Monarch butterflies lay their eggs on just one type of plant–milkweed. This exacting choice of a host plant gives their offspring an unlikely advantage when it comes to predators. The leaves and stems of a Milkweed plant have a milky-white sap that contains cardiac glycoside. This toxin makes the foliage taste bad and protects the milkweed plant from the ravages of most insects and foraging animals like deer.
Monarch caterpillars emerge from the tiny white eggs laid of the undersides of milkweed leaves and start eating their host. The caterpillars not only consume the milkweed's poisonous toxin, but they also adopt the presence of that same compound in their own bodies as a natural means of defence against predators like birds. The residue of the cardiac glycoside even goes on to protect the mature butterflies from predators. It's really a rather clever adaptation.
An adult Monarch sips nectar from Swamp Milkweed growing near the pond in the
backyard of garden writer David Hobson's home.
Swamp milkweed, Asclepias incarnata
Most gardeners are aware that
Monarch butterfly populations have plummeted in recent years. As with bees, there does not seem to be a simple explanation for this decline. There are actually a number of factors that can be blamed for the decreasing numbers of butterflies.
One of the biggest threats to Monarchs is the loss of their winter habitat. In the late summer, adult butterflies fly an amazing 22 miles a day to their winter refuge in the mountaintop forests of central Mexico. This unique ecosystem offers them a cool, moist environment that helps to slow their metabolism and conserve energy through the winter months.
Unfortunately, these forests of Oyamel firs have declined in recent years due to deforestation and climate change.
The Monarch's problems are not limited to their winter home. In Canada and the U.S., the widespread use of pesticides and herbicides in agricultural practices has had a negative impact on food sources. Drought and wildfires have also contributed to a
decline in the numbers of milkweed plants along a Monarch's migratory route north.
What can gardeners do to help?
In spring, Monarch butterflies leave their winter habitat and fly north in search of host plants on which to lay their eggs. Only one type of host plant will due– and that's milkweed. The eggs hatch into larvae and the larvae mature into the second generation of butterflies that carry on to complete the next leg of the journey north.
Finding milkweed along their migratory route is essential to Monarch butterflies both as a source of food for caterpillars and nectar for the mature adults.
By planting milkweed, gardeners can help ensure there's a ready source of food along with their route as the Monarchs make their journey as far north as Canada.
A Bee visiting a Common milkweed flower.
A Monarch sipping nectar from a Common milkweed flower.
Milkweed Basics
There are just
over one hundred of species of milkweed in the U.S. and the
Canadian Wildlife Federation website lists
14 types of milkweed can be found here in Canada.
Habitats in Canada and the U.S. vary. Milkweed can be found in fields, pastures, forests, on roadsides and even along wet shorelines. The foliage varies from thick, oval leaves to narrow, needle-like leaves.
Depending on the species, the foliage can be arranged on opposite sides down the stem or can be whirled around the stem. One thing the various species do have in common is a milky sap that gives the plant its common name.
Milkweed has round or flat clusters of flowers that range in color from greenish-white to pale pink and deep magenta.
Plant type: Perennial
Native habitat: Forms big colonies in fields, open woods, along roadsides and in meadows.
Height: 2-3 ft
Spread: .75-1 ft
Flower: Rose
Bloom period: Summer
Leaf: Thick, oval olive-green foliage
Light: Full sun
Moisture: Dry to average moisture. Drought tolerant once established.
Issues to consider: Spreads rapidly by rhizomes. Liberal self-seeder.
Problems: Aphids
Growing Milkweed
Ideally, you should grow whatever type of milkweed is native to your area. But before you plant anything, there are a couple of issues that you should be aware of with growing Milkweed.
Milkweed is not the most polite garden plant! Common Milkweed, Asclepias syriaca is, in fact, a thug that spreads aggressively via rhizomes. As well as spreading underground, it also produces seeds in great abundance.
Fortunately, there are types of milkweed that spread less aggressively.
Should you worry about the toxic nature of milkweed sap?
Milkweed sap contains cardiac glycoside to varying degrees (dependent on the species). Some plants are more toxic than others, but caution should be employed when handling them. It's a good idea to wear gardening gloves when pruning your milkweed plants. If you do get sap on your hands, wash them right away.
Milkweed is also toxic for dogs, cats, horses, cattle as well as children.
I don't want to overemphasize this issue. Truth be told, the average garden is full of plants that are more or less harmful if eaten. It's always wise to educate young children never to eat
mushrooms, berries or put any part of a plant in their mouth.
I consider myself lucky when it comes to my dogs. They don't like to dig and aren't interested in eating bulbs or plants. The only thing they have snacked on is the odd cherry tomato! If your cat or dog does love to nibble on plants in the yard, milkweed may not be the best choice of plants for your garden.
Finally, milkweed is bitter, so grazing animals are unlikely to eat it unless there is absolutely no other food for them to forage.
Swamp Milkweed growing near the pond in the garden writer David Hobson's backyard.
A photo I took last weekend in David Hobson's garden.
