City dwellers can instantly add life to their digs with a visit to Niche, which offers a diverse selection of perennials, herbs, annuals and vegetables— not to mention pottery, soils and fertilizers—at a cozy shop in the South End and a larger space with a new outdoor garden in Cambridge. No green thumb? No problem. Niche takes a hands-on approach with design consultations, installations and monthly workshops covering topics like seeds, terrariums and air plants.