I have some of those frost-hardy kiwis, three years old (one male, two
female). They have intertwined themselves in the back chain link
fence, and I'd like to train them over a (yet-to-be-built) pergola
instead.

When should I prune the vines, how aggressive should I be? I suspect
that they ought to be no more than a year from starting to set
buds/fruit, so I don't want to thwart my efforts at getting another
"snack bush" for the kids.

Thanks,
Mark

very aggressive, but it is much better if you do it in the fall. you
should not do it in the spring under any circumstances. Pruning in the
summer is OK, but a waste of resources, specially before they have
started fruiting. When you prune them, you can take out 2/3 of the
vines, but of course leave the main branches alone. Make a big pergola,
and do not count your chickens yet, my kiwis took 7 years to fruit.