The Allure of Narrow-leaf Milkweed: A Haven for Monarch Butterflies

Image: Narrowleaf milkweed flowers. Source: David A. Hofmann

Monarch butterflies, those unmistakable symbols of beauty and grace, play a vital role in pollination. However, these magnificent creatures face the risk of endangerment. To safeguard their existence, numerous organizations dedicate their efforts to the conservation of monarch butterflies. One such conservation technique involves the cultivation of narrow-leaf milkweed, scientifically known as Asclepias fascicularis. This distinctive plant species is a crucial element in monarch conservation due to its exclusive status as the primary food source for monarch caterpillars.

Asclepias fascicularis, adorned with clusters of small flowers, boasts an enchanting allure. Whether grown for the sake of the monarch butterflies or simply to enhance the aesthetic appeal of your garden, this plant is sure to be a delightful addition. However, it’s important to exercise caution when handling narrow-leaf milkweed as it is toxic to humans and animals, making it deer-resistant. Refrain from consuming it and remember to wear gloves when tending to it. Let’s delve into the art of growing narrow-leaf milkweed, also known as Mexican whorled milkweed, so that you too can contribute to the well-being of these magnificent creatures.

Quick Care Guide

Narrowleaf milkweed flowers. Source: David A. Hofmann

Common Name: Narrow-leaf Milkweed
Scientific Name: Asclepias fascicularis
Family: Apocynaceae
Height & Spread: Up to 3 feet tall
Light: Full sun
Soil: Well-draining, tolerant of clay and silty soil
Water: About 1 inch per week
Pests & Diseases: Few pest and disease issues, deer-resistant

Unveiling the Charm of Narrow-Leaf Milkweed

Monarch larvae munching A. fascularis. Source: TJ Gehling

The narrowleaf milkweed, also known as Mexican whorled milkweed, is a perennial herb native to North and Central America. It can be found in diverse habitats such as yellow pine forests, moderate slopes, valleys, and grasslands. Serving as a host plant, this milkweed species plays a vital role in the survival of monarch butterflies, beneficial insects, and wildlife by providing caterpillars with a vital food source.

Naturally occurring in regions ranging from northeast Washington to Baja California, this plant is considered native California milkweed. Additionally, it can be found in disturbed areas of Idaho, northern Arizona, and Colorado. Narrow-leaf milkweed thrives in full sun and can withstand some drought, although it does not fare well in desert conditions.

Characterized by erect stems that can reach a height of 3 feet, narrowleaf milkweed features distinctive long, pointed leaves measuring up to 6 inches. Clusters of small flowers, appearing in shades of green, white, purple, or pale pink, adorn the erect umbels. These blooms emerge throughout the summer and fall, with seed pods maturing during the summer months.

The seed pods, resembling horns, emerge after the flowers have faded. These pods carry numerous silky hairs that aid in the plant’s propagation. As the pods split open, the wind often carries the seeds to new locations, facilitating germination elsewhere.

To create a butterfly garden, consider planting Mexican whorled milkweed alongside other Asclepias species or plants that attract monarch butterflies, such as California aster, Indian mallow, and herbs like mint, lavender, and different varieties of sage. Remember, though, that it is only milkweed that serves as the caterpillar’s nourishment.

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It is worth noting that milkweed, including Asclepias fascicularis, contains cardiac glycosides, making it toxic to humans and animals. To ensure safety, it is best to plant them away from edible gardens and areas where livestock may graze. Consider planting them in a fenced area if you have curious children or pets. When handling milkweed, always don gloves and thoroughly wash your hands afterward.

Nurturing Mexican Whorled Milkweed

A. fascicularis foliage without flowers. Source: chuck b.

Once established, Mexican whorled milkweed requires minimal effort to thrive. This low-maintenance plant exhibits remarkable resistance to pests and diseases and is even deer-resistant.

Sun and Temperature

Asclepias fascicularis thrives in sunny conditions, requiring a minimum of 6 hours of direct sunlight. While it can tolerate some shade, it flourishes in full sun. Ideally suited for USDA hardiness zones 6-10, this plant experiences dieback during winter but rejuvenates with the arrival of warmer temperatures. In colder climates, it may be grown as an annual, but regrowth should not be expected after freezing winter temperatures.

