Milkweed: A Brief Photoessay

Craig Holdrege

From In Context #22 (Fall, 2009)

This article is one part of a more intensive study by Craig, with illustrations and photos, of the complex life story of milkweed, “The Story of an Organism: The Common Milkweed.” The full study also depicts how an organism always leads beyond itself to a larger web of relations with other organisms and elements of the environment.

It is early May in upstate New York. When you look onto roadsides and old fields, there is still much of the past to be seen — the dead, brown and gray remnants of last year’s growth. The dry and brittle leaves of some grasses and wildflowers, and the crisscross of matted stems from asters, goldenrods and milkweed provide the immediate surroundings for this year’s fresh green emerging vegetation. The shoots of grasses, goldenrods, and other wildflowers are rising up out of the soil; common milkweed (Asclepias syriaca) shoots emerge only later. You have to get down close to the ground to see the stout little green and brown-reddish spears growing through the leaf and stem litter. And where you find one shoot emerging, you usually find many. Figure 1 shows milkweed shoots from one colony, drawn to scale. (All drawings and photos are by Craig Holdrege.)

Common milkweed grows in colonies that tend to get more populated each year, unless something in the environment inhibits their exuberant growth. There can be hundreds or thousands of shoots, depending on the size of the colony. All these little shoots grow from buds that have overwintered; they developed the previous summer on rhizomes (underground stems) and remain dormant until the following spring.

The rhizome grows and branches each year, never showing itself above ground and forming an extensive network out of which the many individual shoots grow (Figure 2). A colony is thus one large plant — botanically speaking, a clone. The rhizome in the figure was laid bare during the late spring. At the top you see a branching rhizome, with the dime at the right indicating the scale. The lower photo is a close-up.  Key to the labels — a: rhizome; b: roots; c: shoot from previous year; d: buds that were formed in the summer/fall, some of which will unfold in the following spring; e: scars from the base of a shoot from the previous year.

 As a shoot grows upward, the first small lance-shaped leaves begin to unfold. The leaves are arranged in an opposite pattern, meaning that two leaves emerge at the same height out of the stem, opposite each other (Figure 3). The next pair of leaves is offset by about ninety degrees, so that a distinct leaf arrangement (phyllotaxis) emerges. The stem is stout and grows quite straight upward. It is significantly thicker than its counterpart in asters or goldenrod, but not so dense. While the first leaf pairs near the ground are small, the leaves soon became long and broad. Milkweed has much bigger leaves than other plants growing in the roadside/old field community. The leaves have a short thick petiole that becomes the distinct midrib of the leaf blade. The midrib branches off into fairly straight, diagonally ordered side veins that are clearly visible on the leaf underside. The oblong leaves have a smooth, clear margin.

Over the course of May and through June the shoot grows vigorously, reaching a height of about 1 to 1.5 meters. They surpass in length the goldenrods that had begun their development a few weeks before the milkweed. As the shoot extends, new leaf pairs unfold, each pair larger than its predecessor. After a shoot has developed around seven to nine pairs of leaves, the first flower buds become visible among the still unfolding uppermost leaves. The stout little stems (peduncles) that carry the groups of flower buds do not grow out of leaf axils, as is the case in most flowering plants. Rather, in all but one species of the genus Asclepias, the flowers grow out of the main shoot slightly to the side of a leaf. Interestingly, where flowers develop, the leaf arrangement also changes. Subsequent leaf pairs are no longer perpendicular to each other, but shift to an angle of about 120°.

Also — and this is typical in most flowering plants — the leaves become smaller in the flowering region. The uppermost leaves contract to a size comparable to the very first leaves on the shoot, but tend to be more pointed and elliptical in shape. These changes in vegetative structure point us to the next developmental wave in milkweed’s life history — the flower.

