I’ve picked up seed from several different source. Several white fruited fresh berries of Morus alba from around Bedford County (40.0165595,-78.5036327), several white and several black-red berries of M. alba from around Blair County (40.5144636,-78.4068237), and of course the aforementioned M. nigra from JLHudson Seedsman.
Deno describes M. alba as being damaged by starting in cool temperatures, but most other sources recommend starting with cool temperatures for several months, including the inspiration for this project, Lee Reich’s Uncommon Fruits.
I think it is rarely a mistake to plant seeds of fall bearing berries in cool temperatures followed by warm. However I just collected the mulberries this week. I think it is perfectly reasonable to start these fresh seeds directly in warm conditions. That’s what they’d see outside right now. The dry stored seeds from JLHudson have been split between sowing now and sowing in the fall. JLHudson’s catalog and packet both indicate that it should sprout in 1-2 weeks in warm temperatures. The packet was generous, so I sowed the pot of soil fairly heavily.