<em>Asclepias</em>: sow, plant and care for

Description and botany

Asclepias are perennial, herbaceous or shrubby plants, comprising more than 200 species. Found mainly in North and South America, and also in Africa. One species, Asclepias syriaca, native to North America, has naturalised in several regions of France. In the wild, Asclepias syriaca and A. tuberosa grow mainly in open habitats on dry, poor soils, in meadows, by roadsides, uncultivated places, fallow land, wastelands, etc. Asclepias incarnata, by contrast, is found on wetter soils, in marshes, or beside watercourses…

Asclepias formerly had their own family, the Asclepiadaceae, but they are now classed within the Apocynaceae. This large family (more than 5,000 species) includes some ornamental plants such as oleander, periwinkle and star jasmine (Trachelospermum jasminoides). Many species in this group originate from tropical or subtropical regions.

Milkweed was named by Linnaeus in reference to Asclepios, Greek god of medicine. In English, asclepias are known as “milkweed” (“milky herb”, probably a reference to white latex present in their tissues).

Asclepias syriaca is synonymous with Asclepias cornuti. It is surprising that it was called “syriaca”, meaning “of Syria”, whereas it is native to North America — probably a confusion with another species. This milkweed also has several vernacular names: it is called “parrot-weed”, because its fruits recall shape of these birds; “cottonweed”, in reference to seeds bearing a large silky egret; it is also called “gendarme-weed”.

Asclepias curassavica: botanical illustration

Despite its name, gentian asclepiade (Gentiana asclepiadea) has no relation to milkweeds, but was named so because its leaves resemble those of Asclepias.

Asclepias are vigorous plants that grow quickly. They form fairly dense clumps made up of long, upright stems, topped by flowers. These are large perennials, reaching between 80 cm and 2 m in height. Asclepias tuberosa tends to be a little shorter, at between 50 cm and 1 m. Asclepias syriaca has a tendency to spread and can form large colonies. It produces underground suckers and can become invasive.

Like many Apocynaceae, milkweeds contain a whitish latex in their tissues (toxic to most animals), visible when a stem or leaf is broken.

Milkweeds flower in summer, between June and September depending on species. Their flowers are small, but gathered in umbels measuring between 10 and 25 cm in diameter. Each inflorescence consists of a large number of star-shaped flowers.

These flowers can take soft tones: pale pink (Asclepias incarnata), mauve, white (Asclepias ‘Ice Ballet’)… or, conversely, be vivid orange (as in Asclepias tuberosa), yellow or red. Visual effect differs entirely: the former blend well into naturalistic beds, romantic or cottage gardens… while the latter find their place in exotic-style beds or very colourful mixed-borders. Asclepias curassavica offers a beautiful colour gradient from red to orange and yellow. There are also varieties with yellow flowers, such as Asclepias ‘Hello Yellow’.

Milkweed flowers are highly specific, organised in a rather unique way. They consist of five petals reflexed backwards, and five sepals. Each flower bears a crown made of five upright, concave membranes, which could be mistaken for petals, each supporting a curved horn. At the centre is a gynostemium: a column formed by fused stamens and pistils.

Pollen is not in individual grains, as in most plants, but gathered in masses called pollinia. When insects come for nectar, they slide a leg or rostrum into one of the five slots formed by adjacent anthers. Pollinia then attach mechanically to the insect. They can only be carried by the strongest insects; weaker ones risk becoming trapped in the stigma slots, turning flower into a deadly trap.

Generally, milkweed flowers are fragrant (notably Asclepias syriaca and A. incarnata), exhaling a pleasant vanilla-like scent.

Milkweeds are good melliferous plants, producing nectar much appreciated by insects (bees, wasps, butterflies…). Asclepias play an important ecological role, feeding many insects… And in North America, they are exclusive food source for caterpillars of the magnificent Monarch butterfly.

Flowering of Asclepias curassavica (photo Derek Ramsey), Asclepias tuberosa, and Asclepias sullivantii (photo Frank Mayfield)

Milkweed leaves are simple, undivided, with a very ordinary form, lanceolate or ovate. They measure between 7 and 25 cm in length and are more or less narrow depending on species. Asclepias fascicularis, for example, has a very narrow lamina. Many species have opposite leaves (A. curassavica, A. syriaca…), but they can also be alternate or arranged spiralled (A. tuberosa). Leaves are green and marked by a prominent white vein down the centre.

Asclepias syriaca possesses thick, running rootstock and is suckering. These underground stems grow horizontally and produce new shoots emerging from them. This explains how this plant can quickly form large colonies.

As its name suggests, Asclepias tuberosa has tuberous roots, making it more drought-resistant. Asclepias incarnata, meanwhile, has thick white roots specially adapted to wet soils low in oxygen.

