Plant Doctor Archive

Awful oxalis

I LAID newspaper and bark to keep weeds down in my flower garden, but when I returned from being away for some time I was dismayed to find oxalis everywhere. Can you give me suggestions how to get rid of it?

THE main types of oxalis that are problem weeds survive as dormant bulbs below the soil when conditions are not conducive to growth, such as summer drought or winter cold. When the weather suits them, they shoot up new growth, quickly form many more small bulbs and can become a real nuisance. Newspaper under bark suppresses growth for a short while, but once the paper rots down the oxalis will just come through again as the bulbs seem to remain viable in the soil for years.

Instead of newspaper, you could use weedmat, which still allows water and air to get to the soil but will suppress the oxalis much more effectively. Alternatively, when the oxalis is in full leaf, you could spray with Amitrole which is absorbed by the foliage and taken down to the bulbs. You often need to respray new growth as it appears not all the bulbs are killed first time. Another product worth trying is No Weeds Ronstar which comes as granules that you apply to bare soil while the oxalis is dormant, before the foliage emerges. Rain washes the Ronstar into the soil where it prevents germination of weed seeds and the growth of many types of perennial weeds. Follow the instructions carefully as, among other things worth noting, cultivating the soil after application will reduce its effect.

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