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

Description and botany

The Asclepias are perennial plants, herbaceous or shrubby, gathering more than 200 species. They are mainly found in North and South America, as well as in Africa. One species, Asclepias syriaca, native to North America, has naturalized in several regions of France. In the wild, A. syriaca and A. tuberosa grow mainly in open habitats, on dry, poor soils, in meadows, beside roads, on uncultivated ground, fallow land, wasteland, etc. Asclepias incarnata, by contrast, is more often found on moist soils, in marshes or along watercourses…

Asclepias formerly had their own family, the Asclepiadaceae, but they are now classified 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.

The genus was named by Linnaeus in reference to Asclepios, the Greek god of medicine. In English, Asclepias are commonly called “milkweed” (likely a reference to the white latex in their tissues).

The name Asclepias syriaca is synonymous with Asclepias cornuti. It is surprising that it was called “syriaca”, meaning “from Syria”, although it originates from North America — probably a confusion with another species. This milkweed also has several vernacular names: it is called “Parrot-weed” because its fruits recall the shape of these birds, and “cottonweed” in reference to its seeds bearing a large silky egret. It is also known as “Herbe à gendarme”.

Asclepias curassavica: botanical illustration

Despite its name, gentian asclepiadea (Gentiana asclepiadea) has no close relationship with Asclepias; it was named for its leaves, which resemble those of milkweed.

Asclepias are vigorous, fast-growing plants. They form fairly dense clumps of long erect stems topped by the flowers. These are large perennials, reaching between 80 cm and 2 m in height. Asclepias tuberosa is somewhat shorter, at 50 cm to 1 m. Asclepias syriaca tends to spread laterally and can form large colonies. It produces underground suckers and can become invasive.

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

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

These may take soft hues: pale pink (Asclepias incarnata), mauve, white (Asclepias ‘Ice Ballet’)… or, conversely, be bright orange (as in Asclepias tuberosa), yellow, or red. Visual effect differs: the former integrate well into naturalistic borders or romantic cottage gardens… while the latter are better suited to exotic borders or very colourful mixed borders. Asclepias curassavica offers a lovely gradation from red through orange to yellow. There are also yellow-flowered varieties such as Asclepias ‘Hello Yellow’.

Asclepias flowers are very specific, organised in a quite unique way. Each consists of five petals reflexed backwards and five sepals. Each flower bears a crown made of five erect concave membranes that might be mistaken for petals, each carrying an incurved horn. At the centre sits a gynostemium: a column formed by fused stamens and pistils.

Pollen is not released as individual grains, as in most plants, but gathered into masses called pollinia. When insects seek nectar, they slip a leg or rostrum into one of the five slits formed by adjacent anthers. Pollinia then attach mechanically to the insect. Only the strongest insects can carry them away; weaker ones risk becoming trapped in the stigma slits, turning the flower into a deadly trap.

Generally, Asclepias flowers are scented (notably A. syriaca and A. incarnata), emitting a pleasant vanilla fragrance.

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

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

Leaves of Asclepias are simple, undivided, with an ordinary shape, lanceolate or ovate. They range from 7 to 25 cm long and vary in width 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 in a spiralled fashion (A. tuberosa). Leaves are green with a prominent white vein down the centre.

Asclepias syriaca has running, thick rootstocks that produce suckers. These horizontal underground stems give rise to new shoots, explaining how this species can quickly form large colonies.

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

After flowering, Asclepias produce fairly large fruits measuring between 6 and 15 cm long. Fruits can be elongated, oblong or more globose, usually tapered at the tip. They are not smooth but covered with asperities or soft thorns. Fruits are particularly long and narrow in A. tuberosa. Those of A. syriaca are much appreciated because they resemble little parrots, which can, for example, be hung on the rim of a glass. They are highly decorative.

When ripe, fruits open and release small flat brown seeds bearing large silky egret tufts that allow them to fly! It is therefore the wind that disperses Asclepias seeds. The silky hairs can even be used to produce a high-quality, hydrophobic, strong, insulating fibre.

Fruits of Asclepias syriaca, whole then releasing seeds with silky egret tufts

Main milkweed varieties

Most popular varieties

Our favourite varieties

Asclepias syriaca ou cornuti - Herbe aux perruches

This milkweed offers tender, soft-pink flowering. It is also known as 'parakeet herb' because of its decorative fruits resembling parakeets. It tends to spread laterally thanks to its running rootstock.

