Perennial Plant Feature: Milkweeds

milkweed plant

Milkweeds have many benefits—they are hardy, low maintenance, good in rain gardens as well as in dry soils and during droughts, tolerate deer, the mid-summer flowers attract butterflies and the leaves feed monarch butterfly larvae.

Milkweeds are not weeds at all (a plant out of place that you don’t want in your garden) unless, perhaps, you have a formal garden.  The common milkweed (Asclepias syriaca) is perhaps the most common colonizer of gardens and farm fields.  In the latter, this native plant can be a problem, being toxic to grazing animals and poultry when eaten in quantity.  Its cardiac glycosides make it toxic to humans too but, due to the milky sap, it isn’t often eaten.  Most milkweeds have some toxicity, but it varies with species.

Native Americans used native species of this plant for a range of medicinal uses, and taught colonists how to cook and prepare it properly so not to be poisoned.  Uses ranged from skin warts and snake bites to digestive problems, fevers, sore muscles and its main use—lung conditions such as asthma, bronchitis and pleurisy (from whence it gets the common name “pleurisy root”).  In fact, its scientific genus name (Asclepias) honors the Greek god of healing, Asclepius.

With increased recent interest in pollinators, milkweeds have gained more recognition as important to a range of pollinators and particularly to the threatened monarch butterfly.  Butterflies feed on the flowers, but milkweeds are the sole food for their larvae.  The same toxic properties that may harm humans and cattle protect monarch larvae.  By ingesting milkweed leaves, they in turn become toxic to birds and other predators, so are left alone.

Milkweeds are a plant that, like orchids, has evolved a rather unique and complex pollination strategy.  The pollen is contained in pollen sacs (“pollinia”) which attach to pollinators when their feet or mouthparts slip into slits between anther flower parts.  These pollen sacs are then carried to another flower where they get lodged into similar flower slits.  Although the monarch is most associated with milkweeds, it isn’t one of the most effective pollinators.

Milkweed flowers are grouped into domed, slightly drooping clusters (“umbels”) which arise from where upper leaves join stems.  Colors vary with species, but are generally pink, white, or orange.  Depending on region and species, they may bloom from spring through summer on plants ranging from one to five feet high.

The flowers eventually produce warty elongated seed pods (“follicles”), two to four inches long, which turn from green to tan when ripe and split open.  They release many seeds with silky tails (“coma”), often called floss or silk, which aid in wind dispersal.  This floss often has been used in stuffing pillows, as insulation for coats, as acoustic insulation, and oil absorbents.  Pods can be harvested and used in decorations naturally or painted.

Milkweeds grow best in full sun, blooming less and being more leggy in part shade.  Most native species are hardy to USDA zones 3 (minus 30 degrees average winter minimum).  They are slow to emerge in the spring, so require patience and careful attention to early season gardening around them.

They prefer medium to dry soils, tolerating rocky and poor soils, except the rose or swamp milkweed (A. incarnata) which prefers medium to wet soils.  Those species that tolerate drought, once established, do so through deep tap roots.  These roots make most milkweeds difficult to transplant, so place them properly when planting.  If they must be moved, do so when plants are dormant.

Flowers are showy, fragrant in some species, and can be used cut.  Plants are useful massed in natural and native plant gardens, as well as gardens for pollinators, and some species in rain gardens.  In addition to all these benefits, and their low maintenance with few problems is their resistance to deer browsing.

Milkweeds used to be in their own family, but are now grouped in the dogbane family (Apocynaceae).  There are over 140 species, of which at least half are native to eastern North America.  Most are rather rare or seldom seen, but there are a handful that are sold for garden use.

The butterfly weed (A. tuberosa), or orange milkweed, is the most common species that most know.  It has orange flowers in mid to late summer, eventually reaching about two feet tall and wide with an open upright habit.  Leaves lining the stems are stiff, long and narrow.  With its attractive flowers and all the other usual milkweed benefits, it was named the Perennial Plant of the Year in 2017.  ‘Gay Butterflies’ is a mix of yellow-, orange-, and red-flowering plants.

You may find a yellow-flowered cultivar (cultivated variety) of butterfly weed called ‘Hello Yellow’.  In her graduate study in Vermont, Annie White found that over 90 percent of pollinator visits to either this cultivar or the species were by bumblebees.  There was a small, but not very significant, preference with pollinators for the species over this yellow cultivar.

The swamp milkweed, as noted already, prefers more moist soils so would be good for such sites where many perennials don’t grow well.  This makes it a good candidate, too, for rain gardens and clay soils.  It can grow three to four feet high, and about half as wide.  Flowers in late summer are in pink, white, and mauve.  Common cultivars are ‘Cinderella’ with larger rosy pink flowers, ‘Ice Ballet’ with white flowers, and ‘Soulmate’ with rose-purple flowers.

The common milkweed, as noted, is one that colonizes in the wild—fields, along roads and ditches, and even into natural gardens.  The stout, upright stems reach three to four feet, and hold pinkish purple flowers from mid through late summer.

A milkweed that isn’t native but is quite colorful with red and orange flowers, and so is often seen for sale, is the blood flower or tropical milkweed (A. curassavica).  It is native to South America, so only hardy to subtropical USDA zone 9 regions where it often self-sows and becomes weedy.  In most areas it is grown as a well-behaved annual flower, getting two to three feet high, and with blooms through the season. This milkweed is attractive to hummingbirds.

If you aren’t already growing milkweeds, or would like to add more, you can start them from readily-available seeds.  This may take some time, but will be cheaper for producing many plants.  Quicker is to buy plants from nurseries.

You can learn more about helping pollinators with milkweeds and other wildflowers from the Xerces Society (xerces.org/milkweed).  There is much online about monarch butterflies in particular, and how to help them, such as from the U.S. Fish and Wildlife Service (www.fws.gov/savethemonarch).

PERENNIAL PLANT FEATURE: MILKWEEDS

Dr. Leonard Perry, Horticulture Professor Emeritus
University of Vermont

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