Thank you, Jean.
The spores must have gotten into my garden via the wood chips I collected for mulch from the utility company's chipping pile. The source of the wood chips is the limbing under the electric lines throughout Trinity County, much of which is forested.
Based upon the information in the second link, I do not have the right habitat for the mushroom to grow naturally in my garden.
I generally only use shredded leaves for mulch, but this year, due to the drought, the leaf mulch dried up too quickly and did not perform the function of retaining moisture in the soil. So, I re-mulched most of the garden with a combination of shredded leaves and small wood chips. (I screened the chips I collected from the chipping pile.)
Next year, I'll be able to go back to just the shredded leaves because we've gotten significant rain this fall and the soil is wet deep down again, so even tho' the drought is not over, I don't think I will face the same problems I had this year.
Smiles,
Lyn