Bosseye wrote:

Have just (mechanically) cleared 0.25 acres of old and impenetrable
rododendron, and having removed the roots, will rotovate and overlay
with 6" of soil/sand/loam mix before laying a new lawn.
First, I'm aware that the rhododendron is toxic to other plants, and
I'm trying to understand what may not be plantable in the reclaimed
land (ie. lawn, laurel, fruit trees etc). And also what might be done
to neutralise this effect.

This is a weak effect. I think with what you are doing, you will be OK.
You are removing Rhododendron ponticum and it does has some allelopathic
properties. They seem to inhibit germination of seedlings or the
development of seedlings as reported in:

http://www.countrysideinfo.co.uk/rhododen.htm

Second, I'm aware that there is likely to be rhododendron seeds left in
the ground, which will doubtless sprout. Will glyphosate neutral these,
or do I require something different, and will that be fine to seed lawn
in mid September.

Glyphosate has no effect on seeds. It enters green tissue and poisons
the roots. It works best on mature plants that are translocating sugars
to the roots.

Travis wrote:

I don't know where you heard that Rhododendrons are toxic to other
plants. I have many Rhodys planted next to other things and it is not a
problem. I doubt the seeds will be a problem. If perchance a few do
sprout the first mowing will take care of them for good.

I doubt if you plant much seed under your rhododendrons. The
allelopathy affects seedlings, not plants. There is current research
showing that there is some evidence that some rhododendrons including R.
ponticum and R. maximum are allelopathic. It seems to be a substance
that inhibits the germination of competing seeds or prevents mycorrhizal
development of competing roots. Junipers have a similar effect. I have
noted that few weeds start under my rhododendrons or my junipers.
References to current research:

http://64.233.169.104/search?q=cache...ology.uga.edu/
publications/1412.pdf+rhododendron+allelopathic&hl=en&ct=clnk&c d=1&gl=us&
client=firefox-a

http://64.233.169.104/search?q=cache...ology.uga.edu/
publications/122.pdf+rhododendron+allelopathic&hl=en&ct=clnk&cd =2&gl=us&c
lient=firefox-a

http://www.amjbot.org/cgi/content/full/86/11/1597

http://www.amjbot.org/cgi/content/abstract/86/11/1597

http://www.spri.cam.ac.uk/~ojm21/rhodo.htm
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