TROPO's Organic Info Library

Weed control for organic farms

To control weeds successfully you need to use a combination of approaches, according to local grower Sandy Gilmore who passed on some hard-won tips at a TROPO meeting. Sandy, a former Department of Agriculture information officer, has been slowly overcoming weeds on his property at Federal for the past 16 years.

Up to 70% of farm work time for horticulturists can involve activities associated with weed control, Sandy said. "I think that quantity of work can be reduced significantly with appropriate development. The money and energy saved, if put into building soil fertility, is the best long-term means of controlling weeds too."

Clearly what is considered a weed depends on the stage of property development, the land use (eg. kikuyu is a weed to some orchardists but not to graziers) and where you are in the country (Patterson's Curse is considered a weed in central NSW but in central South Australia is considered beneficial).

"I have found that paddocks once dominated by weeds no longer are. I largely attribute that to increased pH (meaning reduced acidity), increased availability of growth-limiting nutrients, maintenance of a dense grassy sward, reduction in the soil weed seed store (so that there is less chance of germination) and species replacement by less troublesome weeds."

Sandy said he felt it unnecessary to spray to control weeds. "If you improve your soil and slash strategically to prevent weed setting seed, other forms of control become unnecessary.

Nonetheless growers needed to consider a lot of variables that affected the type and amount of weeds on their property such as different geology, microclimates and soil moisture. Another complicating factor is the amount of seed available.

Many weed seeds are viable for years. For example, Noogoora burr has germinated when property owners broke up the concrete slabs of old dairies, uncovering burr seed buried 50 years ago. Other weed seeds are spread by birds or blown in from adjacent properties.

Sandy said he had gradually revegetated his property - which didn't have a blade of grass growing when he bought it - at about Iha a year. Each spring he clears off weeds and plants grasses by broadcasting a mix of broadleaf paspalum, setaria and green panic seed. White clover is over seeded the following autumn.

When these are established, he over-sows with kikuyu runners. He said he found that by adding extra lime, rock phosphate and basalt dust the kikuyu will spread and eventually displace the pioneer grasses. He added: "It is interesting to compare the results of seeding in different years, which to a large extent were determined by the degree to which I added liming agents to the soil or by the dryness of the spring as it affected grass germination and survival."

The reason Sandy is so keen on grasses, particularly kikuyu, is because he finds they are one of the best means of building up soil organic matter. "In this area you have 10 tonnes/ha of roots and rhizomes and l0-15 tonnes of above-ground material produced each year. Whilst a large amount of this decomposes without obviously accumulating, it is feeding soil organisms from bacteria to earthworms."

In terms of building up soil organic matter, the more ligneous (woody) stems of setaria provide; in theory, more longer-lived humus than kikuyu. But the horizontal stolons and rhizomes of kikuyu, on the basis of Soil Conservation Service experiences, are 10 times better at preventing surface soil loss compared with tussock grasses.

"Retaining the soil surface layers must be the highest priority for sustainable farmers," Sandy said, "so I am slowly converting to kikuyu and clover since, in our climate, we typically get two or so rainfall events with saturated soil generating massive overland flow. On anything other than flat ground or the densest swards of rhizomatous plants, this can remove hundreds of tonnes of top soil per hectare overnight."

Sandy said the accumulated affects of climate had an important bearing on soil formation and nutrient loss. The chocolate basalt soils west of Lismore formed in a climate where evaporation balanced rainfall and not many cations leached. But the red soils east of Lismore formed when precipitation exceeded evaporation. The extra stream flow over 20 million years removed many more nutrients and much more topsoil.

What appears to be a generally applicable rule is that crop plants and weeds can grow in soils of a fertility better than their lower limits for tolerance. Generally those species best adapted (often most productive) will dominate and out-compete those adapted to poorer fertility sites.

"This means you can't look at the presence of one or a few species to indicate soil conditions - you need to look at the abundance of all species. For example, a few clumps of lantana and Crofton weed in a paddock of kikuyu is not the same as a paddock covered in lantana and Crofton weed and from which the kikuyu has died out."

