As Mark mentioned, municipal compost in this area includes lawn clippings, and I don't use it either for fear of herbicide residue. I do use a mulch containing compost, but the company that provides it tests the ingredients for residual harmful chemicals. I like to use a minor proportion of organic material in bulb potting mix, and most recently I bought bags of "garden topsoil" from certified organic sources for this purpose. The main things I avoid are bark, which appears to be attacked by a fungus with visible mycelia that can also attack the tunics of dormant bulbs, and perlite and vermiculite, which have no value to the plants and tend to rise to the top; the latter are also said to be dangerous if you inhale the dust. When I started growing bulbs seriously around 1990, I had a country place with an alder woodland on part of it (alders are nitrogen fixers). I screened the topsoil to make up part of the bulb mix, along with ground pumice and coarse upriver sand. This worked very well and there seemed to be no problem with disease, even though the leafmold surely contained all sorts of microorganisms. I did not use this mix for seed sowing, but instead used peat as a minor component. I think sterilizing seed soil is pointless unless you can maintain laboratory conditions, since spores, etc., will arrive in the air. I used to grow Meconopsis by surface-sowing on milled sphagnum moss (not peat) as a preventive measure, but since moving to a place where that genus doesn't grow well, I gave that up. Probably the hardy, summer-dormant bulbs I grow are not as vulnerable to disease as the tropical and subtropical species some PBS members have. Surplus bulbs that I've removed to the garden mostly flourish there despite weekly irrigation in most places. It has always seemed to me that cultivating these plants as "hard" as they can tolerate results in healthier populations that appear in character. Coming to bulb growing from the perspective of alpine and rock gardening is no doubt an influence. My bulb house is very like an alpine house, but not even minimally frost-free. Many PBS members might despair at a situation where South African bulbs and tropical amaryllids can't be grown, but I like the relative freedom of this kind of gardening. Jane McGary, Portland, Oregon, USA On 12/17/2023 8:18 AM, Robert Lauf via pbs wrote: > Regarding arborist debris, I'd be curious to know whether the kinds of bacteria and fungi inhabiting half-dead trees would present a problem to bulbs or if they are sufficiently host-specific that they are harmless in potting media. For all I know, they might be the same microbes working in composters. > Any mycologists out there who could weigh in on this? > Bob Zone 7 > _______________________________________________ > pbs mailing list > pbs@lists.pacificbulbsociety.net > https://lists.pacificbulbsociety.net/cgi-bin/… > Unsubscribe: <mailto:pbs-unsubscribe@lists.pacificbulbsociety.net> > PBS Forum https://… _______________________________________________ pbs mailing list pbs@lists.pacificbulbsociety.net https://lists.pacificbulbsociety.net/cgi-bin/… Unsubscribe: <mailto:pbs-unsubscribe@lists.pacificbulbsociety.net> PBS Forum https://…