Asclepias Fascicularis, The Narrow-Leaf Milkweed

Asclepias fascicularis is a common sight in pollinator gardens. Learn more about this narrow-leaf milkweed and how to grow it with our guide!

Monarch butterfly and Asclepias fascicularis

Contents

Monarch butterflies are one of the most easily recognizable butterflies and are important pollinators. They’re at risk of endangerment, so many organizations work hard to keep them thriving and off the endangered list. Narrow-leaf milkweed, Asclepias fascicularis, is a key part of monarch conservation because monarch caterpillars are limited to a strict diet of milkweed.

Asclepias fascicularis is a gorgeous plant that boasts clusters of small flowers. Whether you choose to grow them for the monarch butterflies or for your own enjoyment, you’ll appreciate this addition to the garden. You need to be careful when growing narrow-leaf milkweed because it’s toxic to humans and livestock and, therefore, deer-resistant. Never eat it, and always wear gloves when handling it. Save the munching for the caterpillars!

Let’s look at how to grow narrow-leaf milkweed – also known as Mexican whorled milkweed – so you can add this sun-loving flower to your butterfly gardens. The monarchs and other critters will thank you for it.

Quick Care Guide

Narrowleaf milkweed flowers
Narrowleaf milkweed flowers. Source: David A. Hofmann
Common NameNarrowleaf milkweed, Mexican whorled milkweed, narrow-leaved milkweed
Scientific NameAsclepias fascicularis (aka Asclepias mexicana)
FamilyApocynaceae
Height & Spread1-3 feet tall, 1 foot wide
LightFull sun
SoilWell-draining, tolerant of poor soils
Water1 inch per week, drought-tolerant
Pests & DiseasesAphids, blue milkweed beetles, milkweed bugs, fungal diseases

All About Narrow-Leaf Milkweed

Monarch larvae munching A. fascularis
A monarch larvae munching A. fascicularis. Source: TJ Gehling

The narrowleaf milkweed (Asclepias fascicularis, aka its earlier scientific name, Asclepias mexicana), also known as Mexican whorled milkweed, is a perennial herb native to North America and Central America in the yellow pine forest, moderate slopes, valleys, and grasslands. Like other milkweeds, it is an important host plant for monarch butterflies, beneficial insects, and wildlife and is necessary for monarch caterpillars to feed on. 

Asclepias fascicularis can naturally be found in northeast Washington down to Baja California, where it is considered native California milkweed. It can also be found naturally in the disturbed grounds of some parts of Idaho, northern Arizona, and Colorado. The plant loves full sun and can withstand some drought, but it doesn’t grow well in deserts.

Because it is a perennial herb, the narrowleaf milkweed is a plant that you can expect to come back. It also self-seeds, so once you establish them in your garden, they should split open, spill seeds, and reliably come back each year. Look out for the erect narrow leaves of these milkweeds and distinctive flower to distinguish it from nearby grass plants. 

The plant has erect stems that can grow up to 3 feet tall. Surrounding the erect stems are distinctive, long pointed leaves that are thin and grow up to 6 inches long. The umbels stand erect on the stem as well and are made up of several small flowers that can be slightly green, white, purple, or pale pink. These several umbels bloom throughout summer and fall, and you can harvest the seed pods in summer.

After the pale pink or white flowers of this perennial herb bloom and fade, large seed pods that look like horns protrude from the distinctive long pointed leaves. Within these seed pods are plentiful silky hairs that assist the plant in propagation. The seed color is light brown, and their shape is disc-like and attached to the plentiful silky hairs. As the pods split open to spill seeds on the earth, they are sometimes swept up in the wind to germinate elsewhere.  

Mexican whorled milkweed (Asclepias fascicularis) isn’t the only kind of milkweed out there, so you can plant it with other Asclepias species or with other plants that attract monarch butterflies like the California aster, Indian mallow, or herbs like mint, lavender, and different types of sage. These other plants will bring monarchs to your garden, but remember, it’s only the milkweed the caterpillars will eat.

The milkweed family, and all species within the family, are toxic to humans and animals, as they contain cardiac glycosides. So it’s best to plant them away from your edible garden or fields where livestock graze. If you have curious kids and pets, consider planting them in a gated area. It’s good practice to always wear gloves when you handle milkweed and wash your hands thoroughly after handling them.

