Root weed control in special weather conditions

When the window to control root weeds becomes smaller or larger due to wet autumns or droughts and early harvests, it may be necessary to include weed control solutions that are not normally used.

By Karen Munk Hansen

Disclaimer: This article has been machine translated from Danish to English, so please note that there may be errors in the translation.

Wet autumns or dry summers can make it necessary to adopt different strategies for controlling root weeds than you would otherwise. This article brings together results from country trials and experiences from interviewed farmers.

Wet autumn - double ploughing with cover crops

Double ploughing can be an option for controlling field bindweed and pigweed in particular in wet autumns. Previous trials show that double ploughing is the most effective mechanical control of Canada thistle. Ploughing immediately after harvest, followed by a fast-growing catch crop, followed by a second ploughing in late winter or spring can significantly reduce the incidence of thistle. The weed species mentioned go dormant or slow down growth in winter, so there is no effect of repeated treatments in autumn.

Advantages and disadvantages of the method

Advantages of the method	Disadvantages
there is a post-harvest treatment time good opportunities to establish a catch crop and retain nutrients	Ploughing time right after harvest can clash with other urgent tasks

Bare frost and high pressure - drying out chickweed

Drying out during periods of frost and high pressure will have a good effect on vetch. The likelihood of frost in the winter months allows for the drying out of chickweed runners if they are brought to the soil surface in advance. During periods of frost, frost cracking and desiccation can destroy exposed roots and runners. When drying out, it is important to pull the roots up to the soil surface without breaking them up. Rotary tools are particularly well suited for this purpose, as as many bare roots as possible are freed from the soil after driving over them. This is followed by careful ploughing.

Dry summers - mini summer fallow

In dry summers, where harvesting is done early and where there can be a good effect of desiccation, mini summer fallow can be used on most root weed species without major yield loss. Mini summer fallow is effective regardless of the weather, but in more humid conditions the yield loss by harvesting early will be greater.

With a massive and mixed presence of weed species, the ultimate strategy is mini summer fallow. Here it may be necessary to shorten a crop's growing season by, for example, cutting grain for green or whole crop. This is the only control method that has been proven to effectively reduce the population of thistles, pigweed, coltsfoot and vetch. Results from previous trials show a reduction of the species of 85-90 per cent. It is important to be aware that there may be a large nitrogen impact that should be absorbed by a robust cover crop.

Effect of mini summer fallow on different root weeds.

Experiences from interviewed farmers

In 2019, SEGES Organic Innovation interviewed a number of organic farmers about challenges with root weeds in extreme weather conditions. Experiences from five farmers are gathered here for inspiration.

Wet fields - especially in autumn

Several farmers mention that it can be a possible advantage to use a Marsk Stig harrow rather than a Kvik-Up harrow, Kvikkiller or similar when the soil is soft and wet. Experience shows that there is a greater chance of getting into the field with a Marsk Stig harrow. It has a particularly good effect on chickweed because it is possible to implement a strategy while you may only make the first of several planned crossings with the heavier equipment. Harrowing has been most effective when ploughed beforehand so that roots and runners are laid out on the soil surface. Follow-up harrowing is often done again in the spring, followed by ploughing before sowing.

Drying out during frost in early spring

If there is dry weather and night frost in early spring, many farmers have good experience of harrowing roots and runners on the soil surface to utilise the frost for drying out. This can either be the conclusion of a strategy started in the autumn or an independent, targeted effort. There is experience of spot treatment with a Kvik-up harrow during dry periods with frost, so that the frost is utilised for control in the areas of the field where root weeds are a problem. The repeated harrowing is often followed by ploughing prior to sowing.

Late sowing in dry spring

The opportunity to utilise the larger window of opportunity for repeated harrowing in a dry spring is taken advantage of and a later sowing is carried out. These experiences are based on the fact that effective weed control can outweigh the disadvantages of a later sowing.

Dry summers

With a dry summer like 2018, where the harvest was early and there were good conditions for drying by repeated harrowing, the large window for effective drying of root weeds has been extended and utilised to good effect.

Green fields

In many people's experience, a heavy crop or catch crop is the best way to control root weeds. It can be better to have a catch crop after harvest that can outcompete the root weeds than to do a lot of repeated harrowing. By including crops in the crop rotation that are to be harvested early, it is possible to establish a good and timely establishment of cover crops and thus utilise them to compete against root weeds. There is experience with spot treatments in the catch crop with e.g. Kvik-up harrow, so control is still achieved even if the field is green. One farmer has established the cover crop by ploughing in advance at a depth of 18-20 cm.

Choose the right strategy to control root weeds in spring

Choose the right strategy for post-harvest root weed control