Last year I "cleverly" made my indoor seed-starting mix faster-drainer by adding a lot of screened pine bark from a clean high-quality mulch I had high germination rates, zero damping off, and overwatering ceased to cause problems. Everything did much better ...
except for petunias.
In a 128-cell tray, almost NO petuinias sprouted. i think that was around 12 different varieties from trades, but near-ZERO germination had to be my fault.
I think the coarse surface of the mix let tiny petunia seeds fall deeply into the mix where they got no light. Possibly petunias LIKE a very damp surface and the coarse mix allowed the surface to drain too well, even though I used a dome or plastic film cover to maintain humidity, and sprayed the surface every few days.
In past years, I killed many seeds of other species but was always able to start petunias easily on top of soggy, peaty soiless mixes. This was in a cold room (55-60 F, maybe 65 F some days).
I did add a heat mat on top of a piece of drywall, but even if it rasied the soil temp by 15F, that would only bring it to 70-75 F or rarely 80. Also, since I shared one heat mat among several trays, only the center part of the tray got the full warmth, and some days no extra warmth at all.
I think it was the coarse, faster-draining surface of the mix that meant "no petunias" for me last year. This year I'm going to use the fine part of what I screened out to top-dress my petunia tray ... and maybe start two trays, one of them early enoguh that if it fails, I have time to start a second tray!
