Plant Doctor Archive

Old trick

I AM trying to track down details of the use of (I think) 3 per cent sulphuric acid applied as a spray to onion beds to reduce the need for hand weeding. I think this is an old-fashioned method, but at least it is organic. Can you help?

TEN per cent sulphuric acid was once used many years ago to control weeds in onion beds, but the hazards of diluting the concentrated acid with water were so dangerous its use has been discontinued. Even if you could get hold of the acid, I wouldn't recommend you try it. The shape and biological make-up of onion leaves are such that the acid did not damage them, but it did affect the weeds.

Modern-day organic weedkillers, like Nature's Way Greenscape, work on a similar leaf-destructive basis. They are fatty acids and, if applied at strong enough concentrations, will damage all plants they come in contact with. However, it is perhaps possible that you may be able to dilute one of these weedkillers to a point where it may have no effect on onions but still affect seedling weeds. You could carry out some trials to see if the idea works, but be prepared to lose some onions! Let me know what happens.

Reproduced with permission from the former Weekend Gardener magazine. The views expressed here are not necessarily those of the RNZIH.