Cultivation Principles – Sow Thistle
Sowthistles prefer to germinate on the soil surface and predominantly found on crops such as oilseed rape and beans where there are large areas of bare soil. Their survival is favoured by non-till systems and where shallow cultivations are used that keep the seed near the soil surface.
Burial of seed will increase seed persistence in the soil but regular cultivations will increase the rate of seed decline.
Perennial sowthistle seedlings should be treated as for annual sowthistles.
Established stands of the weed should be subjected to frequent cultivation to exhaust food reserves. Cultivations chop up the roots and cause new shoots to grow from the pieces. The root reserves are decreased more by spring cultivation than at later growth stages. Fallowing for a year beginning in the autumn and cultivating every 3 weeks in the spring reduces perennial sowthistle populations considerably.
Rotation – Inclusion of cereal and grass crops in the rotation is key to their control. Narrow crop rows and high crop densities helps to suppress them.
Residue Management: Ensure even spread of residues across field and a good chop of straw to maximise herbicide efficacy and avoid unnecessary cultivations. Straw chopper knives need changing/reversing regularly – often 3 or more times a season.
Where appropriate (e.g. when direct drilling with appropriate drill into high residue levels) leave a longer stubble height. Stubble mulching of high stubbles can be used where straw needs time to become brittle after harvest.
Residue spreading and shallow stale seedbed cultivation by shallow discing/rolling (multiple passes followed by glyphosate as needed) is effective to manage residues, control slugs, and create the best conditions to stimulate weeds outside the crop. Depth to be only just greater than drilling depth of following crop, ideally ≤5cm. Consolidate after tillage to maintain moisture in the stale seedbed. Surface consolidation is essential after shallow tillage, and through to depth pressing plus surface consolidation is needed following deeper tillage. Timing of tillage is preferably later in autumn or spring.
Maintain seed bank horizons by controlled surface disturbance of all operations. Mixing weed seeds to variable depths reduces efficacy of residual herbicides and prolongs the germination period for weeds.
Loosening as needed by low rake angle winged tines with adequate lift height for conditions and depth worked. This controlled vertical fissuring also minimises disrupting the weed seed bank horizons. Aim to establish a following crop ASAP after loosening (cover crop if spring drilling next main crop) to stabilise the resulting structure created.
Plough where severe infestations are present – note ensure policy suits other weed species if present. Skimmers need to be set adequately and depth, furrow width and speed need managing to ensure all weed seeds are buried below 5cm. Pre- discing and rolling ahead of ploughing will help to ensure this process is as effective as possible.
AT ALL COSTS avoid: (i) deep non- inversion mixing – especially disc or disc/tine based; (ii) re-inversion before 5 or preferably more years; (iii) lower depths of disturbance when cultivating than during follow-on spring drilling, (iv) failure to consolidate after soil disturbance as this will lose moisture and adversely affect germination potential of the weed seeds.