Why wouldn't this be past perfect, have pruned as opposed to exclusively a continuous 'been pruning' sense? How would one say "We have pruned the grass?"
You're correct. It's present perfect. The present perfect continuous would be "nós temos estado podando a grama" (but you'll hardly ever hear that)
Nós podamos a grama.
We prune the grass.
We pruned the grass.
We have pruned the grass.
Nós temos podado a grama.
We've been pruning the grass.
By the way, "podar" is usually used for trees rather than grass.