September 2006. A commercial onion grower on the main island of Tonga was concerned about the weed (illustrated) in his crop, and would like an identification.

Several species of Oxalis were suggested: O corymbosa (pink wood sorrel), O articulata, O debilis. O corymbosa was formerly O corymbosa. It is quite difficult to control as herbicides against Oxalis would affect onions.

Oxalis forms bulbils at the base, and any cultivation of the soil breaks up the clusters of bulbils and spreads them. By far the most effective control method is careful hand weeding, involving gently digging up each plant with minimum disturbance and picking every bulbil out of the disturbed soil before moving on to deal with the next plant. It is extremely labour-intensive, but very effective. This can be done while the onions are growing, provided there is sufficient space between the rows to permit working in the crop without damaging the onion plants.

A word of warning – any soil on the roots of lifted onions from that field is likely to contain Oxalis bulbils, and is likely to help spread the problem.