Raising a wild garter and have questions.

[Stefan-A]

Doesn't natter how "fresh" it is, dirt is dirty.

No. "Dirt" has multiple definitions and "dirty" is vague to the point of uselessness, as it can cover everything from the presence of harmful bacteria to harmless bacteria and inert particles and essentially anything that negatively affects aesthetics.

In spite of where they live in the wild you will want to keep substrate clean and dry in captivity.

This is true.

Aspen shavings/shredded are economical and a good choice. Do not use pine or cedar.

Aspen shavings are dusty and can cause respiratory issues, as well as impaction if ingested and in practice it's not necessarily any cleaner than dirt.