10% + 2' is the formula typically used for earth supported posts
from power poles to fence posts. I cannot see any reason to
worry about frost depth for a fence or a grape vine.

Ran many miles of high power line in western Nebraska on 120'
poles. 14' setting depth.



--
______________________________
Keep the whole world singing . . . .
DanG (remove the sevens)




"JoeSpareBedroom" wrote in message
...
I need to install three 4x4 posts to support grape vines.
There'll be some sort of wire stretched horizontally between the
posts, as done at vineyards. However, I'd prefer NOT to pour
cement. Reason: This project could be a total failure if the deer
decide grape vines=salad. They ate my neighbor's car last week,
so anything's possible. So, if I'm using 8 foot posts, should 3
feet under the ground be sufficient?