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

Jane McGary janemcgary@earthlink.net
Sun, 04 Oct 2009 10:18:01 PDT
Tom wrote,
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.

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

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