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Weed Wisdom April 2017

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By Dr. Bob Blackshaw

Scentless Chamomile in bloom

Southern Alberta landowners will want to keep all eyes open in the fields this season as Scentless chamomile (Tripleurospermum perforatum) trickles out of  the Black and Grey soil zones where it has a long history as a weed. Increased rainfall in recent years means its showing up in Dark Brown and Brown soils.

It is sometimes confused with ox-eye daisy (Chrysanthemum leucanthemum) or pineappleweed (Matricada matricarioldes), but there are morphological characteristics that distinguish these species. Ox-eye daisy has broad, linear, coarsely-toothed leaves while scentless chamomile has finely divided, fern-like leaves. Pineappleweed has a pineapple-like odour when crushed (scentless chamomile is odourless) and pineappleweed doesn’t have noticeable white flowers like scentless chamomile. 

Scentless chamomile can exist as an annual, biennial or short-lived perennial. It reproduces by seed and can produce up to 1 million seeds per plant. Optimum seed germination occurs at soil temperatures of 5-20 C; thus it can easily establish in late fall or early spring. It readily germinates on or near the soil surface, so it is well adapted to pasture, hayland, and reduced tillage cropping systems. It grows well in moist soils and is commonly found in low-lying areas or around slough margins.

Scentless Chamomile prior to bloom

Scentless chamomile stems are often reddish in colour and plants typically attain a height of 60-100 cm. Flowers have a white daisy-like appearance. Seeds are 2 mm long and dark brown or black with three distinct light-brown ribs. Seed is widely dispersed by wind and water and as a contaminant in crop seed or animal feed. Seed has little initial dormancy, but certain environmental conditions can induce secondary dormancy allowing the seed to persist in the soil for many years.

Scentless chamomile can reduce forage and crop yields depending on its infestation density and establishment timing. Its dense fibrous root system can extract considerable amounts of soil nutrients and water that otherwise would be available to the crop. Documented spring wheat yield reductions due to scentless chamomile have been 20-55%.

Emphasis should be placed on controlling seedlings in fall or early spring as larger plants are more difficult to control. Glyphosate is very effective, but there are also several other herbicides that can control it. Higher seeding rates of competitive crops such as barley and winter wheat can suppress scentless chamomile. 

In this cross section of a seed head, you can see how many seeds each Scentless Chamomile flower produces.

Two insects from Europe have been reared and released in Canada as biocontrol agents. A seed feeding weevil (Omphalapion hookeri) and a gall midge (Rhopalomyia tripleurospermi) are providing some measure of control in undisturbed, non-crop areas.