Growers are reporting increasing issues dealing with scentless chamomile in ginseng, which has become a problem weed in many crops. Scentless chamomile (Matricaria perforata), also known as scentless mayweed, is a short-lived perennial or sometimes annual, closely resembling stinking mayweed, with its large yellow-centered flower heads with white ray florets, but it is usually taller (up to 75 cm, 30 in.) and more branched (Figure 1). It also resembles pineappleweed. Its stems below the flower heads are smooth and hairless, and the whole plant is virtually without odour (unlike pineappleweed or stinking mayweed). Scentless chamomile can grow effectively at very low temperatures, so plants that are small at this time of year can be very large by the time the snow is gone in the spring.
Figure 1. Scentless chamomile in bloom.
Due to its extensive fibrous root system, hand-weeding of scentless chamomile can result in large clumps of soil and straw being removed along with the weed, uprooting ginseng plants nearby (Figure 2). Competition with the ginseng also results in stunting to the ginseng. Although there are no reports that these plants can directly damage nearby plants (allelopathy) in other crops, there may be some negative effects to ginseng based on observations.
Figure 2. Hand-pulling of scentless chamomile results in a large clump of soil being removed, disrupting the straw and nearby ginseng plants.
Scentless chamomile is spread entirely by seeds, often entering the ginseng field within the rye straw. The first step in management is to control it within the rye crop itself. This is best done in the fall with the use of 2,4-D Amine 600 at 600-900 mL/ha applied when the scentless chamomile is in the seedling stage (Figure 3). Applications should be done in October when plants are still active.
Figure 3. Young scentless chamomile plants within a rye crop (photo taken Oct. 18). Herbicide applications should be made at this stage or earlier in the fall for best control.
Within the ginseng crop, there are no options for control of scentless chamomile in the fall. Since the weed grows effectively at very low temperatures, the best option for control is to go in with a high rate of glyphosate as early as possible in the spring (mid-late March if snow is gone at that time). Effective control of this weed in other crops has required up to 5 L/ha (2 L/ac) of the 360 g/L a.i. formulations. Consult your glyphosate label for what is permitted on ginseng. This timing is likely too early to control some weeds that are not active until April, so a second application at your normal application timing may be required.
Special thanks to Kristen Obeid, OMAFRA Weed Management Specialist for Hort Crops, for input on weed control options.