I bought a house last year and built my first garden bed last month. I just dug up dirt and sod and built a raised bed by digging a trench around the bed and piling dirt inside the trenches. Then I dumped about 1 inch of stuff from a bag labeled "black mulch" on the bed and planted onion seeds in it. (This mulch seems to be partly decomposed landscaping waste including woody branches or twigs.) We had days and days and maybe 10 inches of rain over the next two weeks. Not a single onion has sprouted. Now I'm thinking I was wrong to put this black mulch on the bed. Do you think the bed will be okay if I scrape the mulch off? What do I need to put on the bed once I get the mulch off, if anything, to help seeds sprout?
A related question is what to put in planters that I want to use for germinating tomato and maybe eggplant seeds? Do I have to go out and buy seed-starting mix? Could I just use soil from my yard? Thanks in advance.