Several herbicide treatments were evaluated in a 2019 on-farm research trial to determine best management options for scentless chamomile, a weed with a reputation for being difficult to control. Although 10 treatments provided greater than 80 per cent visual control of emerged plants, new seedlings continued to emerge several weeks after application, resulting in a significant amount of plants at the end of August. In order to significantly reduce the population, producers will need to manage new seedlings that emerge in the spring (prior to planting), summer (in-crop) and fall (post-harvest). Failure to manage summer- and fall-emerged plants increases the likelihood that they will develop as biennials or perennials and be more difficult to control.
Biology: In Ontario, seedlings emerge from April through to October, with the greatest amount of seedling emergence occurring in October. Whether emerged plants will express an annual, biennial or short-lived perennial life cycle will depend on germination timing. Seedlings emerging in April to June often develop as annuals, while seedlings that emerge after June typically develop as biennials or short-lived perennials.
Longevity of viable seed: The seed appears to retain a high level of viability for four years of dispersal but its viability is significantly reduced after 10 years.
This background knowledge is important because biennial and short-lived perennial plants are much more difficult to take out with herbicides since they are anchored with a dense mass of fibrous roots. Unless the applied herbicide can control the top-growth and translocate to and kill the entire root mass, the plant will grow back. Seedlings, by comparison, are much more susceptible to herbicides as there is less root and shoot mass to control.
Key lessons from the 2019 field trial: When seedling plants were under 15 cm (six in.) in height, all rates of glyphosate provided effective control. When plants exceeded 15 cm in height, a 1.34 l/ac. rate of glyphosate (540 g/l) was required to achieve adequate control (Table 1 below). The addition of ammonium sulphate (AMS) as a water conditioner prior to mixing in glyphosate neither improved nor decreased the level of weed control. Tank mixing other herbicides with the 0.67 l/ac. rate of glyphosate did not provide control that was equivalent to the 1.34 l/ac. rate of glyphosate. Some tank-mix combinations caused reduced weed control from antagonism.