Asclepias incarnata- A Clump Forming Milkweed
Though I take a keen interest in helping Monarch populations increase, I would never grow Common milkweed in my garden. It's just far too aggressive and I have enough problems with goutweed and lily-of-the-valley! The milkweed I grow is Swamp milkweed,
Asclepias incarnata. It too is a food and nectar source for Monarchs, but it is a clump-forming perennial.
Swamp milkweed is native to most of the U.S. and Eastern Canada. Though it is found in swamps and wet meadows, is adaptable to average, well-drained garden soil. This plant has a tall, slender, vase-like shape and pinkish-magenta flowers that have a faint vanilla fragrance. It
likes full sun, but I have also grown it in light shade. The one thing this plant does not like is over-fertilization which can inhibit flowering.
Swamp milkweed is easily grown from seed. Once mature, it has a deep taproot that makes it difficult to move.
When ripe milkweed seed pods split open revealing tightly packed brown seeds
with a tail of silky, white fluff.
Swamp milkweed does have one drawback. It can be a rather prolific self-seeder. To combat the problem of nuisance seedlings, I make a point of going around the garden and deadheading the flowers.
One last thing to mention-
Swamp milkweed is quite slow to emerge in spring. It's a good idea to mark its location when you first plant it.
Plant type: Clump-forming perennial
Native habitat: Swamps and wet meadows
Height: 4-6 ft
Spread: 2-3
Flower: Pinkish-magenta
Bloom period: Summer
Leaf: Narrow, lance-shaped green leaves
Light: Full sun to light shade
Moisture: Dry to average moisture.
Issues to consider: Liberal self-seeder.
Problems: Aphids
USDA zones: 3-9
Asclepias incarnata 'Ice Ballet' is a white cultivar. It is slightly shorter than the
native species and is also clump-forming.
Asclepias incarnata 'Ice Ballet' has white flowers with a light vanilla scent. It is both a food and nectar source for Monarch butterflies. Full sun or light shade. Moist to average growing conditions. Height: 3-4 ft, Spread: 1.5-2 ft. USDA zones 3-9.
I know this post is getting long, but I wanted to make a quick mention of a milkweed cousin.
Butterfly weed, Asclepias tuberosa can also be a food and nectar source for Monarchs. It is easy to grow in
poor to average, well-drained soil. Butterfly weed can be grown from seed, but plants are somewhat
slow to get established.
Unlike milkweed, this plant does not have a milky sap. It is still harmful if eaten, however.
Bright orange flowers mature into long seedpods. Be warned–like milkweed, Butterfly weed can be a prolific self-seeder. To ward off issues, remove the seed heads before they open.
Asclepias tuberosa 'Gay Butterflies' has clusters of yellow, gold, orange and scarlet flowers. Asclepias tuberosa 'Hello Yellow' has showy golden-yellow flowers.
Plant type: Perennial
Native Habitat: Swamps and wet meadows
Height: 1-2 ft
Spread: 1-1.5 ft
Flower: Orange
Bloom period: Summer
Leaf: Narrow, lance-shaped green leaves
Light: Full sun
Moisture: Dry
Issues to consider: Liberal self-seeder.
Problems: Aphids
USDA zones: 4-9
Monarchs at the Toronto Botanical Garden.
At one time Monarchs numbering in the thousands could be seen along the north shores of Lake Ontario and Lake Erie.
Monarch visiting my favourite Bee Balm, Monarda that is just beginning to flower.
These days large numbers of butterflies are not a common sight, but I do feel encouraged by larger numbers of Monarchs in my garden this summer.
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References, Links and Further Reading:
Canadian Wildlife Federation's information on milkweed
National Wildlife Federation (U.S.)- 12 native milkweeds for Monarchs
Mission Monarch- An important part of Mission Monarch is to identify and map milkweed in Canada. There are about a dozen species of milkweed in Canada, in nine of the ten provinces.
Milkweed Watch- is a program asks members of the public to assist researchers and citizen groups concerned with the health of monarch butterfly populations by identifying the location of milkweed plants, which are crucial for monarch reproduction in Canada.
Monarchbutterflies.ca - The objective of this group is to educate the public of their fragile life cycle and to raise healthy, beautiful butterflies. As a proud member of the International Butterfly Breeders Association (IBBA), we are professional monarch butterfly breeders that commit to ethical and honest practices. IBBA members must commit to raising butterflies according to legal regulations and shipping with proper permits. Whether you are purchasing these butterflies for an event or educational purposes; the release of every butterfly helps bring back the population. Our promise is to provide you with the highest quality, hand raised and disease free butterflies.
Monarch Joint Venture is a national partnership of federal and state agencies, non-governmental organizations, businesses and academic programs working together to conserve the monarch butterfly migration.
Monarch Watch.org- Education, conservation and research.
Journey North- Journey North provides an easy entry point to citizen science, with simple protocols, strong online support, and immediate results. Reported sightings are mapped in real-time as waves of migrations move across the continent. People report sightings from the field, view maps, take pictures and leave comments.
USA Today-The effect of drought and wildfires in Texas
Yale School of Forestry & Environmental Studies- Read about the plant to save the sacred Oyamel Firs.
