Frangipani plants don't produce seed every year nor do they produce seed after every flush of flowers. The flowers must receive successful pollination to produce seed; otherwise the flowers drop without forming a seed pod. The pollen-producing anthers don't always release the pollen, which leads to poor seed formation. Flowers produced in late summer or fall are more likely to experience successful pollination to produce seed. Hand pollination increases the chances of seed formation. Insert a cotton swab into the center of the flower completely, then twist it around to break the pollen free from the anthers. The flowers then self-pollinate and produce seed.
The seeds begin forming as soon as the flowers wilt on plants that experience successful pollination. The pods grow up to 7 inches in length and are green during the first few months after they begin to grow. The seeds take up to eight months to reach maturity from the time of pollination, so plants pollinated in fall produce seed in late spring or early summer. In the plant's native tropical climate, spring or summer flowers may also successfully set seeds that are ready for harvest in late fall or early winter.
The pods begin to split open once the seeds are fully mature and ready for harvest. The pods develop their full dark brown, nearly black color. Mature pods are hard and brittle when they split open. The pods each contain up to 60 seeds. The seeds have small wings and are also hard and dry once mature. Harvest the seeds only after the pods begin to open naturally but before they split completely open and spill the seed.
Cut or pluck the pods from the frangipani bush, taking care not to break the stem holding the pod. Holding a bowl beneath the pod as you cut it free with the other hand catches any seeds that spill out during harvest. Frangipani seeds only remain viable for a short time after harvest, so it's best to plant them immediately to ensure the best germination rate. The seeds may not produce flowers the same color as the parent plant. For example, a pink parent plant may produce a plant with blooms that are white, red or pink.