Scentless chamomile (Tripleurospermum perforatum) spreads quickly through extensive seed production - up to one million seeds per plant! It is native to Europe and was introduced to North America as an ornamental.
Stems are highly branched, reddish in colour at maturity and grow up to 1m tall.
Leaves are finely divided (like dill) and odourless when crushed (hence the name 'scentless' chamomile). This distinguishes it from pineappleweed, which is a non-legislated weed whose dill-like leaves have a strong pineapple scent when crushed.
Flowers and Fruit
Scentless chamomile has white, daisy-like flowers whose petals turn backwards (like a badminton shuttlecock) as they age. One of the reasons this plant is so successful is because it flowers continuously all season from May (hence the common name 'mayweed') until frost and the flowers contain viable seed as soon as they open. Seeds are dark brown, very small, and can germinate as soon as they fall from the plant. They may also persist in the soil for up to 20 years.
Scentless chamomile is not highly competitive. It specializes in colonizing disturbed sites such as urban developments or overgrazed pastures.
Due to its high seed production, scentless chamomile is difficult to eradicate once the soil seedbank is infested. This weed is not palatable to livestock and lowers pasture quality.
Never plant scentless chamomile as an ornamental in your yard. Pick, bag and burn/bury all visible plants and encourage strong competition from turfgrass to prevent seeds from germinating.
Mowing is not effective on scentless chamomile; the plant reacts by forming flowers at every point of breakage, so multiple mowing operations can actually increase seed production! Handpulling and bagging is very effective in young infestations that haven't dropped seed yet. Do not leave flowering plants on-site, as this spreads seed.
Chemical applications can be effective against growing plants but do not reduce seed viability in the soil. Several products are registered for control of scentless chamomile; consult your local Agricultural Fieldman for recommendations.
The City of Grande Prairie has established two biological controls for scentless chamomile. Omphalapion hookeri is a weevil that feeds on seedheads and reduces seed set, and Rhopalomyia tripleurospermi is a midge that forces plants to produce galls and reduces flowering.