Milkweed Species for the Garden and Insectary

Over the past several months, Marjorie and I have shared the past summer’s success in attracting monarch butterflies with readers of my weekly Bangor Daily News column (http://gardeningintunewithnature.bangordailynews.com/2014/09/02/ecologicaly-functional-garden/a-monarch-summer) and with gardeners who came to hear us speak on our favorite topic: “Gardening with Insects, the Little Things that Run the Garden”.  Following in the wake of each event, we were always asked, “Where can I find milkweed seeds (or plants) for my garden?”  Often this question was followed with, “Which type of milkweed is best for attracting monarchs?”  This column is devoted primarily to the second of these questions, although you will find a list of sources for seeds and plants at the end of this column.

The gardener has a choice as to where to plant milkweeds.  They may be planted in beds and borders, their showy flowers as lovely as those of other herbaceous perennials, or they may be added to the insectary, that small patch of plants grown to attract beneficial insects, including pollinators and predators, by providing the pollen and nectar resources that they require. (See http://gardeningintunewithnature.bangordailynews.com/2014/07/22/ecologicaly-functional-garden/july-brings-major-changes-in-the-garden-insectary/ for more information on garden insectaries).  We have opted to grow our milkweeds in the garden proper.  True enough, once lepidopteran larvae and aphids have had their way, milkweeds are little more than naked stems topped by seed pods, but they make up for this in providing terrific caterpillar watching.

Other than the tropical milkweed (A. curassavica), milkweed species discussed below are perennial species native to eastern North America.  With the exception of butterfly weed (A. tubersosa), milkweed stems contain a milky sap that can be irritating to the skin and gardeners are advised to wear gloves when handling the plants.

Annual Tropical Milkweed

Monarch caterpillars munching on the annual tropical milkweed.

Monarch caterpillars munching on the annual tropical milkweed.

If there had been no plants of tropical milkweed (Asclepias curassavica) in Marjorie’s Garden last summer, it is doubtful that we would have seen a monarch.  Newly-planted swamp milkweeds, a perennial species, were too young, needing another year or two before they became butterfly magnets, so we sowed the tropical milkweed seeds indoors in early April, transplanted the seedlings to the garden in early-June, and had plants beginning to flower by the end of June, some in large tubs and some in garden beds.  (I’ve since learned that soaking the seeds in water for 24 hours before planting will speed up germination.)

On a warm early-July morning, while working in the garden, we watched a monarch soar over the deer fence and into the vegetable garden – the first monarch butterfly to ever visit Marjorie’s Garden.  Before the day was out there were two monarchs floating around the garden, twin orange and black beauties.  We never saw them touch down, yet two weeks later there were monarch caterpillars chewing on the leaves of the tropical milkweed.

Tropical milkweed (also called bloodflower) grows best in light, rich, evenly moist soil in full sun, although it does tolerate a bit of shade and temporary soil dryness.  Plants typically grow to three feet in height with upright stems bearing pointed, medium green leaves.  The showy flowers, red-orange with yellow hoods, are borne in rounded axillary clusters from late spring to early autumn.  In addition to monarchs, other butterfly species as well as bees and hummingbirds are attracted to plants in bloom.

Only a few seed pods developed on our tropical milkweed plants due to the short growing season.  In more southern gardens, seed development is more likely and there have been reports from gardeners in USDA Zone 7 of seeds that successfully overwintered.  Tropical milkweed grows as a tender perennial in central and southern Florida.

There are a few cultivars of tropical milkweed available, including the yellow-flowered ‘Silky Gold’.  Others include ‘Silky Deep Red’ and ‘Red Butterfly’, both with darker red flowers than the species, and ‘Apollo Orange’ and ‘Apollo Yellow’, each with the appropriately colored blossoms.  I doubt that any of these are better than the species at attracting monarchs.

Perennial Milkweed Species

In addition to butterflies, swamp milkweed serves as a nectar source for bumble bees, native solitary bees, and other beneficial insects.

In addition to butterflies, swamp milkweed serves as a nectar source for bumble bees, native solitary bees, and other beneficial insects.

In terms of tolerance for a wide range of growing conditions as well as effectiveness in attracting monarchs and other pollinators, the best perennial milkweed for home gardens is the swamp milkweed, Asclepias incarnata.  It is far better suited to the home garden than the common milkweed (A. syriaca) or purple milkweed (A. purpurascens), two species that will quickly take over the garden with aggressive rhizomatous growth habits.  Swamp milkweed, on the other hand, has an erect clump-forming habit, grows three to four feet tall with branching stems, and never grows out of bounds.  In summer the end of each stem bears a cluster of small, fragrant, pink to mauve flowers that are a favorite of butterflies, bees, and other pollinating insects.  These flowers are followed in late summer by attractive seed pods, each about four inches long, which split open when ripe to release silky-haired seeds to the wind.

