How do you spell the past tense of silo?

The verb "silo" doesn't have a past tense because it's not a verb. It's a noun that refers to a tall structure used for storing grain or other agricultural products.

If you're looking for a verb related to the concept of "silo," you might be thinking of "to silo," which means to isolate or separate something, often information or people. The past tense of "to silo" is siloed.