Rooftop farming is a promising new trend to bring healthy and cheap food to urban areas. This type of farming all but removes soil from the equation, with each plant housed in plastic containers hovering above a tank, their stringy roots dangling into a circulating pool of nutrient-boosted tap water. A recent paper reports that growing leafy greens in rooftop hydroponic systems can not only produce a steady supply of vegetables—it can also be cheaper than buying store-bought alternatives.