Water and Humidity

Narrowleaf milkweed is considered drought-tolerant and requires minimal watering. A weekly watering session with approximately 1 inch of water should suffice, although it can withstand dry climates. However, it is important to note that this plant does not thrive in desert conditions. Regular watering helps maintain optimal soil moisture levels, promoting healthy growth. When watering, direct water at the base of the plant to prevent leaves and flowers from getting wet. Wet foliage can lead to fungal infections and other diseases, which may harm monarch butterflies. Soaker hoses, drip irrigation, or careful hand irrigation are recommended during the summer months.


Well-draining soil is crucial for Asclepias fascicularis to prevent waterlogging. Although this plant can tolerate poor soil conditions, including clay or silty soil, it thrives best in well-drained soil. Its adaptability allows it to grow in various soil types, provided water drainage is efficient. Slopes and valleys are ideal locations as they facilitate water runoff away from the roots. A neutral pH level is preferred, but this milkweed species can tolerate slightly acidic or alkaline clay soil.


Milkweed, including Asclepias fascicularis, does not typically require fertilization. Native to North America, these plants are well-adapted to poor soil conditions. If planting in a nutrient-depleted flower bed, incorporating compost into the soil may prove beneficial. However, caution must be exercised to avoid excessive nutrient levels, particularly nitrogen. Excessive nitrogen can hinder lavender or purple flower clusters from blooming, instead promoting the growth of thin, whorled leaves on the stem. Optimal flower production is achieved when milkweed is cultivated with minimal fertilizer application.


While not essential for the plant’s health, pruning narrow leaves or lavender flowers during winter can help prevent the development of Ophryocystis elektroscirrha (OE), a harmful parasite that affects monarch butterflies, causing deformed wings. During winter, cutting the stem back to approximately 6 inches promotes new growth in the spring.

Narrowleaf Milkweed Propagation

Narrowleaf milkweed is commonly propagated through seed. Seed collection is relatively simple, as the seeds develop within seed pods that hang from the plant. These pods can be harvested in late July after the flowers have bloomed and faded, amidst the distinctive narrow whorled leaf pattern. It is important to wear gloves when handling any part of the Asclepias plant.

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Similar to other milkweed species, the narrowleaf milkweed seed requires a period of cold exposure to facilitate germination. In regions where temperatures do not drop significantly, storing the seeds in sand inside a plastic bag in the freezer for two months replicates the cold exposure necessary for germination. Afterward, spread the seeds in your garden during early spring for optimal growth.

Troubleshooting Asclepias Fascicularis

Asclepias fascicularis. Source: Mathesont

Asclepias fascicularis encounters minimal pest and disease issues. Nonetheless, a few noteworthy problems may arise, endangering the well-being of monarch butterflies. Familiarize yourself with these issues and their corresponding preventive measures:

Growing Problems

Unsuccessful germination of narrowleaf milkweed seeds may be attributed to insufficient exposure to cold temperatures or excessively high temperatures. For successful germination, the seeds need exposure to cold weather during winter months and cannot germinate if temperatures surpass 85°F. Patience is key, as newly planted milkweed species often prioritize root growth during the initial year, with flowering commencing in subsequent years.

If no flowers are observed in the first year of planting milkweed seeds, it is likely due to the plant’s focus on root development. As a perennial plant, milkweed species establish strong roots during the initial year, ensuring future flower production. Therefore, lack of blooms in the first year should be no cause for concern.

Excessive nitrogen may also inhibit milkweed flowering. If you notice an absence of blooms despite not planting milkweed seeds in the current year, it is advisable to reduce nitrogen levels. Blending compost into the soil, particularly if it lacks nutrients, can be beneficial. However, it is important to strike the right balance to prevent excessive nitrogen levels.


Aside from monarch caterpillars, other critters may feast on narrowleaf milkweed. Aphids, notorious for sucking sap from plants, pose a significant threat to milkweed if present in large numbers. Washing off these pests with water or attracting ladybugs, known to consume aphids, can help control their population. Planting milkweed away from the rest of your garden can prevent aphid outbreaks from spreading.