Among the unfolding upper leaves (Figure 4) you can see small grayish-green balls of tightly grouped flower buds. As the buds grow and gradually turn pink (illustrations on next page), the stout stem that carries them away from the main shoot extends diagonally upward. Each flower bud has, in turn, its own delicate stalk, which also lengthens, and the tight ball becomes a looser and larger sphere. All the individual flower stalks (pedicels) originate at the apex of their common stem, which means that in botanical terms the milkweed inflorescence is an umbel. There can be anywhere between 10 to nearly 200 flowers in one umbel. While in many umbel-forming plants, as in most members of the carrot family, the flowers spread out into a plane to form a disk, the umbel of the common milkweed maintains its spherical form until the flowers wilt.

The lowermost umbel on a given shoot opens first, then the one next higher up, and so on. A shoot has an average of 3 to 6 umbels, but some shoots have none and others up to 10. Usually when the lowermost umbel is already wilting, the uppermost one is still in bloom (Figure 5). Milkweed flowers are long-lived in comparison to the flowers of other plants, since any single flower can be open for over a week. The whole phase of flowering in a colony lasts about four weeks.

Long before you come close enough to a colony to be able to see its flowers, you can smell that it is in full bloom. The sweet scent of the nectar-filled flowers carries far. As you approach the colony you see the rich-pink flowering spheres and a multitude of insects crawling and flying around. Honey bees, native bees such as bumbles bees, ants, and a variety of butterflies move from flower to flower, umbel to umbel, drinking nectar from the blossoms. Along the way they pollinate the flowers, a strangely intricate process related to the complex anatomy of milkweed flowers that I will discuss in another essay.

With the multitude of flowers and pollinators, you would think that many fruits (pods) would develop. But this is not the case. Although one shoot may have on average between 300 and 500 flowers, only about 1 flower out of a hundred develops into a pod. Most flowers in an umbel wilt and fall away, while the occasional fruit shows itself through its thickening stalk and the white, fuzzy swelling that is the developing pod.

While vegetative growth is rapid and expansive, and flowering is a period of bursting productivity, pod development is slow and extended. During July, August, and September the pods grow, and in their inner cavity the seeds develop (Figure 6).  One interesting feature of pod development is that the stalk that carries the pod — regardless of the angle at which it originally extends from the umbel’s stalk — twists and curves into a position such that the pod becomes oriented vertically.

The pod expands some laterally, but mainly grows in length. By October or early November the pods have reached their full size and maturity. The suture along the convex side of the pod splits open and the neatly ordered, tightly packed seeds become visible. It almost looks as if an artist had laid the seeds out. With further opening of the pod, the seeds begin to fall and float away (Figure 7). Each seed has lovely white silky extensions (comas) that allow them to be carried away by a breeze — even though, as seeds go, common milkweed seeds are significantly larger and heavier that those of other old field species. Interestingly, although individual shoots within a colony may flower three weeks later than others, the fruits in that colony release their seeds at the same time.

I mentioned above the paucity of pods in comparison to the wealth of flowers in a milkweed shoot or colony. But each pod is full of many seeds — one study gives an average of 226 — so that in a colony of, say, 1,000 shoots, hundreds of thousands of seeds will spread into the environment. You would, as a result, expect to easily find seedlings of milkweed plants in areas around existing colonies. As I began to study common milkweed, I searched for pictures of seedlings so I could identify them. To my surprise I could find none, although you can find images of the seedlings of most agricultural field weeds. Nor have I come across any studies describing milkweed seedlings in the wild. Evidently, milkweed seeds are not proliferate germinators.

Since, however, new and young colonies can be observed, at least once in a while seeds must germinate and some seedlings take hold. As field experiments show, already in the second year a plant can produce multiple stems, some over a meter apart from each other, showing the vigorous growth of the rhizome.

In this brief sketch of its life history we can see how, up through flowering, common milkweed is characterized by exuberant vitality: vigorous underground rhizome growth; yearly expansion of many long, large-leafed shoots; production of numerous flowers that secrete copious amounts of nectar. After flowering, it is as if milkweed pulls back and concentrates its vitality into formation of a relatively small number of pods, but each swells into a large pod that houses a multitude of apparently viable seeds that spread into the larger environs. Yet only a few of them form new colonies.

In issue #23 of In Context we present the remarkable story of the milkweed flower, with its intricate and unusual anatomy and equally remarkable pollination by various insects.