Asclepias syriaca Guide: Grow & Identify Common Milkweed

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Introduction

Asclepias syriaca, commonly known as common milkweed, is a perennial plant native to North America. It is well-known for its role in supporting monarch butterflies as a host plant for their larvae and for its fragrant, nectar-rich flowers. Common milkweed is also referred to regionally as butterfly flower, silkweed, and Virginia silkweed. This plant is valued for its ecological importance, ornamental beauty, and has been used traditionally for various purposes, although it is considered invasive in some areas outside its native range.

Classification

• Kingdom: Plantae
• Phylum: Tracheophyta
• Class: Magnoliopsida
• Order: Gentianales
• Family: Apocynaceae
• Genus: Asclepias
• Species: A. syriaca

Description

Asclepias syriaca is a herbaceous plant that is known for its robust growth and ability to thrive in a variety of environments. It is particularly recognized for its milky sap, which contains toxic compounds known as cardenolides. These compounds are sequestered by monarch butterfly larvae, providing them with a defense against predators. The plant has ornamental value due to its attractive flowers and is also of interest to hobbyists who wish to attract and support monarch butterflies. While it has some commercial and horticultural value, particularly in native plant landscaping, common milkweed can become problematic if it spreads aggressively in areas where it is not wanted.

Size

Common milkweed typically reaches heights of 4 to 6 feet, with a maximum known height of around 8 feet under ideal conditions. The plant has a moderate growth rate, averaging a few inches per month during the growing season. Its roots are rhizomatous, spreading horizontally and capable of producing new shoots, which contributes to its ability to form colonies. While the root system is extensive, it is generally not known to cause significant problems.

Leaves

The leaves of Asclepias syriaca are broad, ovate, and range from 4 to 10 inches in length. They have a velvety texture and are arranged oppositely on the stem. The leaves exhibit a prominent central vein with a network of smaller veins branching out, and their margins are smooth. Botanically, the leaves are described as simple, entire, and with a thick, waxy cuticle that helps in water retention.

Flower

Common milkweed flowers are complex and highly specialized. They are borne in spherical clusters known as umbels, with each individual flower possessing a unique structure adapted for pollination by insects. The flowers are pink to mauve in color and have a sweet scent, which attracts a variety of pollinators. Blooming typically occurs from late spring to mid-summer. Each flower has five reflexed petals and a crown-like structure called the corona.

Fruit

The fruit of Asclepias syriaca is a large, warty follicle that contains numerous seeds. Each seed is attached to a tuft of silky hairs, known as a coma, which aids in wind dispersal. The pods split open when mature, releasing the seeds to the environment.

Stem

The stem of common milkweed is stout, upright, and covered with fine hairs. It is green to reddish-purple in color and contains the characteristic milky sap when broken.

Cultivation and Care

Common milkweed is a low-maintenance plant that is easy to cultivate, making it a good choice for native plant gardens and restoration projects.

Planting

To plant Asclepias syriaca, choose a location with full sun exposure. The seeds can be sown directly into the ground in the fall, allowing them to stratify naturally over the winter. Alternatively, cold stratification can be done manually before spring planting. Plant the seeds 1/4 inch deep in the soil and space them about 18 inches apart to allow for spreading.

Soil Preference

Common milkweed is adaptable to a range of soil types, including sand, silt, clay, and loam. It prefers well-drained soil and can tolerate drought conditions. The preferred pH range for this plant is between 6.0 and 7.0.

Watering

Once established, common milkweed is drought-tolerant and requires minimal watering. During the establishment phase, water the plant regularly to ensure healthy root development. Afterward, watering can be reduced, and the plant will typically thrive with natural rainfall.

Sun Requirements

Asclepias syriaca requires full sun to grow well and produce abundant flowers. Planting it in open areas, meadows, or the back of a border where it can receive at least 6 hours of direct sunlight daily is ideal.

Pruning

Pruning is not typically necessary for common milkweed, but it can be cut back in late fall or early spring to maintain a tidy appearance and control spreading. Use sharp pruning shears to make clean cuts.

Propagation

Propagation of Asclepias syriaca can be done through seed collection and sowing or by dividing the rhizomes in early spring or fall.

Health & Safety

The milky sap of common milkweed contains toxic compounds that can be poisonous if ingested by humans or animals. It is particularly toxic to dogs and cats if consumed. The plant is not known to be thorny or to cause contact dermatitis.

Pests and Problems

Common milkweed can be susceptible to pests such as the oleander aphid (Aphis nerii) and the milkweed bug (Oncopeltus fasciatus). These insects feed on the sap of the plant, which can weaken it and reduce its vigor. Treatment includes the removal of heavily infested parts of the plant or the use of insecticidal soap. Common milkweed can also suffer from root rot if grown in poorly drained soil. To prevent this, ensure that the soil is well-draining and avoid overwatering.