Blue milkweed beetles, characterized by their shiny blue-green appearance, can decimate a milkweed garden within days. Red and black milkweed bugs, namely Lygaeus kalmii and Oncopeltus fasciatus, respectively, pose another threat, as they consume milkweed seeds. While these pests may not cause the complete demise of your milkweed garden, they can significantly impact seed production.

Handpicking these pests is the most effective method of control. Chemical pesticides should be avoided, as they may harm monarch butterflies. If opting for insecticidal soaps, exercise caution to ensure their compatibility with butterfly populations.

A milkweed muncher hard at work! Source: TJ Gehling

Diseases of Asclepias Fascicularis

Ophryocystis elektroscirrha (OE), a microscopic parasite that manifests in the form of spores on milkweed, poses a significant threat to monarch butterflies. This parasite infects butterfly eggs and larvae, often resulting in deformed wings and impaired flight capabilities. By pruning milkweed during winter and specifically selecting native milkweed species for planting, the spread of OE can be prevented. Tropical milkweed varieties, such as Asclepias curassavica, while visually appealing, are not native to the US and are more likely to harbor OE.

Fungal infections may develop if narrowleaf milkweed leaves are consistently wet or if the plant is subjected to excessive moisture. Wet conditions, including heavy rainfall, can facilitate the onset of leaf infections. To prevent fungal infections, it is crucial to water only when the soil is dry and to direct water to the base of the plant. Adequate airflow is essential in preventing the rapid spread of diseases; thus, thinning out overcrowded plants is advisable.

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Should a diseased plant be identified, immediate removal of affected leaf matter is recommended. In severe cases, complete plant removal may be necessary. Fungal diseases cannot be cured; therefore, prevention is paramount. Fungal sprays may be employed, but caution should be exercised to avoid harming butterfly species.

Frequently Asked Questions

Monarch butterfly and Asclepias fascicularis. Source: TJ Gehling

Q: Is Asclepias fascicularis invasive?
A: Asclepias fascicularis, while native to the US, can exhibit weedy growth and become aggressive under certain conditions. If planted in a designated butterfly garden, this characteristic can be advantageous.

Q: Is Asclepias fascicularis native to California?
A: Asclepias fascicularis is native to most of California, as well as parts of Washington, Idaho, and Arizona.

Q: How do you take care of Asclepias fascicularis?
A: Asclepias fascicularis thrives in full sun and requires approximately 1 inch of water per week. It is drought-tolerant and does not demand excessive maintenance when provided with suitable growing conditions.

Q: How do you plant Asclepias fascicularis?
A: Press the seeds into the soil and water generously, ensuring that the seeds are not covered by soil. Since the seeds require light for germination, refrain from burying them. Germination may continue for up to two months after planting, so consistent watering is crucial until the plants are fully established.

Q: Where should you not plant milkweed?
A: It is advisable to avoid planting milkweed near monarch butterfly overwintering sites, as this may disrupt their life cycle. Additionally, consider planting milkweed away from vegetable beds to prevent aphid infestation. It is essential to keep milkweed out of reach of pets and small children, as it contains toxic compounds.

Q: Will milkweed take over my garden?
A: In some instances, milkweed may spread and dominate a garden. To prevent this, designate a specific area for milkweed cultivation to allow it to grow freely.

Q: Is Asclepias fascicularis good for monarch butterflies?
A: Asclepias fascicularis is an excellent choice for supporting monarch butterflies. Its native status in the US ensures a safe and suitable food source for monarch caterpillars.

Q: Which milkweed is best for monarchs in Southern California?
A: Asclepias fascicularis is highly recommended for monarchs in Southern California, as it is native to the region and thrives in its climate.

Q: How fast does milkweed grow?
A: Milkweed exhibits rapid growth, with germination typically occurring within 7-10 days. The plants are ready for caterpillar consumption approximately one month after planting.

By embracing the cultivation of narrow-leaf milkweed, you not only add an enchanting touch to your garden but also contribute to the conservation of monarch butterflies. With its inherent allure and significance to these magnificent creatures, Asclepias fascicularis serves as a testament to the wonders of nature’s intricate web of life. Let your garden become a sanctuary for monarch butterflies – a testament to the beauty and vitality they bring to our world.

For more information on narrow-leaf milkweed and other gardening tips, visit the Ames Farm Center.