Pumice
Jane McGary (Wed, 21 Oct 2015 16:59:34 PDT)

Sarah asked

On 10/21/2015 2:02 PM, Sarah Hinckley wrote:

Here in the maritime Pacific Northwest, EVERYTHING gets green stuff on
it. We just hope most of the green stuff has pretty flowers.

Almost every kind of grit I've used for topdressing in seed pots
eventually acquires algae and moss, but pumice holds out for 2 or 3
years against them. Perlite of course would not be used on the surface
of a pot, but if you don't apply a topdressing it will "float" to the
surface of a mix that has other lightweight components (such as peat and
bark).

Jane McGary
Portland, Oregon, USA