Well-drained growing media (was Oh No! A Boophone disaster!)

Leo A. Martin leo@possi.org
Mon, 05 Oct 2009 18:02:22 PDT
> On a related point, I wonder if anyone with experience of using it
> would advise me which grade or particle size of pumice to use for
> horticultural applications. I've never used it but am keen to experiment
> with it after reading the recent correspondence on Boophone and other SA
> Amaryllidaceae.

So much of this depends on your climate and the plants you are trying to
grow. People in wetter and more humid areas generally succeed with more
open mixes of larger particle size. People in drier, more arid areas look
for smaller particles. Bulb seedlings usually need a more water-retentive
mix. But the general idea is to use a combination of soil mix plus
container that almost dries in the same time frame as your preferred
watering interval.

For example, Phoenix has very low winter humidity and warm days unless
it's raining--often under 10% for weeks on end. I use non-porous
containers twice as deep as wide and a soil mix of sand mixed with native
granite soil for my winter-growing bulbs. I suspect in Oregon with this
mix and containers my seedlings would rot quickly, but my problem in
Phoenix is keeping seedlings damp enough.

Perlite works well under many circumstances but horticultural perlite
available here is of quite small particle size and it stays wet a very
long time. With watering, perlite, if mixed with anything else, soon
floats out of the heavier stuff and forms a zone of pure perlite at the
top of the container, with the heavier stuff below this. Then the perlite
blows out of the pot in the wind, or it is splashed out with watering. I
have wanted to try some of the larger-particle perlite, often used for
insulation, but I have been warned much of this is very high in fluoride
and toxic to plants. I don't know where to find it in any event.

I would definitely encourage people to wash the pumice to remove the
powder unless you wish a more water-retentive mix. And remember to wear a
mask when working with pumice or perlite; the dust can cause lung disease.
That goes for pretty much anything that is dusty--peat moss, coir, scoria,
etc. etc. etc.

Leo Martin
Phoenix Arizona USA

More information about the pbs mailing list