Milkweed: An Introduction

by Rose Franklin              February 12, 2001

   I grew up on a dairy farm in central Pennsylvania. On our farm, milkweed grew in patchy colonies among the crops, along the fence rows, and in the cow pasture. The milkweed grew about four feet tall and produced rounded clusters of tiny mauve-pink flowers which smelled sweet and attracted an array of colorful butterflies. In late fall the milkweed produced thick-walled seed pods which eventually cracked open to reveal hundreds of little brown seeds, each of which was attached to its own silky parachute. On windy November days, thousands of these white silky, spider-looking parachutes dotted the sky as they carried seeds off to far away places. As a child I knew of only one milkweed and assumed that this was the only milkweed there was.
   Today, almost fifty now, I still live in central Pennsylvania, just a few miles from where I grew up. I still love the sweet fragrance of milkweed and look forward to its blooming, knowing the milkweed nectar will attract numerous species of butterflies: Swallowtails, Monarchs, American Ladies, Hairstreaks, and more. Now though I understand that the milkweed which grew on the family farm isn't the only milkweed which exists. I  know that that milkweed was 'common milkweed' and that, actually, ten or so other milkweed species grow wild right here in Pennsylvania. As a child I had no idea that the short, orange-flowering 'butterfly weed' which grew along the highway was really a milkweed. Nor did I realize that the yard-high, beautiful pink flowering weed which  attracted butterflies to swampy locations was a milkweed too.
   The scientific name for milkweed is Asclepias (pronounced 'as-KLEE-pea-us'). Named to commemorate Asklepias, Greek god of medicine, Asclepias produces star-shaped flowers arranged in round or flat clusters called umbels. Most, but not all, milkweeds produce a milky sap.
   Worldwide, about 200 species of Asclepias grow and in the United States alone, over 100 species grow native. The American natives are mostly erect, coarse growing perennials which flower in summer and then develop distinctive seed pods. In late fall, the seed pods open to disperse their seeds. Each milkweed seed bears a tuft of long, silky hairs which readily carry the seed great distances.
   Among the milkweed species which grow in the United States are Asclepias syriaca (common milkweed, the one I knew as a child), Asclepias tuberosa (butterfly weed, a.k.a. pleurisy root), Asclepias incarnata (swamp milkweed), Asclepias viridis (spider milkweed), Asclepias purpurascens (purple milkweed), Asclepias quadrifolia (four-leaved milkweed), Asclepias exaltata (poke milkweed), Asclepias variegata (white milkweed), Asclepias sullivantii (prairie milkweed), Asclepias verticillata (whorled milkweed), Asclepias hirtella (green milkweed), Asclepias arenaria (sand milkweed), Asclepias latifolia (broadleaf milkweed), Asclepias speciosa (showy milkweed), and Asclepias floridana (Florida milkweed). Another 80+ milkweed species and this list of U.S. native milkweeds will be complete!
   There are two major groups of milkweeds: narrow-leaved (with linear or narrowly lanceolate leaves) and broad-leaved (with leaves usually more than two inches wide throughout most of their length). Some milkweeds make beautiful specimens for the formal flower garden while others are considered dull, invasive weeds.
   Aside from being a great nectar source for numerous butterfly species, milkweed is the host plant for Monarch butterflies. If there was no milkweed, there would be no Monarchs  to adorn our flowers with their bright orange and black wings. Milkweed contains cardenolides (cardiac-active steroids) which are consumed by monarch caterpillars during feeding and then sequestered in their bodies even after the adult monarchs emerge from their pupae. These cardenolides make monarchs toxic and bitter-tasting to birds and other vertebrate predators.
   With so many milkweed species, how does the butterfly enthusiast decide which ones to include in his or her garden? Well, the choices are dramatically reduced when you take availability into consideration. While there are about 200 species of milkweeds growing worldwide, only a few are typically offered in the plant selections at nurseries, greenhouses, and garden centers.
   Among the milkweed species commonly offered to the home gardener are butterfly weed (Asclepias tuberosa), swamp milkweed (Asclepias incarnata), and tropical milkweed (Asclepias curassavica). Each of these species are worthy of planting the the formal garden and all entice butterflies to visit.
   Butterfly weed (Asclepias tuberosa) grows 18"-24" high, produces clusters of bright orange flowers from late June through July, and is highly utilized as a nectar source. It grows best in sandy soil that drains well. Butterfly weed is hardy in zones 4-9.
   Swamp milkweed (Asclepias incarnata) grows 30"-36" tall, bears pink flowers, and usually blooms from late June through July. 'Ice Ballet' swamp milkweed (Asclepias incarnata 'Ice Ballet') produces white flowers rather than pink. Swamp milkweed is utilized by many butterfly species (monarchs, red admirals, American ladies, painted ladies, fritillaries, and more) as a nectar source. It is also utilized by monarch butterflies for egg-laying. Swamp milkweed is hardy in zones 3-8.
   Tropical milkweed (Asclepias curassavica) is not an American native. It is believed to have originated in South America, and thus, must be grown in the United States as an annual. Tropical milkweed matures at 30"-36" high, serves as a nectar source for numerous butterfly species, and is highly utilized as a host plant for monarchs. When monarchs are offered a palate of milkweed species for egg-laying, they usually choose tropical milkweed when it is among the choices.
   Through the eyes of a child, milkweed was to me a plant that yielded a milky sap, attracted butterflies for nectaring, and produced pods of downy seeds that floated far away following disbursement from the pods. As an adult though, I see milkweed as so much more. It's a nectar source for not only butterflies but also bees and hummingbirds. It's the host plant for monarch butterflies (and milkweed tussock moths too). And some milkweeds make showy garden specimens. Milkweed to me could never again be just milkweed. And by any other name it couldn't be any sweeter!

