Soil Systems Farm

Controlling weeds in broadacre and vegetable crops

Controlling weeds in broadacre and vegetable crops requires the manager to consider a number of important decisions. In organic farming, many of these decisions are proactive in nature – what you implement today has a big bearing on what happens in the weeks and months ahead. You don´t have the luxury of pulling out a spray and killing the weeds.

Understanding the depth of germinating weeds

Most of the cropping weeds tend to germinate from a depth of 2.5 to 5cms. In general, the smaller the weed seed the shallower the depth of germination. With this in mind, weed control in direct sown crops is all about timing, the length of time from the last cultivation till the time the crop emerges. Before the crop emerges, weeds can then be killed with shallow cultivation, spring tynes or heat treatment. Once the plants or transplants have established, further weed control is followed up until the canopy closes in. Of course, this depends a lot on the weather conditions, health of the crop, nutrition and friability of the soil.

Weeds vary according to fertiliser history

Every farmer knows that the fertiliser history plays a big role in what weeds emerge. The first time I ever came across this was when I was a District Agronomist at Hilston for NSW DPI. The old farmer after sowing his wheat crop with a sulphur based fertiliser said, “that is the first time I have seen those weeds in over 30 years”. I did not remember the weeds in question but the principle rings true even today – change the nutrition of the soil and the weed spectrum changes. From a practical point of view the farmer must focus on increasing the fertility of the soil so that the crop quickly fills in the canopy. This is why composting and remineralisation of the soil is so critical.

Never let the weeds seed

This is often easier to say than do but it must be the aim of every producer to stop the weed seeding. Depending on the weed species, each weed can produce up to a few thousand seeds. In some cases hand removal of invasive weeds is required.

Choosing the right equipment

A very good publication on choosing the right equipment for your crop is “Steel in the Field” – A farmers guide to weed management tools. In this book they go through all the equipment required for row, dryland and horticultural crops. Many of the pieces of equipment are explained in detail together with case studies outlining their use. Blind cultivation using spring tynes is another way of weeding cereal crops or specialty crops not sown in a row. Here the secret is to cultivate into loose soil prior to sowing then again once the crop has established its roots. There is a small loss of plants but the end result is a 90-95% reduction of weeds. An example of this is the reduction of barnyard grass. Finally, there are a number of companies that tailor the equipment to your specific horticultural or cropping requirement.

Crop rotation, pastures & green manure crops

Crop rotation and the use of pastures or green manure crops is a cultural practice that dates back over 3000 years. The secret here is to provide the right soil nutrition and maximise the number of plant species on the farm. By doing this root exudates from each species will increase thus providing the foundation for healthy root development. Rotations of crops vary enormously from country to country and depend on the soil type, microclimate and cultural background of the farmer. Dogma has no place here, just observation, diversity and recording what happens.

Pastures should be encouraged as they are the fastest way to build organic matter, stable soil humus and reduce the weed burden in paddocks. Make sure the species selected are numerous and include grasses, legumes and herbs.

Multi species green manure crops should also be viewed as an important mechanism for breaking the weed cycle. Examples of autumn green manure crops include cereal rye, oats, barley, mustards (for biofumigation), vetch, clover, peas and medics. Spring and Summer green manure crops include forage sorghum, Japanese millet, buckwheat, mung beans, cowpeas and lab lab. An excellent text on this subject is “Managing Cover crops Profitably”.


There are a number of agricultural crops that produce chemicals that inhibit other weeds from germinating. Classic examples of this are lab lab, rye, hairy vetch, red clover, mustards and sorghum. This is why it is important to diversify the crops you grow and keep  records of what happens to weed burden in the following crop.

Mulches – plastic and natural

Mulches like traditional straw are commonly used to form a physical barrier on the surface of the soil. Other examples include plastic in strawberries and wood shavings in ginger.

No till organics

This is a new science developed at the Rhodale Institute and a number of universities around the US. In practice it involves growing a green manure crop (cereal rye before soybeans or hairy vetch before corn) and crushing the stem as the head begins to develop. The crushing of the stem occurs as you plant the following crop using an on-farm welded crumple roller. The dead mulch creates a physical barrier that weeds cannot penetrate. Like all green manure crops and pasture stands, its success depends on the nutrition you provide.

Adam Willson has 20 years’ experience in agronomy, horticulture, soil and on-farm commercial composting. He runs a small, certified organic market garden in Mount Tamborine, Queensland. He has a Bachelor of Science in Agriculture from Sydney University and is the director of Soil Systems Australia,


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