After flowering, milkweeds produce quite large fruits measuring between 6 and 15 cm in length. They can be elongated, oblong or more globose, and are generally tapered at the tip. They are not smooth but covered with bumps or soft thorns. Fruits are particularly long and thin in Asclepias tuberosa. Those of Asclepias syriaca are much appreciated because they resemble parakeets, which can be hung, for example, on rim of a glass. They are very decorative.

When ripe, fruits split open and release small, flat brown seeds. These seeds bear large, silky egrets, white in colour, which allow them to fly away! It is therefore wind that disperses milkweed seeds. Silks can even be used to produce a very high-quality fibre: hydrophobic, strong and insulating.

Fruits of Asclepias syriaca, whole then releasing seeds with silky egrets

Main milkweed varieties

Most popular varieties

Our favourite varieties

Asclepias syriaca ou cornuti - Herbe aux perruches

This milkweed offers a soft, pale pink flowering. It is also known as 'parrot plant' because of its decorative fruits resembling parakeets. It tends to spread laterally via running rootstock.

• Flowering time July to September
• Height at maturity 1,20 m

See the product sheet

Asclepias tuberosa - Milkweed

This species is prized for its vivid orange flowers. It brings vitality and dynamism to borders and easily fits into an exotic-style garden. It grows well in dry, poor soils.

• Flowering time August to October
• Height at maturity 60 cm

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Asclepias incarnata - Milkweed

This milkweed offers a soft pink–mauve flowering and fairly fine green foliage. Unlike most species, it prefers moist soils. It fits easily into naturalistic gardens.

• Flowering time July to September
• Height at maturity 1,20 m

See the product sheet

Asclepias incarnata Ice Ballet - Milkweed

This variety bears beautiful white inflorescences in summer. Its delicate, elegant flowering makes it suitable for cottage gardens or romantic gardens. You can also plant it at the edge of a pond.

• Flowering time July to September
• Height at maturity 1,20 m

See the product sheet

Asclepias curassavica - Milkweed

A species prized for its bright, vivid flowering in lovely yellow and red shades, sometimes tinged with orange. It has an attractive exotic, tropical look. Its only drawback is that it is not very hardy: it will need protection from cold in winter.

• Flowering time July to November
• Height at maturity 90 cm

See the product sheet

Planting

Where to plant?

Milkweeds prefer heat and full sun, essential for generous flowering. Choose a well-exposed location with excellent light. If you live in southern France, you can place them in partial shade.

They need free-draining soil, where water does not stagnate but soaks away quickly. Asclepias syriaca and Asclepias tuberosa prefer rather dry, poor soils. Do not hesitate to add some coarse sand before planting. Conversely, some species such as Asclepias incarnata and Asclepias speciosa enjoy moist soil. You can plant them at the edge of a pond. Overall, milkweeds are fairly tolerant of pH. However, it is preferable to site them in a spot sheltered from strong winds.

You can also grow milkweeds in pots. This allows bringing less hardy specimens under cover and containing spread of a species such as Asclepias syriaca.

Be cautious with Asclepias syriaca, which has already naturalised in several French regions. You can plant it in a pot, or install a barrier against rootstocks to limit its spread.

It is preferable to choose planting site carefully, because once established milkweeds do not like being moved. Their root system, which penetrates deeply into the soil, could be damaged by transplanting.

When to plant?

Plant milkweeds preferably in spring, from May onwards, once frost risk has passed.

How to plant?

If planting several specimens, allow 40–60 cm between plants, depending on variety.

1. Place plant in a tub filled with water to thoroughly moisten the rootball.
2. Dig a planting hole at least twice the size of the rootball. Add draining material if soil tends to retain water (except for Asclepias incarnata). If growing Asclepias syriaca, you can install a barrier against rootstocks to prevent excessive spread.
3. Place rootball in planting hole.
4. Backfill with soil.
5. Water thoroughly.

Continue to water regularly until plant establishes and resumes growth.

It is also possible to plant milkweeds in a pot or container. For this, choose a large, deep container (milkweeds have a well-developed root system). Add a drainage layer, then a light potting compost, and plant. Backfill around and water.

Asclepias tuberosa

Care

Milkweeds are easy plants that prosper (and sometimes spread) readily. Their care is limited to fairly basic tasks.

Once established, Asclepias syriaca and tuberosa tolerate drought fairly well. You can, however, carry out occasional waterings in summer to support flowering. Asclepias incarnata needs slightly cooler soil than other species and will therefore require more regular watering.

For Asclepias syriaca, you can leave faded flowerheads in place to allow the plant to produce unusual, decorative fruits shaped like parakeets; but harvest these fruits before they reach full ripeness to prevent them dispersing seeds – this species can become somewhat invasive. For other milkweeds, you can either leave faded flowerheads to obtain fruits and seeds, or cut them off (which relieves the plant and can prolong flowering).