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

See the product sheet

Asclepias tuberosa - Milkweed

This species is appreciated for its bright orange flowers. It brings vitality and dynamism to borders and can easily be incorporated 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 tender pink–mauve flowering and fairly fine green foliage. Unlike most species, it likes moist soils. It integrates 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 lovely white inflorescences in summer. Its delicate, elegant flowering makes it suitable for cottage 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 valued for its bright, vivid flowering in beautiful yellow and red shades, sometimes orangey. 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 are plants that enjoy heat and full sun, essential to ensure generous flowering. Choose an open spot 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 drains 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 do well in moist ground. You can plant them at the edge of a pond. Overall, milkweeds are not very sensitive to pH. However, it is preferable to site them in a spot sheltered from strong winds.

You can also grow milkweeds in a pot. This allows you to move the more tender ones under cover and to contain the 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 rootstock barrier to limit its development.

Choose the planting location carefully, because once established milkweeds do not like being moved. Their root system, which penetrates deeply into soil, could be damaged by transplanting.

When to plant?

Plant milkweeds preferably in spring, from May onwards, once frosts are no longer a risk.

How to plant?

If planting several plants, leave 40 to 60 cm between them, depending on variety.

1. Place the 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 drainage material if your soil tends to retain water (except for Asclepias incarnata). If you grow Asclepias syriaca, you can install a rootstock barrier to prevent it spreading too much.
3. Place the rootball in the planting hole.
4. Backfill with soil.
5. Water thoroughly.

Continue watering regularly until the plant establishes and resumes growth.

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

Asclepias tuberosa

Care

Milkweeds are easy plants that thrive (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, water occasionally in summer to support flowering. Asclepias incarnata needs slightly fresher soil than other species, and will therefore require more regular watering.

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

We recommend pruning milkweeds in autumn, around November, by cutting foliage back to near ground level.

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

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

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

Propagation

To propagate milkweeds, the best technique seems to be sowing. It is also possible to divide clumps, to take stem cuttings, or, for Asclepias syriaca, to propagate rootstocks.

Sowing

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

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

Remember to water regularly after sowing, and during the first year of cultivation. Pot on young seedlings as soon as they reach a size that allows handling.

Dividing clumps

You can divide milkweeds in late autumn, once the plant has entered dormancy. Start by carefully lifting a clump of milkweeds, then split it into several pieces using a spade. Replant each piece in a new location, then water well.

Companion planting milkweed in the garden

Because their flowers are appreciated by pollinating insects, you can incorporate Asclepias into a garden favourable to biodiversity, alongside other melliferous young 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.

Species with soft pink flowers, such as Asclepias syriaca, fit very well into naturalistic gardens. It offers gentle flowering and has a wild habit that integrates easily into a landscape. Plant it with airy, freely growing plants such as Verbena bonariensis, Gaura lindheimeri, Veronicastrum, or willowherbs… In this type of garden you can also choose Asclepias incarnata, especially if your soil is cool or damp.

Pink- or mauve‑tinged 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. Pair them with plants that have bright, dynamic, warm colours. Consider kniphofias, daylilies, crocosmias or eucomis. Add some decorative foliage, such as Phormium, Imperata cylindrica ‘Red Baron’, or ricinus.

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

Vegetate pond edges or a body of water with Asclepias incarnata. Plant it with meadowsweet, purple loosestrife, willowherbs, ligularias, Cirsium rivulare, Eupatorium… This Asclepias has the advantage of offering very natural‑looking flowering and foliage, making it possible to create a rather wild area composed of vegetation that appears spontaneous, as one might find naturally at the water’s edge.

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

Did you know?

• Milkweeds and monarchs

In North America, the magnificent monarch butterfly, which undertakes long migrations in numbers reaching millions, depends on milkweeds for its life cycle. Indeed, its larvae feed almost exclusively on milkweed leaves. And although the plant is toxic to most animals, monarchs are able to eat the leaves and sequester the toxic substances, which in turn makes them unpalatable to predators. The butterfly has exploited the same toxicity that protects the plant from herbivores.

• Uses

Silky egrets attached to the seeds of Asclepias syriaca can be used to obtain a high-quality fibre: hydrophobic, strong, resistant and insulating. Asclepias syriaca is cultivated in Canada for these qualities, which is why this plant is sometimes called “Quebec 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 following accidental oil spills.