Top' Robinias causing headache
have three Robinia 'Mop Tops' planted about two metres apart.
They were all planted at the same time by the previous owner several
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?
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.
Gardener, Issue 143, 2004, Page 27
Reproduced with permission from the former Weekend Gardener magazine. The views expressed here are not necessarily those of the RNZIH.