"Nick Maclaren" wrote in message
...

I have Web access, but had to retype :-( Here is the relevant source
in Cross, which shows how these urban myths develop by the simplification
and extrapolation of ambiguous scientific results.

Doekson (1964) found that ground up leaves of rhododendrons caused a
reduction in the number of earthworms (Lumbricus rubellus Hoffmeister)
in peaty soil. The substance responsible was soluble in acetone, but
not in ethanol, ether or benzene. He also observed a reduction in the
numbers of L. rubellus and Allolobophora callignosa Savigny in the
soil in which rhododendrons were planted, even without the addition
of ground-up leaves. However, Lumbricus rubellus is common in the
Rhododendron humus in the Killarney woods.

Although the soils at sites 18-10 would have been greatly influenced
by Rhododendron, being almost pure stands, there is no obvious
difference with other soils. It is, however, probable that in common
with other ericaceous species, Rhododendron has a deleterious effect
on the soil, mobilising cations, directly or indirectly by the production
of phenols. Rangaswami & Verkatswarku (1966) report the presence of
polyphenols in other species of the genus, and Raudnitz (1957) has
found a humic acid in the leaves to be a water-soluble, surface-active
polyphoric ester. ...

Yeah, well. One person has found a reduction in earthworms, and another
has found little or no reduction. Several people have detected common
plant toxins (surprise, surprise!) Not a lot of evidence there.

In several other places in that paper (quoted), it referred to the
physical effects I described, and one that I did not - rhododendron
humus apparently forms a hard surface when baked in the sun.

Frankly, I don't believe that the toxin effects of either R. ponticum
or J. niger are important - whereas the physical ones of many thicket-
and canopy-forming plants with large leaves most definitely are. You
just have to look for yourself to see the latter! But it is amazing
how few scientists ever do look for themselves, as Rackham points out!


Regards,
Nick Maclaren.

I am totally with you on this one.
At my previous house we used the leafmould in volume from beneath the
Rhododendrons as a seed and potting compost without any bad effects.
It was also noticeable that the areas that had been scraped clear of debris
became host to a majority of the common garden weeds.