Plant Doctor Archive

'Mop Top' Robinias causing headache

We have three Robinia 'Mop Tops' planted about two metres apart. They were all planted at the same time by the previous owner several years ago.

Two of the trees are wonderful, with healthy foliage - they have hostas and rhodos planted beneath. The other robinia is not quite as tall and looks like it's getting ready for winter already.

The foliage looks poor and it has some dead branches. It has Viburnum davidii planted beneath it. Do you think it might be something caused by the plants beneath, or is it lack of water? Can you please help?

I DOUBT if it's anything to do with the viburnums. It is possible your poor-looking robinia is in a drier spot than the others and may be receiving fewer nutrients.

However, the more likely cause of the symptoms you describe is the larvae of the lemon tree borer - a native insect - which, contrary to the image conjured up by its name, attacks a range of native and ornamental plants as well as citrus.

Lemon tree borers do seem rather partial to robinias and if they get into the trunk of a very young tree they can knock it back severely.

Check out the dying branches for small holes which may even have blobs of dried sawdust-looking material around them. These are excretions from the larvae as they bore into the stems.

Split a dead branch up the centre and you may find a tunnel with the larvae still inside. They can spend almost two years inside the stems before emerging as an adult beetle.

There's little you can do in spring to stop the adult laying its eggs and once inside the stem the larvae are well protected from sprays.

Some gardeners recommend injecting kerosene into the holes then plugging them, but that's often not very effective.

My advice is to prune out the damaged branches, cut them back to healthy wood and destroy the prunings to kill any larvae inside. Then fertilise the tree well with a general fertiliser, apply mulch and water during dry periods to encourage lots of new growth.

With a bit of luck, your tree will recover, though you'll need to keep a wary eye on all three trees for future attacks.

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