Plant Doctor Archive

Bulb fly

I HAVE several clumps of smallish vallota lily bulbs, also known as cyrtanthus or Scarborough lily, but they never flower.

I have spread them around the garden hoping a different spot might help, but no luck. As they're from South Africa, I have been sparing with the fertiliser. They are in sandy soil, so I don't think drainage is a problem. Can you give me any tips?

MY first reaction was that perhaps you've been digging them up and moving them so often they haven't had a chance to reach flowering size, but that seems unlikely. I checked with bulb expert Lindsay Hatch from Joy Plants and he suggests the problem could be bulb fly larvae which can damage the bulb, weakening it so it doesn't flower but is still capable of producing small bulbs.

The adult bulb fly lays eggs at the base of leaves just before the bulbs go dormant or they crawl into the neck of the bulb after the leaves have died off to lay their eggs. The maggots eat into the bulb, destroying the tissue inside.

Lindsay suggests digging up the bulbs each year soon after the foliage has died down and if you find any holes in them to poke a fine wire in to kill the grub. Store the bulbs and replant the following season. Alternatively, if you are sure bulb fly is the culprit, you could apply a soil insecticide, like Diazinon granules, sprinkled around the bulbs as the leaves begin to die down.

Lindsay recommends planting vallotas in a sunny, well-drained spot, with the neck of the bulb just below or even showing at the surface (which, unfortunately, leaves it more exposed to bulb flies) and to feed with a high potassium bulb fertiliser. Other suggestions are to cover the bulbs temporarily over the dormant months with a mulch of good soil or finely ground bark immediately after the leaves have died off, or to allow a fine-leafed groundcover to grow over the bulbs, both of which make it harder for the adult fly to lay its eggs in the bulb.

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