Mexican Whorled Milkweed Care

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

Once Mexican whorled milkweed (Asclepias fascicularis) is established in your garden, you won’t have to think too hard about keeping it alive! It’s low maintenance and has few pest and disease issues. It’s even deer-resistant!

Sun and Temperature

Asclepias fascicularis requires sun and warm weather to thrive. It needs at least 6 hours of direct sun and but is somewhat shade tolerant. Narrowleaf milkweed plants are best suited for hardiness zones 6-10, where they will die back in winter but will come back once the temperatures get warm again. You may be able to grow it as an annual in colder climates, but don’t expect them to come back after a long winter of freezing temperatures.

Water and Humidity

Narrowleaf milkweed plants are drought-tolerant and don’t require much water. It will only need about 1 inch of water per week, but it can handle some dry climates. However, it doesn’t grow well in deserts, so try to supply your milkweed with water each week to keep them happy. Moist soils are best.

When you water your plants, water at the base to prevent the leaves and lavender or purple flowers from getting wet. Wet leaves can increase the chance of fungal infections or other diseases, and some of the diseases they can contract can harm monarchs. Therefore, in the summer, water with soaker hoses or drip irrigation. Careful hand irrigation is also acceptable. 

Soil

The Asclepias fascicularis plant needs well-draining soil to prevent standing water. These Asclepias plants are tolerant of poor soil, like clay soils or silty ones, making them easy to care for even in neglected flower beds. These plants grow well in clay soil but can tolerate other kinds, as long as water can drain. They grow well on slopes and valleys since water can easily drain away from the roots. A neutral pH level is best, but they can tolerate slightly acidic or alkaline clay soil, as well.

Fertilizing

Milkweeds generally don’t need fertilizing since they’re native to North America and can tolerate poor soils. If you have a flower bed that’s completely devoid of nutrients, adding compost to the soil may be beneficial. Keep in mind that too many nutrients, especially nitrogen, can prevent the flowers’ lavender or purple clusters from blooming. Nitrogen-fertilized narrow milkweeds will produce more whorled thin leaf growth on their stem rather than lovely clusters of blooms. 

Pruning

Pruning of the narrow leaves or lavender flowers isn’t necessary for this Asclepias to thrive, but doing so in the winter can prevent the development of a harmful parasite called Ophryocystis elektroscirrha, or OE, that infects the monarch butterfly and causes deformed wings. You can cut the thin stem of the plant back to 6 inches in the winter. This will also help promote new growth in the spring.

Narrowleaf Milkweed Propagation

Narrowleaf milkweed plants are best propagated by seed. The seed of these Asclepias plants are easy to collect because they develop in seed pods that hang off the plant, which can be picked late July after the flower has bloomed and faded amidst the narrow whorled leaf pattern. Be sure to wear gloves when you handle any part of the Asclepias plant.

Like all milkweeds, seed germination will only occur if the seed has been exposed to cold. If it doesn’t get cold in your region, try storing the narrow Asclepias fascicularis seed in sand in a plastic bag in the freezer for two months, and then spread them in your garden in early spring. 

Troubleshooting Asclepias fascicularis

Asclepias fascicularis
Asclepias fascicularis. Source: Mathesont

Asclepias fascicularis has very few pests and disease problems, but there are some you need to look out for that can be detrimental to the monarch butterfly. Let’s take a look at what these problems are and how you can fix or prevent them.

Growing Problems

There are a couple of reasons why your narrowleaf milkweed plants won’t germinate from seed. It’s possible that they weren’t exposed to cold enough weather or that it’s too hot. Seeds need some exposure to cold temperatures so they can germinate in the spring, and they can’t germinate if the temperature is above 85°F.

If any species of milkweeds you plant aren’t growing any flowers the first year, it’s probably because they’re focused on root growth. They’re a perennial plant that will come back year after year, and they need strong roots to do so. If they’re not blooming flowers the year you planted them, they likely will be next year.

If you didn’t plant milkweed seeds this year and you don’t see any blooms, they may have too much nitrogen. Nitrogen encourages leaf growth and can inhibit the growth of flowers on plants. Milkweed species need little to no fertilizer, so be sure you don’t accidentally overfertilize them.

