Perennial vegetable gardens build soil the way nature intended—by allowing the plants to add more and more organic matter without tillage, and letting the worms do the work of mixing it all together. Perennials provide ecosystem benefits.
"An annual vegetable garden, as we all know, requires much more watering, weeding, and work to get a good crop. Once established, perennial vegetables are often more resistant to the attacks of pests, due to their reserves of energy stored in their roots. ...
Their deep roots and soil building abilities make them more self-sufficient in terms of watering, and their canopies which leaf out so much earlier than annuals better suppress the growth of weeds. What else can these remarkable plants do?"
Via Bronwen Evans