Vivero means nursery or greenhouse. It can be open to the elements or covered. In the world of mezcal, this has become a more common way to grow agave starts, from either seed or hijuelo.
Viveros have become an important part of the mezcal making process and there are several programs to support the building of more viveros. It can be especially important in projects that are focused on how to semi cultivate wild agaves. By growing wild agaves from seed or hijuelo in a protected environment when they are very young (starts), they have a much greater chance of reaching maturity when transplanted out into the fields or countryside.