Gomphocarpus physocarpus and Gomphocarpus fruticosus
Gomphocarpus goes by many names, including hairy balls, monkey balls, family jewels, balloon plant, bladderbush, swan plant and cottonbush. Even the Latin names are confusing, since the species used to be known as Asclepias physocarpa and A. fruticosa, respectively. Regardless of what it’s called, Gomphocarpus has become a popular novelty annual for cut flower growers. G. fruticosus has even been a movie star, seen in Aeon Flux.
Overview of G. physocarpus
Gomphocarpus physocarpus is native to southern Africa, but is widely cultivated in the warmer regions of all continents. Gomphocarpus grows like a weed because it is, at least in many parts of the world, including Australia, Mexico and Central and South America. In these frost-free zones, the plants grow 8 feet tall and will be slightly woody at the base. In the U.S., plants range from 4 to 6’. Gomphocarpus is grown as an annual, although Seattle growers (USDA Hardiness Zone 8) report that it perennates.
Beginning in early summer, the plants produce umbels of 5 to12 creamy white, nodding flowers, and these will continue to be produced throughout the season. They are followed by the bladder-like seed pods, which are the real interest for growers and consumers alike. The pods, which are 2 to 3” in diameter, look like prickly, pale green balloons, since they are covered with short, rather stiff hairs. At the 2005 HortiFair in Amsterdam, we saw them used both in their natural green state and painted.
Propagation: Plants are easy to start from seed; any soilless mix will work. Cover seeds lightly with the mix, keep moist but not wet, and germination will occur in about 21 days. Germination percent can be increased and time decreased by starting seeds in wet paper. Presoaking seeds can also increase germination. Seedlings must be removed from plug trays on time, since the plants produce a large taproot and resent transplanting at later stages. Some growers have been successful with direct seeding.
Seed for next year’s crop can be gathered when the pods split open easily. There will be hundreds of seeds per pod. Remove the tassels before planting.
Spacing: The closest recommended spacing is 12” x 12”.
Growth habit: Gomphocarpus grows as a broom-like herb with a woody base. The stem is generally unbranched towards the base, but highly branched towards the top, where flowers are produced abundantly on the branches. Pinching should encourage low branching. Because of plant height and habit, netting is recommended.
Growing on: The plants grow well in almost any soil. In the wild, they even grow in poorly drained areas. Plant in full sun and irrigate for optimum pod production. Fertilizer is not necessary and may even decrease flowering.
Like other members of the Asclepias family, flowers do not self-pollinate and pollinators may be necessary. Honeybees and native bumblebees should be adequate.
Pests: As with all milkweeds, orange aphids and green aphids are potential pests. Milkweed bugs may infest pods.
Harvest and Postharvest
Harvest stage: The pods require about 4 months to mature and can be cut beginning in late summer. Cut pod-bearing branches at their origin from the main stem. Stems should be cut while the pods are green. There will be 4 to 6 pods per branch. Because the stems continue to flower throughout summer, the pods will be borne about 2/3 of the way up the stem, with another 12 to 24” of foliage and flowers above this. This top 1/3 is difficult to keep hydrated; Dutch growers cut it off.
After the first frost, pods will turn slightly red. Don’t wait too long to cut, though, since frost is also the trigger for pod opening.
Postharvest treatment: When cut, the stem exudes a milky latex sap, which causes an allergic reaction for many people. Wear gloves when harvesting, and inform your customers that the sap can cause dermatitis. For best results, cut stems early in the morning and place in buckets filled with water. Keep the stems as cool as possible. Recut into warm water and place in the cooler overnight.
Gomphocarpus fruticosus can also be used for seed pod production. Although the two species may be easily confused, G. physocarpus has larger, more rounded and ornamental seed pods than G. fruticosus. G. fruticosus produces the same pale green, prickly pods, but they are pointed and less symmetrically round. G. fruticosus branches abundantly from near the base of the plant yielding a more shrub-like form, reminiscent of a shrub willow. It may have a greater potential for pod production because these lower branches may become much longer than the upper branches of G. physocarpus, and should be cut at or near the base of the plant. All production and harvest procedures are the same as for G. physocarpus. The two species can hybridize.
Gomphocarpus physocarpus is available from Johnny’s Selected Seeds [sold as Asclepias physocarpus (Oscar)] and GeoSeed.