'Ice Ballet' is a lovely cultivar of swamp milkweed.

‘Ice Ballet’ is a lovely cultivar of swamp milkweed.

‘Ice Ballet’ is a compact, white-flowered cultivar of swamp milkweed that grows to three feet in height.  I saw this cultivar two years ago in a Cape Cod garden, tucked amid the deep green leaves of northern barberry along the winding entrance drive.  Beautiful!

The common name, swamp milkweed, belies this plant’s ability to grow equally well in wet or well-drained areas.  Full sun is required for optimum growth.  We’ve learned that the plants have deep taproots and are best left undisturbed once established, and that new growth is slow to emerge in spring.

Butterfly weed is an excellent nectar plant for monarchs.

Butterfly weed is an excellent nectar plant for monarchs.

Until last year I was familiar with butterfly milkweed (A. tuberosa) only from seeing it in other perennial gardens and, while it is often listed as an excellent nectar source for adult monarchs, its leaves are apparently to thick and hairy for the caterpillars to relish.  Monarchs have been known to lay eggs on it, but not if swamp milkweed or tropical milkweed are around.  We gave it a try last year, planting a few seedlings among the tropical milkweed plants, but we never found any caterpillars munching the leaves.

 A. tuberosa,  native throughout eastern North America, grows in a loose clump to about two feet tall and bears clusters of bright orange to yellow-orange flowers.  Mature spindle-shaped seed pods release the silky-haired seeds characteristic of the genus.  Unlike other milkweeds, the hairy stems of this species do not have milky-sap.  A. tuberosa ‘Gay Butterflies’, a group of cultivars with flower colors of red, orange, or yellow, as well as a mixture of all three colors (Gay Butterflies Mix), can be purchased from several mail-order houses.

All milkweed species produce silky-haired seeds borne in terminal seed pods.

All milkweed species produce silky-haired seeds borne in terminal seed pods.

Butterfly weed is worth growing as a source of nectar for pollinators and other beneficial insects.  While easily grown from seed, I recommend starting with nursery-grown plants as seedlings will take two to three years to flower.  Easy to grow in average well-drained soil and full sun, butterfly weed develops a deep taproot and should be left undisturbed once established.

Sources for Milkweed Seeds and Plants

The following list of mail-order sources for milkweed seeds and plants is based on 2015 catalogs that I have received to date and is by no means exhaustive.  If you need a source of plants, either seedlings or transplants, always start with a phone call to local garden centers and nurseries and be sure to use the scientific name for the species you are trying to find.

Asclepias curassavica (tropical milkweed):

seeds – Johnny’s Selected Seeds (www.Johnnyseeds.com)
Territorial Seed Company (www.TerritorialSeed.com)

Asclepias incarnata (swamp milkweed):

seeds – Prairie Moon Nursery (www.prairiemoon.com)
Botanical Interests (www.botanicalinterests.com)

plants – Prairie Moon Nursery (www.prairiemoon.com)
Burpee (www.burpee.com) (‘Ice Ballet’)

Asclepias tuberosa (butterfly weed):

seeds – Botanical Interests (www.botanicalinterests.com)
Select Seeds (www.selectseeds.com)
Burpee (www.burpee.com)

plants – Burpee (www.burpee.com)
White Flower Farm (www.whiteflowerfarm.com) (Gay Butterflies Mix)

Mid-coast Maine gardeners interested in finding first-year seedlings of A. incarnata and A. curassavica will find them at the UMaine Cooperative Extension Annual Plant Sale in Ellsworth on Saturday morning, May 16, 2015.  In addition to these milkweeds, a wide assortment of robust native and non-native herbaceous perennials, all grown by Master Gardener Volunteers, will be available as potted divisions.  (All profits from this sale support the UMaine Extension Master Gardener Volunteer Program.  Directions to the sale can be found at: http://umaine.edu/hancock/directions/)

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About Reeser Manley

I was born in Laramie Wyoming but moved to the southeast at an early age. I was educated through my B.S. in Biology in the Columbus, Georgia area, then crossed the Chattahoochee River to earn my M.S. in Botany at Auburn University. For the next ten years, I worked as Horticultural Manager for the George W. Park Seed Co. in Greenwood, S.C. At 40 years of age, I decided to return to graduate school and in 1994, I earned a Ph.D. in Horticultural Science from Washington State University, Pullman, Washington. Fast forward through 10 years at university (7 at UMaine, Orono) and you find me teaching high school science in Eastport, Maine, the edge of the world, and writing a weekly garden column for the Bangor Daily News. My new book, The New England Gardener's Year, a Month-by-Month Guide for Maine, New Hampshire, Vermont, Massachusetts, Rhode Island, Connecticut, and Upstate New York”, will be published later this year by Cadent Publishing. You can learn more about the book by visiting its Facebook page: http://www.facebook.com/pages/The-New-England-Gardeners-Year/187285218055676.)