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ThistlesKey Points
Introduction BiologyThough 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 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 90cm 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 12.5cm 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. Product Choice 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 0.25 litres per hectare in 200 to 250 litres of water, made when the weeds are at the rosette growth stage and the crop has at least three expanded leaves. A second application of 0.5 litres per hectare must be made three to four weeks later, again using 200 to 250 litres per hectare of water, to maximise control. Learn more about Dow Shield 400 |
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