COMPLETE WEEDKILLER MAY NEVER EXIST

Press, Volume XCVIII, Issue 28896, 16 May 1959, Page 8

COMPLETE WEEDKILLER MAY NEVER EXIST

There was no perfect chemical for total weed control under all conditions of climate and vegetation, and probably there never would be. This was stated by Mr Warren Johnston, technical service officer of Imperial Chemical Industries (New Zealand), Ltd., Wellington, when speaking on the difficulties of total weed control to the annual conference of the New Zealand Association of Inspectors of Noxious Weeds, at Nelson.

Nearly everybody thought it was simple to spread a few handfuls of chemicals, watch grass and weeds die overnight, and see the area stay bare for ever. But no such chemical existed, and Mr Johnston said he doubted if one would ever be found. For the present, at least, it was impossible to keep the ground permanently bare of vegetation after one application of any chemical.

Mr Johnston said that the chemists should not be blamed for the failure to find the complete weedkiller; it was not their fault. The reason was that the fungi and bacteria always present in the soil would forever break down any synthetic chemicals used in weedkiller mixtures. If the soil could be sterilised weedkillers would last much longer. But there was little hope of doing this, so it just had to be accepted that fungi and bacteria would continue to decrease the life of weedkillers in the soil.

Mr Johnston was critical of those who waited until vegetation

was about three to five feet high before getting sprays to kill it. How could they expect any chemical to wet such a tangled mass, kill it completely and leave sufficient residue in the soil to have an effect for up to a year? No chemical made would do this. And, not only was such a request unreasonable, but it could be dangerous as high dead vegetation was a bad fire risk.

Mr Johnston said he considered it much better to spray in late winter—early spring and again in late summer—early autumn rather than wait until summer or early autumn as most people did. During the late winter —early spring period there was plenty of moisture in the soil to help the weedkiller penetrate. There was also enough rain following application to assist the weedkiller to sink in. At this time of the year the weeds were small and easier to kill while the fire hazard was negligible. If spraying was again carried out in late summer—early autumn Weeds which had germinated since the earlier dosing had not grown much. The weeds would have gone through the driest period of the year and little effort would be needed to kill them. Mr Johnston said that it also had to be realised that weed control was an ever-recurring job. Contracts for this type of work should be let on a two to fiveyear basis. Too many people applied only one dose of weedkiller and failed to follow up the initial results.