3 September 1999


CONSUMER demand outstrips the UKs domestic supply of organic vegetables.

But weed control deters many potential converts from conventional farming from trying to meet it.

"When it comes to weed control nothing is as effective as a herbicide," says Bill Bond, a weed scientist at HRI. But chemical weed-killers cannot be used in organic systems, so growers ideas must change to find the optimum system for each farm and field.

Non-chemical solutions are being explored in a MAFF funded project at HRI and on ten converting reference farms across the UK.

Weed problems are often laid during the conversion period, warns Mr Bond. The first fertility building grass and/or clover ley needs cutting before any weed seed is set, and careful checks must be made before first cropping to pinpoint other potential troubles.

Weed seeds can blow in from outside the farm. Poorly dressed seed, especially home-saved, can be another source, as can composted green waste not heated enough to kill weed seeds, he adds.

Rotation is important in determining weed risk, but tillage is the main way of keeping weed numbers down and preventing any one species becoming dominant.

"Post harvest you have to decide whether the seeds persist longer on the surface or when buried. You have to know what the problem is."

Ploughs have a big impact, and following work can be used to kill or control weeds using stale seed-beds or by cultivating in darkness.

Cover crops and inter-cropping are useful but must deliver more than just weed control or they are not economic. They need to offer nutrients, organic matter or pest control as well, says Mr Bond.

The potential for weed control through modifying crop density is often limited by market quality demands. Required plant spacings often leave bare soil which is ideal for weeds, as do the row spacings needed for mechanical weeding. But modules can tip the balance in favour of the crop by planting after a stale seed-bed cleaned up by mechanical or flame weeding.

A full fallow to control perennial weeds is too costly, he maintains. So it is important to keep on top of such problems from the start by using short fallows or fast-growing crops such as radish. &#42

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