How to Grow and Care for African Milkweed

The African milkweed, known scientifically as Gomphocarpus physocarpus, is a plant that captures attention in any garden. It goes by several common names, including balloon plant and hairy balls, that allude to its most distinctive feature. While it produces clusters of small, white flowers, the main ornamental appeal comes from the subsequent seed pods. These pods are large, inflated, and spherical, covered in soft, bristle-like hairs.

Role in the Garden

This plant’s primary ecological function is serving as a host for monarch butterfly caterpillars. Adult monarch butterflies seek out this plant to lay their eggs, and the emerging caterpillars feed exclusively on its leaves. This food source is necessary for the larval stage, allowing the caterpillars to grow and eventually transform into chrysalises.

As a non-native species from South Africa, its use in North American gardens requires some consideration. One concern is its potential to host a protozoan parasite called Ophryocystis elektroscirrha (OE). This parasite can accumulate on the plant’s surfaces and infect monarch caterpillars that ingest it, leading to weakened adult butterflies with reduced lifespans and flying ability.

In warmer climates where Gomphocarpus physocarpus does not die back from frost, it can persist year-round. This availability may encourage monarch butterflies to overwinter in areas outside their natural migration routes, potentially disrupting their migratory patterns. Continuous breeding on these plants in southern regions can also lead to a greater buildup of OE spores, increasing the infection rate among local monarch populations.

Growing and Care Requirements

To cultivate African milkweed successfully is straightforward as it is not a demanding plant. It thrives in full sun, requiring at least six hours of direct sunlight daily to produce strong stems and abundant seed pods.

The plant prefers well-draining soil and is adaptable to different soil types, including sandy and loamy conditions. While it appreciates consistent moisture when first establishing its root system, it becomes quite drought-tolerant once mature. It is typically grown as a fast-growing annual in colder climates but behaves as a tender perennial in USDA zones 8 through 10. African milkweed generally does not require supplemental fertilizer.

Propagation Methods

Propagating African milkweed is most commonly done from seed. The seeds develop inside the large, puffy pods, and each seed is attached to a tuft of silky floss that aids in wind dispersal. To collect them, wait until the pods have dried on the plant and are just beginning to split open. It is helpful to place a small bag over the ripening pods to catch the seeds.

For planting, you can start the seeds indoors about six to eight weeks before the last expected frost date. Alternatively, seeds can be sown directly into the garden soil after all danger of frost has passed. Plant the seeds shallowly, about 1/4 inch deep.

Potential Garden Issues

One of the most common pests to affect African milkweed is the oleander aphid, a small, bright yellow insect that congregates in dense clusters on the stems and leaves. While they can be unsightly, they rarely cause significant harm to a healthy, established plant. A strong jet of water from a hose can be effective at dislodging them.

In regions with long growing seasons, Gomphocarpus physocarpus can self-seed readily and may become weedy if not managed. The wind-dispersed seeds can travel and sprout in other areas of the garden. Removing the seed pods before they split open is an effective way to control its spread.

It is important to handle this plant with awareness, as all parts contain cardiac glycosides, which are toxic if ingested. The milky white sap that exudes from cut stems or leaves can also cause skin irritation in some individuals. Plant it in areas away from the reach of curious children and pets.