Mayweeds — Matricaria spp. and Tripleurospermum spp. — are common and high profile annual or hardy-annual composite weeds. There are several widely distributed species including scented mayweed (Matricaria recutita), scentless mayweed (Tripleurospermum inodorum) and pineappleweed (Matricaria matricarioides).
Mature plants are large, fleshy and robust and can be competitive with the crop. Their presence can have a significant impact on crop harvesting, moisture of the seed at harvesting and the level of admixture left in the harvested sample.
Scented mayweed germinates all year, with particular flushes in spring and late summer to early autumn. It has a small seed that can only germinate from shallow depth and its prevalence may be favoured by minimum tillage cultivation systems. Characteristic flowers with yellow centres and white petals are normally seen in late spring and early summer. One plant can produce 10,000 seeds.
Scentless mayweed will germinate at most times of year. Peaks of germination are normally March and August to October. Germination of seed only occurs if they are near the soil surface. Flowering can occur from late spring through to autumn with each plant capable of producing up to 5,000 seeds.
The germination pattern of pineappleweed is similar to that for scented mayweed but with the main flush being in spring. It appears to thrive in soil that is constantly disturbed and compacted, often being particularly prevalent in gateways and on field headlands. Plants flower in summer, without the characteristic white petals of other Mayweed species, and can produce 5,000 seeds per plant.
Control of Mayweeds in oilseed rape can be achieved before 1st February in the year of harvest with ASTROkerb. Alternatively Mayweeds can be controlled in the spring with Galera or Dow Shield 400. Dow Shield 400 contains clopyralid as the active ingredient and Galera contains clopyralid plus picloram. The particular strength of including picloram with clopyralid is the addition of activity on Cleavers.
Mayweeds up to 10 cm in diameter can be treated with 0.25 litres per hectare of Dow Shield 400 in the spring (after 1 March), or if weeds are small, not hardened-off by frost and growing well, with 0.175 litres per hectare.
If Dow Shield 400 is applied after a pre-emergence herbicide that reduces the crop’s wax layer, or during frosty weather, slight leaf distortion and chlorosis can occur. This will be outgrown and will not affect final yield of the crop.
Galera can be applied after 1st March and once the crop has 4 true leaves. The ideal timing for application is when the weeds are small and actively growing. One application of 0.35 litres per hectare will control all susceptible emerged weeds. To achieve optimum levels of weed control good spray coverage is required using a minimum of 200 litres per hectare of water.