Due to the regular occurrence of drought in southern Africa, there is need for more research on drought tolerant fodder plants such as Opuntia species. The influence of different water applications was evaluated in terms of root and cladode mass, root length, and water-use efficiency for one-year-old cactus pear plants of the species Opuntia ficus-indica (L.) Miller (Morado cultivar) and O. robusta Wendl. (Monterey cultivar). One-year-old cladodes were planted in pots (210 mm diameter × 550 mm deep soil) and grown in the greenhouse at day/night temperatures of 25–30/15–18°C. The water treatments applied were 0–25%, 25–50%, 50–75% and 75–100% depletion of total plant available water. Water-use efficiency (WUE) was defined as the cladode or root dry-mass production per unit of water used. Root mass decreased and root length increased significantly (p < 0.01) for both species with water stress. Due to the finer root system of O. robusta, the root mass was smaller than that of O. ficus-indica. In contrast, the root length/root mass ratio was higher for O. robusta than for O. ficus-indica. The influence of water stress on root die-back was clearly observed in O. ficus-indica but was less noticeable in O. robusta. The lateral roots per taproot increased significantly (p < 0.01) with water stress, with as many as 35 and 70 side roots per taproot for O. ficus-indica and O. robusta, respectively. On average, for all water treatments, the roots of O. ficus-indica and O. robusta composed only 12% and 10%, respectively, of the total plant biomass. In terms of root production, both species used water less efficiently with water stress, but if expressed in terms of cladode production, the WUE decreased for O. ficus-indica and increased for O. robusta with water stress. A water application of only 11 mm was enough to completely hydrate the cladodes of both species at the lowest water treatment. It is clear that O. ficus-indica is more sensitive to water stress than O. robusta. The unique root systems of these species make them adaptable under especially dry conditions.