like flowers that have raised yellow centres surrounded by white petals and are two to four cm (3/4 - 1 1/2 in.) in diameter. Flowering occurs in late June through September. Crushed flowers are practically odourless as compared to wild or German chamomile (Matricaria recutita) which smells strongly of the herbal chamomile tea.
Seeds: The seed is a small ribbed, dark brown achene.
Scentless chamomile can produce from 300,000 to 500,000 seeds per plant or 1,000,000 seeds per square meter of densely infested area. Seeds are easily spread by wind and water. Seeds are dark brown in colour and are light in weight, thus the common areas of infestation tend to be around water sources where seeds have been floating.
Scentless chamomile was used for ornamental purposes only. While not poisonous, it has little feed value.
Scentless chamomile can be found in all provinces throughout Canada, particularly in headlands, waste areas, ditches and forage crops. It has an extensive, fibrous root system enabling it to cling to and maintain the integrity of large clumps of soil, making it possible to survive cultivation especially in both heavy and solonetzic soils. It grows best in high moisture areas, spreading rapidly during wet periods.
Scentless chamomile can germinate in soil that is completely flooded. When invading a new area, it often begins growth around stream banks, flooded ditch bottoms or slough edges. From newly invaded areas, it moves to other disturbed areas such as transmission lines, pipelines, newly constructed roads, right-of-ways and drainage ditches, finally invading forage stands, native pastures and cultivated fields.
Scentless chamomile seeds will survive in the soil for long periods (up to 10 to 15 years), but most will germinate in the first three to four years after being shed. Seeds requires light to germinate. Therefore, frequent tillage may promote infestations.
Its coarse, rank growth is unpalatable to livestock. It increases especially in overgrazed conditions. Plants that over-winter are very difficult to control as they mature early and the adjacent vegetation cannot offer much competition.
Since scentless chamomile responds so well to moisture, a wet year means more contaminated feed and larger yield losses. Field work done when moisture levels are high would serve to transplant the weed.
Mowing non-cropped areas often prevents seed production. In most years, multiple mowings will be needed. Mowing promotes scentless chamomile to flower below the cut line, therefore a strategy of mowing that begins at the highest possible cut height with each cut made progressively lower will be more effective at reducing seed production.
A solid healthy stand of forage will help to suppress scentless chamomile growth and establishment. Seeding to smooth bromegrass for several years should reduce the infestation. Several years of alfalfa production has been shown to reduce scentless chamomile populations. Cut forage crops prior to chamomile petal emergence to prevent production of viable seed.
Since scentless chamomile seeds are spread easily by farm equipment, moving water and snow make sure to thoroughly clean equipment when moving to a different field. Tarping loads of grain and wrapping bales when transporting helps to prevent the spread of scentless chamomile seeds. Summer and winter road maintenance equipment will further the spread of scentless chamomile.
Hand pulling, hoeing and tillage are effective on small areas. These methods are most effective when performed before scentless chamomile flowers.
Classical biological control agents are available for heavily infested areas.
The scentless chamomile seed head weevil (Omphalapion hookeri) is a tiny black weevil that lays it eggs in the flowers prior to petal emergence and larva consume developing seeds. Feeding continues until the larva pupates and eventually emerges from the flower as an adult which over-winters in the trash on the soil surface.
The scentless chamomile gall midge (Rhopalomyia tripleurospermi) lays eggs in the growing points of the plant where irritation to undifferentiated tissues results in the formation of a gall to enclose and protect the developing midge larva, instead of normal stem, leaf or flower tissues. The adults only survive for 24 to 48 hours outside of the gall, and this insect will produce one to three generations per year. The insect over-winters in the galls.
A third agent, the scentless chamomile stem boring weevil (Microplontus edentulous), lays eggs on the stem of scentless chamomile and larva burrow within the soft pith inside the stem, then pupates and matures inside the stem where it over-winters and emerges as an adult in the spring
Both the seed head weevil and the gall midge have become well established in Saskatchewan, but the stem boring weevil has not been very successful.