Pests

Monarch caterpillars aren’t the only critter that likes to munch on narrowleaf milkweed. Aphids like to suck the sap out of the plant, which can kill the milkweed in large numbers. You can wash off aphids with water or attract ladybugs to the area so they can eat them. Aphids love milkweed, so you may want to plant milkweed away from the rest of your garden to prevent outbreaks from spreading.

Blue milkweed beetles are shiny blue-green beetles that can decimate a milkweed garden in just a few days. Red and black milkweed bugs, Lygaeus kalmii and Oncopeltus fasciatus, are two types of pests that eat milkweed seed. They may not cause your entire milkweed garden to die, but they can severely affect the spread by limiting how much seed you have.

The best way to remove these pests is by hand. We don’t recommend using any chemicals like pesticides since they may also harm the monarch butterfly. Insecticidal soaps are another method, but be sure they won’t hurt the butterflies before you use them.

Milkweed muncher
A milkweed muncher hard at work! Source: TJ Gehling

Diseases Of Asclepias Fascicularis

The parasite mentioned earlier, Ophryocystis elektroscirrha (OE), is a microscopic parasite that lives on milkweed in the form of spores. They attach to the monarch butterfly and can infect its eggs and larvae. The infected insects will likely have deformed wings and won’t be able to fly normally. Some monarchs may appear healthy but still be infected. 

Cutting back milkweed in the winter can prevent the spread of this parasite to the beloved butterfly. You can also prevent the spread by only planting milkweed species that are native to where you live. The tropical milkweed plant (Asclepias curassavica) is a gorgeous variety but isn’t native to the US and is more likely to carry OE.

A narrowleaf milkweed plant may develop fungal infections if their leaves get wet when watered or if they receive too much water. Too much rain can also cause leaf infections, which is why well-draining soil is a must. You can prevent fungal infections by only watering when the soil is dry and only watering at the base of the plant. If plants are overcrowded, remove some to increase airflow so diseases can’t spread as quickly.

Remove any diseased leaf matter if you find a diseased plant as soon as possible. It may be best to remove the entire plant. Fungal diseases can’t be cured, so all you can do is prevent the spread. You can prevent fungal diseases with fungal sprays, but these may pose a threat to butterfly species, so be careful if you decide to go this route.

Frequently Asked Questions

Monarch butterfly and Asclepias fascicularis
Monarch butterfly and Asclepias fascicularis. Source: TJ Gehling

Q: Is Asclepias fascicularis invasive?

A: The Asclepias fascicularis plant is native to the US and usually isn’t considered invasive. However, it can become weedy and grow aggressively if the conditions are right. If you have a designated space for them in a butterfly garden, this isn’t necessarily a bad thing!

Q: Is Asclepias fascicularis native to California?

A: The species 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 needs full sun and about an inch of water each week. It’s drought-tolerant and can mostly be left alone when grown in the right conditions, making it an easy plant to care for.

Q: How do you plant Asclepias fascicularis?

A: Press the seeds into the soil and water them generously. The seeds need light to germinate, so don’t cover them up with soil. Seeds may continue germinating for up to two months after planting, so keep the area well-watered until the plants are established.

Q: Where should you not plant milkweed?

A: You shouldn’t plant milkweed near monarch butterfly overwintering sites. The presence of milkweed may encourage them to continue mating, which can disrupt their lifecycle. For home gardeners who don’t live near these sites, you may want to avoid planting them near your vegetable beds since they often attract aphids. Also, do not plant them where pets or small children can reach them as they contain toxic compounds. 

Q: Will milkweed take over my garden?

A: In some cases, milkweed might take over your garden. It’s best to choose a designated spot for milkweed so it can grow freely.

Q: Is Asclepias fascicularis good for monarch butterflies?

A: Asclepias fascicularis is great for monarchs! The caterpillars only eat milkweed, and Asclepias fascicularis is native to the US, so it’s a safe option to grow.

Q: Which milkweed is best for monarchs in Southern California?

A: Asclepias fascicularis is best for monarchs in Southern California because it’s native to the area and grows well in the climate.

Q: How fast does milkweed grow?

A: Milkweed grows fairly quickly. It should germinate in 7-10 days and will be ready for larvae to eat about one month after planting.

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