Re: Eucalyptus Questions
Nan Sterman wrote:
>
> >An old gardener down the road told me that the roots of Eucalyptus, probably
> >thinking of, for example, E. citridora and E. globulus, produce some growth
> >inhibiting materials. The leaves, however, certainly do make a good mulch.
>
> Karl and others, if you go back in the medit plant archives, I think
> you will find a messge that someone sent about a research project
> that had been done on euc leaf mulch. The results showed that the
> mulch actually stimulated growth rather than inhibited, disproving
> the long-held myth that leaves produce growth inducing hormones.
> The study also (and I don't recall how) suggested that the roots
> inhibit growth by way of their being so dense and close to the
> surface rather than by synthesizing an inhibitory compound.
>
Nan
I presume in this contex growth inducing was a slip for growth
inhibiting?
I am sure you are right that in many cases failure of growth under
various trees and shrubs is due to the close-packed roots rather than ny
allelpathic chemical.
However, curiously, some plants which are perfctly friendly in life can
produce allelopathic chemicals after death in the roots which are
rotting in the ground. I have seen two examples of this myself. The
first was when I planted tomatoes following a cover-crop of barley. Not
knowing that unrotted barley roots can be poisonous, I planted my
tomatoes before the barley had completly gone and had a very sad crop
with yellow, brown spotted leaves which got nowhere.
The second example happened equally unexpectedly to a friend of mine who
made a new shrub border on the site of an old apple tree. Most of the
plants grew perfectly normally, but a camellia in the bed showed similar
symptoms to my tomato plants. My friend thinking she had been sold a
"lemon" indignantly went back to the nursery and talked them into giving
her another plant, with exactly the same result. Reading what I had said
about the barley in a gardening column I wrote at the time, she
approached me to see what I thought and at my suggestion she moved the
camellia to another bed, where it very rapidly recovered.
I suppose the interesting thing was that the apple roots had so specific
an effect, but of course this applies also to that very well-documented
poisoner the black walnut, as while there are a wide selection of plants
which cannot tolerate its secretions, an almost equal number of species
can safely be grown in its shade.
As to gum leaves as mulch, I am sure the Australian contributors could
tell us if they are universally safe or whether there are some species
on which they should not be used or some gums which are less benign of
leaf than others.
And apropos of walnut leaves and twigs, while they can be lethal when
fresh, it seems once they are composted the juglone rapidly breaks down
rendering them safe.
--
Tony & Moira Ryan <theryans@xtra.co.nz>
Wainuiomata (near Wellington, capital city of New Zealand)
