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milk weed pod seeds transparent

A smooth, clump-free, weeded soil bed will virtually guarantee a successful start for germination and seedling establishment. If vegetation exists in the future habitat location, it can be removed by using a tiller or by hoeing the area. To reduce clumping, do not work the soil when it is wet. The soil should be worked to a fine consistency to ensure good soil to seed contact.

Soil Types
If you have a choice, light soils are better than those with heavy clay. Well-drained soils are generally best but there are some species, e.g. A. incarnata (swamp milkweed) and A. sullivantii, which do well in saturated conditions.

Milkweeds can be propagated from seeds, cuttings, and, in some cases, from root divisions. This account will deal with storage, treatment and planting of milkweeds seeds and will briefly touch on propagation from cuttings.

Where to Plant
Most milkweed species evolved in open areas where they were exposed to full sunlight and they will do best if they are planted in the sunniest areas of your gardens. A few species, such as A. purpurascens, appear to require partial shade.

When small seeds are sown, they are often mixed with sand or fine soil to have better seed distribution. However, this method does not completely prevent crowding of seedlings and thinning will be necessary. Thinning provides more space between plants, increasing the amount of light reaching the plants and the air circulation around them. Seedlings may need to be thinned several times beginning 1-2 weeks after germination. Without proper thinning, you will end up with weaker plants.

Growing Milkweeds from Cuttings
All milkweeds are perennials and some can be grown from cuttings. Cuttings provide a way producing new plants in a relatively short time and it avoids some of the difficulties of starting plants from seeds. To start cuttings, cut the stems underwater, then coat the bottom of the stem with a strong rooting hormone. The stems should be placed in sand, vermiculite, or potting soil that is kept continuously moist. Cuttings can usually be transplanted in 6-10 weeks. Survival is best when cuttings are made from green stems (1/3 inch diameter) obtained from plants fertilized two weeks earlier.

After the seeds are sown in the flats, cover each flat with a clear plastic cover or a plastic bag to keep the seeds from drying out while germinating. Then, place the flat under grow lights, in a warm sunny window, or in a greenhouse. Most seeds will germinate in 7-10 days if the flats are maintained at 75˚F. After the seeds have germinated, remove the plastic covering from the flats. Once the seedlings have emerged, the soil should be kept moist by watering the flat from the bottom. You can water from the bottom by placing the flat in a sink or a larger flat filled with 2 inches of water until moisture appears on the soil surface. The soil should be kept moist but some care is needed to keep the seedlings from getting too wet – such conditions contribute to fungal growth that can kill the young seedlings (“damping off”). Thinning (see below) can reduce damping off.

Gomphocarpus physocarpus
also known as Swan Plant
Source: KENPIE reproduced under
Creative Commons

The terminology for this plant is a little confusing. It is a native milkweed of South Africa and in 2001 its name was changed from Asclepias physocarpa to Gomphocarpus physocarpus to reflect that it is in the family of African milkweeds. Now its true scientific name is Gomphocarpus physocarpus but many people still know it and refer to it as Asclepias physocarpa.

Why Grow Gomphocarpus physocarpus?

Since the Balloon plant is native to South Africa, it is only a perennial in USDA Zones 8-10 (some say 9-10). It can be weedy in tropical areas. In the cooler zones it is grown as an annual. It is fast growing but gets large so this is one you may want to start indoors before the last frost or buy an already growing plant. This milkweed will get about 4-6 feet tall and about 3 feet wide.

Balloon plant seeds can be collected in the fall for replanting in the spring and for sharing with friends, family, or butterfly projects. To collect the seed, let the pod dry and turn rosy-tan colored so the seed can mature. Before it opens, or right as it starts to open, break it the rest of the way and strip the seeds from the white “silks”. The seeds should be brown/black and dry looking. Dry completely before storing.

The other source of confusion is that it is very similar in appearance to Asclepias fruticosa (which has also been renamed to Gomphocarpus fruticosus). The difference is that G. fruticosus is a smaller plant than G. physocarpus and the seed pod is shaped a little different. For G. fruticosus, the seed pod is slightly oblong and can resemble a swan which is where the name “Swan plant” came from. G. physocarpus has a seed pod that is more rounded and thus became known as “Balloon plant”. Now the two common names of “swan” and “balloon” plant are interchanged between both species.