Zone: 3 to 8
Soil: sand to clay - see notes
Light: Full sun to part sun
Bloom colour: White, purple or orange
Bloom period: June-August
Height: 2 to 5 feet
Moisture:Dry or moist - see notes
Attracts: Many butterflies and bees.
Notes: Milkweeds are fascinating plants that are well known for being a host plant for Monarchs. There are actually quite a few insects that specifically use these plants for food. There are also many butterflies and bees for which milkweeds are a major nectar source. These plants produce a milky sap that may contain significant concentrations of cardiac glycosides. The Monarch butterfly ingests these chemicals and consequently becomes toxic as well. All stages of the Monarch's life cycle are toxic including the eggs and this is rather unusual. When a Monarch is eaten by a bird, the bird throws up and is put out of action for a while when it could be building up fat supplies for its migration or to survive the winter. The long-term decline of the Monarch butterfly populations east of the rockies is thought to be connected to the declining milkweed population in the mid-western United States. Additional milkweed in Canada and stronger enforcement of logging in the overwintering areas in Mexico would also certainly help.
The individual flowers look different to a regular flower because the stamens are fused together to form the central part of the flower. Around this, you can see five cup-like structures that are called hoods. Insects have to reach into the hoods to get access to the nectar and in so doing, they may place their feet onto the reproductive parts and effect pollination. While most plants produce individual pollen grains that attach to the insect, milkweeds produce a pair of pollinia, which are sacs containing the pollen. The pairs of pollinia are often visible on a pollinator's leg.
Most species are great wildlife plants attracting a range of bees and beneficial wasps. In general, they are slow to emerge after the winter and could well be the last plant you see starting to grow in your garden. They grow quickly and typically flower in the early part of the summer. If you cut the dead flowerheads off, you may well be able to get them to rebloom. Butterfly weed, in particular, will rebloom when the seed heads have not been able to develop as this increases seed production. Milkweed reseed quite easily; just pat the seeds into the ground in the fall. All the species that I have grown require some sunshine in order to start growing. Do not plant any in total shade.
The members of this genus are often subject to attack by Milkweed Aphids. You can clean them off or if you are more patient, then you can wait for ladybird and hoverfly larvae to clean them off for you. This latter option is better for ecological reasons and the plants will still do fairly well.
The most commonly seen member of this species in open areas is Asclepias syriaca. This is the common milkweed that you see growing in fields. It is not really suitable for gardens because it has a rhizomatous root system that allows it to outcompete other plants. What may look like a bunch of separate plants growing together is more than likely to be a single individual. If you have a larger garden and can dedicate some space to these plants combined with some grasses like Little Bluestem, then you will get to enjoy all the wildlife that these plants attract.
The best garden species of milkweeds are those that clump instead of spreading via rhizomatous root systems. Swamp milkweed and butterflyweed are clumping. Pokeweed does not seem to spread either.
For dry sandy soils, grow Asclepias tuberosa - butterfly weed. It develops a long taproot that helps it survive drought, but this also makes it difficult (but not impossible) to transplant. Do not be fooled by the name. The other species of milkweed mentioned here are just as effective at attracting butterflies. Butterflyweed is commonly available in nurseries. If you do not see it on sale, try asking for a common cultivar called Hello Yellow, which is a selection of the wild plant. It makes a lovely flower combination with regular orange butterfly weed and it is just as attractive to pollinators. This species makes a good companion plant in front of purple milkweed. The unusually strong colour of this flower and the coverage of the bloom makes this one of the most striking plants available, but it will not look its best if grown in clay. If you cut off the flowerheads as soon as they are done, you will get a second bloom in late summer.
Swamp milkweed (Asclepias incarnata) is a fantastic plant for either informal or formal gardens. Its profuse pink flowers come out in mid-June before many others appear and remain for a good six weeks. Their stems are strong and their leaves are ornamental in looks. These plants are covered in bees and beneficial wasps; they deserve to be planted a lot more often. This plant grows to around 48 inches whereas Asclepias tuberosa is typically two feet in height. This species grows well in ordinary garden soil or clay and tolerates some drought despite its name. If you only have room for one species of milkweed, then choose this one. It is also the most popular species of milkweed in my garden for Monarch butterflies that are laying eggs. Swamp milkweed naturally grows in swales and in interdune environments where drought follows flooding. It is quite adaptable in a garden.
In confined gardens with plenty of sun, you could also try Purple Milkweed in dry sandy soils or Sullivant's Milkweed in mesic/dry clay soils. Purple milkweed grows to about 3 ft high and is one of the most fragrant milkweeds in Ontario. It takes three years to flower from seed and three weeks to finish blooming, so be patient. In Ontario, it is a rare plant. These plants spread moderately through rhizomes.
Whorled milkweed is sometimes available. It is a little less showy than other species and seems to attract less wildlife. However, its fine leaves and smaller stature makes it a nice addition to a wildlife garden. It flowers a little later than other milkweeds in August. It does well in dry sandy soil and is best grown en masse. It grows about 2 feet high. It is an excellent milkweed at the front of the border and only spreads moderately through rhizomes.
In Canada, Bloodroot (or Tropical Milkweed) can be grown as an annual that is about 3 - 4 feet high. It will grow in average garden soil with a decent amount of sunshine. The flowers are notable because of the red/yellow bicolouration. This is not a plant that should be grown anywhere in the southern US as research has shown that the plant becomes a vector of parasites that affect migrating Monarchs if it can overwinter.
Poke milkweed is more shade tolerant than other species but it will still need some sunshine. Aa well drained garden soil with some watering will provide excellent growing conditions. The flowers are not tightly clustered like other milkweeds. Instead they grow in loose umbels and hang downwards.
The Canadian species include:
Asclepias exaltata - Poke milkweed
Asclepias incarnata - Swamp milkweed
Asclepias ovalifolia - Dwarf milkweed
Asclepias purpurascens - Purple milkweed
Asclepias speciosa* - Showy milkweed
Asclepias sullivantii - Prairie milkweed
Asclepias syriaca - Common milkweed
Asclepias tuberosa - Butterfly milkweed
Asclepias variegata - White milkweed
Asclepias verticillata - Whorled milkweed
Asclepias viridiflora - Green milkweed