Sandy said many Australian organic growers had read in the early literature of organic and biodynamic authorities in Europe and North America that particular weeds indicated or improved soil fertility.

"But in the light of the history of glaciation and soil renewal in cold temperate climates, and the fact that they often deal with soils where the cations actually increase with depth, in contrast to most Australian and tropical climate soils, some of their comments about particular weeds bringing nutrients to the surface need to be treated with great caution."

"It has become clear to me that much of the agricultural and horticultural literature from Taiwan, India and South Africa is much more applicable to Australia in general and subtropical farming in particular."

Sandy encourages local growers to delve into the SE Asian literature, most of which is written in English. He quoted research which showed that many of the aromatic weeds like Crofton weed, farmers' friends, stinking roger and blue goat weed contain chemicals that can reduce the growth of crop plants.

He also said he hoped more people would take an interest in local flora, and suggested that farmers keep records of the abundance and species of weeds in each paddock each year so that they can monitor changes and add to local knowledge.

Sandy's comments on some local weed species

Blackberry nightshade(Solanum nigrum): Some references say the berries are toxic, others that they are edible. People seem to be affected differently.

Blue goat weed (Ageratum houstonianum): In a pasture, blue goat weed indicates intermediate fertility. A lot of people don't like it but Sandy finds that as the grasses increase it just gets out-competed.

Carpet grass (Axononopus affinis): Not a real problem. It reflects reduced soil fertility.

Crofton weed and mistflower (Agerarina adenophora and Airiparia): A sign of acid, rundown conditions. Can be out-competed.

Crowsfoot grass (Eleusine indica): Tends to grow in the hot dry months before the wet season on bare, compacted earth.

Docks (Polyogonum spp): Tend to colonise as soil pH increases, appearing when pH is about 5.

Inkweed (Phytolacca americana): Seeds are dispersed by birds. It indicates a middling condition of soil but the plants get bigger and lusher as you improve the soil. Ultimately a grassy sward will out-compete it.

Lantana (Lantana camara): As well as competing for light and moisture, lantana wages "chemical warfare" against nearby plants.

Native rose-leafed raspberry (Rubus rosifolius): Sandy said in his experience cutting it once or twice was enough to send it into a decline.

Paddy's lucerne (Sida rhombifolia): If it appears in greater numbers than previously, it is a sign of increasing pH but often low organic matter. It's good for building up long lasting (ligneous) organic matter.

Red natal grass (Rhynchelytrum repens): It produces very little biomass, recycles very few nutrients where it occurs, typically cropped paddocks now low in organic matter.

Setaria palmifolia: A native of New Guinea with palm-like foliage. Stems and leaves are covered in 4mm hairs that stick to your skin and causes itchiness. Cattle will eat it. It tolerates fairly low nutrients, acid conditions and some shade.

Sow thistle (Sonchus deraceus): Indicates intermediate to high fertility levels. Often grows under orchard trees.

Spear thistle (Cirsium vulgare): Indicates high - often over high nitrogen levels and often high organic levels.

Tobacco bush (Solanum mauritianum): Don't worry about it unless it is growing next to orchard trees. If you are doing rainforest regeneration, encourage it at the forest edge. It does a good job in .recycling nutrients which will benefit the native trees. Foliage is very high in nitrogen - more than 4%.

Tropical chick weed (Drymaria cordata): Sandy said he had found it quite competitive with pasture swards, particularly areas which are slightly moist or shady. It can go from a high biomass, high turnover groundcover to something that provides very little nutrient turnover. The soil becomes soggy and looses its crumb structure, it doesn't transpire much water and it appears to cause a lot of problems in the soil.

Verbena species (Verbena spp): Four species are naturalised on the North Coast. Sandy said he had seen them build up in numbers, then decline. He interpret this to mean they indicate intermediate state of soil fertility.

Wandering jew (Tradescantia albiflora and others): Six or seven species grow in the TROPO area. Soil beneath it becomes very gluggy it appears wandering jew prevents soil aeration. It also reflect high nitrogen levels. Striped wandering jew is a potential pest in rainforests.

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