To ensure cross-pollination in Plumeria, plant multiple Plumeria varieties or species in close proximity to encourage natural cross-pollination. The presence of diverse pollinators, such as bees and butterflies, will increase the chances of pollen transfer between different flowers. Alternatively, use hand pollination techniques to manually transfer pollen between flowers of different varieties. Ensuring genetic diversity through cross-pollination can lead to healthier and more robust Plumeria offspring with unique traits and characteristics.