Plant Doctor Archive

Dreaded onion weed

Please can you tell me what I can spray to kill off onion weed? They are only small now, as I have been digging them up for months, but I just can't get rid of them.

I'VE found Amitrole most effective for onion weed. It's best to spray when the weeds are in full leaf for good control, which means you need to curb your desire to dig them out, leaving them to grow quite large and leafy before spraying. This is so there is maximum foliage to absorb enough weedkiller to kill off the bulbs. If you spray too soon you may just kill the leaves but not harm the bulbs. Even then you will probably still get some regrowth next season from a few bulbs that didn't die off, which will need to be treated again.

Make sure you mix the spray according to the label recommendations, including the addition of a "wetting agent" such as Sprayfix for a better kill rate.

Don't panic if nothing seems to happen for a while, as Amitrole works quite slowly and it might even be next season before you realise your onion weed problem has greatly diminished.

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