Well-drained growing media (was Oh No! A Boophone disaster!)
Jane McGary (Sun, 04 Oct 2009 10:18:01 PDT)

Tom wrote,
On a related point, I wonder if anyone with experience of using it

I live where horticultural pumice is readily available. In general we
can get two grades, washed and unwashed. Most nurseries use the
washed product and mix it with composted fine bark, which is also
very cheap and lightweight. Because I grow summer-dormant bulbs, I
avoid using bark in the mix because I think it hosts microorganisms
that also attack the bulbs, or at least their tunics. The washed
product, which has a fairly large average particle size (I think
about 4 to 5 mm), will "float" to the top if mixed with heavier
components, such as the coarse sand I also use, so I prefer the
unwashed product, which has particles from about 7 mm down to dust. I
think the fines in it make the significant nutrients in pumice more
available to the plants. Not everyone agrees that having fines of any
kind in a potting mix is desirable, however; you do have to manage
your watering very carefully so as not to waterlog the mix. Having
maintained many of my bulbs in this mix for more than 15 years, I
like it anyway.

I don't know if the unwashed (unscreened) product is available
anywhere outside the immediate area where pumice is quarried. This is
white to gray pumice, by the way; the term "pumice" is also applied
in the trade to a dark red volcanic rock which, I am told, is more
properly called "scoria." The red pumice, or scoria, is mostly used
in large particle sizes (3-4 cm) as mulch. Loren Russell has informed
us that it contains very significant nutrient levels, and he has gone
to rock yards and scooped up the fine detritus in the bottom of the
scoria bin to use in growing alpines. I use the smallest size of
scoria that can be bought here, about 1 cm diameter, as a
top-dressing for some bulbs.

Our NARGS chapter in Portland recently heard a talk by Truls Jensen
of Wild Ginger Farm nursery called "Fresh air," which featured his
techniques for increasing air space in container soils in order to
grow xeric plants successfully. Wild Ginger, a mail-order nursery,
has a very interesting plant list: WildGingerFarm.com

Jane McGary
Northwestern Oregon, USA