Growing Monarch Caterpillars in Your Garden

   If your goal is to draw monarch females to your garden for egg-laying, my suggestion is that you plant two species of milkweed: tropical milkweed and swamp milkweed.  It would be a wise choice to include tropical milkweed (Asclepias curassavica) in your garden because it has tender leaves that monarch caterpillars love to feed on (and egg-laying females choose often for laying eggs on). Tropical milkweed is easy to grow and matures quickly under the long days of summer. But because it is a tropical plant, tropical milkweed must be replanted every year.
   Swamp milkweed (Asclepias incarnata) is another milkweed that should be considered for massive planting in the butterfly garden. Swamp milkweed produces leaves which are about the same size as tropical milkweed leaves and almost as tender as tropical milkweed. Once swamp milkweed is three years old, it generally shoots 5 to 10 stems up from its root system, producing lots of foliage for caterpillar consumption. Swamp milkweed is available in pink-flowering or white-flowering varieties.
   We periodically search for monarch eggs and caterpillars on the milkweeds growing in our gardens, just to see what milkweed species the females are choosing for egg-laying. Our observations have drawn us to conclude that about 50% of the eggs are laid on tropical milkweed, 40% on swamp milkweed, and the remaining 10% on the other milkweed varieties growing in our garden, these including Asclepias tuberosa (butterfly weed), Asclepias syriaca (common milkweed), and Asclepias vetrticilla (whorled milkweed).
   By planting several (or many) plants of each tropical milkweed and swamp milkweed, you will likely catch the attention of any female monarch who is nearby and looking for a location to deposit her eggs. In our yard, we have several dozen milkweed plants and always lure monarchs in for egg-laying. Typically we count over 100 caterpillars on our milkweed in a single growing season. You too can ready your property for becoming a monarch nursery. Just plant milkweed.....lots of milkweed!

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Rose Franklin's Perennials
107 Butterfly Lane         Spring Mills, PA  16875
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Copyright 2001  [Rose Franklin's Perennials]. All rights reserved.
Revised: July 02, 2007