- Thistle species continue to cause management problems for oilseed rape growers
- Thistles compete with the crop and look unsightly
- Sow-thistles block combine sieves and markedly reduce speed of harvesting
- Control cannot be achieved through cultural techniques alone
Dow Shield® 400 and Galera® both have excellent activity on Sow-thistles and Creeping Thistles
Yield loss and harvesting difficulties can be caused in oilseed rape by infestations of sow-thistle — Sonchus spp. — and creeping thistle — Cirsium arvense. Annual sow-thistle, of which there are two widely distributed species, is rapidly increasing in importance. The most damaging and widespread of all thistle species is creeping thistle. This species affects arable and grassland farming enterprises. Both weeds can be controlled successfully using either Dow Shield 400 or Galera.
Though innocuous as seedlings, annual sow-thistles can produce dense stands of tall sappy stems, bearing yellow flowers that protrude from the crop at harvest. High populations can cause combine sieves to become blocked leading to markedly slower progress at harvest. Prickly sow-thistle (Sonchus asper) can produce 6000 seeds per plant and smooth sow-thistle (Sonchus oleraceus) up to 5000 seeds per plant. Both plants produce a yellow flower that appears in early summer.
Creeping thistle is a highly competitive perennial weed capable of growing to 90 cm in height. Reproduction is primarily through specialised creeping lateral roots that are capable of surviving winter. Roots normally reach a depth of 1.0 – 2.0 metres but in well aerated soils this can be as much as 5.0 metres. Vertical roots that seek out the water table, combined with the high proportion of lateral roots below tillage depth, ensure the parent colony survives, despite deep cultivation.
Creeping thistles can survive indefinitely as a consequence of their root system. Studies using 12.5 cm lengths of root fragments confirmed that 100% of those tested were capable of producing new plants. Shorter fragments are also capable of giving rise to new plants on many occasions. These shoots arise from root buds, thus field cultivations are often responsible for breaking up root structures, spreading and increasing infestations.
As temperatures rise to above 5°C in the spring, these roots produce aerial shoots that develop in to distinctive patches of creeping thistle. As the initial young leaves develop in to rosettes, rapid colonisation of large areas is seen to occur. Colonies can expand at rates of up to 6 metres per annum.
An additional method of colonisation is through seed dispersal. In contrast with other Cirsium species, creeping thistles carry male and female flowers on separate plants. Plants are pollinated by insects, with high levels of cross-pollination occurring when plants are within 30 metres of each other. This method of reproduction ensures biological variation in the species. Seeds are usually produced in July to September and shed from August onwards. Seed numbers depend on the proximity of pollinating plants, the number of flower heads and the number of seeds per head. Though 5000 seeds per plant have been recorded the usual number is around 1500 per plant. The plumes that carry the seed are easily detached.
Researchers have questioned the significance of seed as a dispersal method for creeping thistle since, in one study, 99.8% of plumes found within 1.0 km of parent plants did not have a seed attached. Other methods of long distance dispersal, such as being carried in irrigation water, have been proposed as alternatives. The agronomic impact of seed may be its potential to lie dormant. Seed generally germinates within a matter of days, particularly at high temperatures, and germination efficiency varies with subspecies. Alternatively, seed may remain dormant, for up to 6 months in water and for up to 21 years when at depth in soil.
Many cultural techniques have been employed in an effort to control creeping thistles, including deep ploughing, shallow cultivations, repeated mowing, smothering with straw and even burning. Current opinion is that no single method is likely to give complete control. The strategy of starving the roots is now well developed and focuses on ploughing, mowing, grazing and herbicide strategies to weaken top growth and reduce the root bud bank.
Annual sow-thistles and creeping thistles can be effectively controlled in oilseed rape using Dow Shield 400 or Galera herbicides. Applications to control sow-thistles should always be made when the weeds are small. Dow Shield 400 applied at 0.25 L/ha, or Galera at 0.35 L/ha, will control annual sow-thistles present at the 2 true leaf stage when applications are made.
For optimum control of creeping thistles a sequence of Dow Shield 400 applications is the most appropriate approach. The first application of Dow Shield 400 should be at 0.5 L/ha in 200 to 250 litres of water, made when the weeds are at the rosette growth stage and no larger than 30 cm across. If further thistles emerge after treatment with Dow Shield 400 a second application, normally 0.25 L/ha, should be made.