We recommend pruning milkweeds in autumn, around November, cutting foliage back close to the ground.

Milkweeds, if exposed to wind, may need staking to prevent stems from flopping over.

If you grow less hardy species (such as Asclepias curassavica), consider protecting them in winter by bringing them under cover. If your milkweeds remain outdoors, even if relatively hardy, you can apply a thick mulch around the stump to protect it.

Milkweed is sometimes attacked by whiteflies, also called « mouches blanches ». These small insects can be seen flying up when leaves are disturbed. They weaken the plant by feeding on sap. You can treat them using black soap diluted in water. Overall, Asclepias have few enemies (at least in France; in North America their leaves are eaten by monarch caterpillars), because they are toxic to most animals!

Propagation

To propagate milkweeds, the best method appears to be sowing. It is also possible to divide clumps, to take stem cuttings, or, for Asclepias syriaca, to propagate from the rootstock.

Sowing

You can sow milkweeds under cover in late winter (February–March), or in early spring. It is best to place seeds in the refrigerator for 3 to 6 weeks before sowing (cold stratification).

1. Prepare a pot by filling it with a light potting compost (use a seed compost, or a mix of compost and sand). Firm and level the surface.
2. Place seeds on the surface.
3. Cover them with a thin layer of sieved compost.
4. Water with a fine spray.
5. Place the pot under cover, in a bright spot out of direct sun.

Remember to water regularly after sowing and during the first year of growth. Pot on young seedlings as soon as large enough to handle.

Dividing clumps

You can divide milkweeds in late autumn, once the plant has entered dormancy. Begin by carefully lifting a clump of milkweed, then divide it into several sections using a spade. Replant each section in a new location and water well.

Pairing milkweed in the garden

Because their flowers are appreciated by insect pollinators, you can include asclepias in a biodiversity-friendly garden alongside other melliferous plants. Choose, for example, scabious, sages, phacelia, Verbena bonariensis, nepeta, lavender… Create a very natural, wild-looking border, favouring flowers rich in nectar that visiting insects enjoy. In this type of garden, it is useful to stagger flowering over a long period so insects can feed throughout the year.

Soft-pink-flowered species, such as Asclepias syriaca, fit very well into naturalistic gardens. It offers a gentle flowering and has a wild habit that blends easily into the landscape. Plant it with plants of a free, airy habit, such as Verbena bonariensis, Gaura lindheimeri, Veronicastrum, or Epilobium… In this type of garden, you can also choose Asclepias incarnata, especially if soil is cool or damp.

Pink- or mauve-tinted asclepias can be included in naturalistic borders. Asclepias incarnata (photo Andrew C.), Gaura lindheimeri ‘Whirling Butterflies’, Pennisetum setaceum and Verbena bonariensis (photo Acabashi)

Conversely, orange-flowered asclepias, such as Asclepias tuberosa or Asclepias curassavica, will find their place in an exotic-style garden. To accompany them, favour blooms in bright, dynamic, warm tones. Choose kniphofias, daylilies, crocosmias, or eucomis. Add some decorative foliage, such as Phormium, Imperata cylindrica ‘Red Baron’, or castor bean.

Bright orange, yellow or red asclepias are better suited to exotic-style gardens. Asclepias tuberosa (photo Aaron Carlson), Phormium ‘Pink Panther’, Crocosmia and daylilies

Plant the edges of a pond or water feature with Asclepias incarnata. You can plant it with meadowsweet, loosestrife, Epilobium, ligularias, Cirsium rivulare, Eupatorium… This asclepias has the advantage of offering very natural-looking flowering and foliage, allowing creation of a rather wild area made up of vegetation that appears spontaneous, as one might find in nature at the edge of a stream.

→ Discover other ideas for pairing with Asclepias in our advice sheet!

Did you know?

• Milkweeds and monarchs

In North America, the magnificent monarch butterfly, which undertakes long migrations in its millions, depends on milkweeds for its life cycle. Its larvae feed almost exclusively on milkweed leaves. And although a plant toxic to most animals, monarchs can eat the leaves and sequester the toxic compounds, making them unpalatable to predators. The butterfly has thus taken advantage of the very same toxicity that protects the plant from herbivores.

• Uses

The silky egrets attached to the seeds of Asclepias syriaca can be used to obtain an excellent-quality fibre: hydrophobic, strong, durable and insulating. It is cultivated in Canada for these qualities, which is why this plant is sometimes called “Quebec’s silk”. It can be used, for example, to make warm clothing or to fill pillows. It is also notable for its ability to absorb hydrocarbons, but not water, and can therefore be used to clean up after accidental oil spills.