How to Plant, Grow, and Care for Asclepias Fascicularis

Monarch butterflies are among the most easily recognizable and 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 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.

What is Asclepias Fascicularis?

The narrowleaf milkweed (Asclepias fascicularis, aka its earlier scientific name, Asclepias mexicana), also known as Mexican whorled milkweed, is a perennial herb. 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. 

Native Area

This herb is native to North America and Central America, where it grows in yellow pine forests, moderate slopes, valleys, and grasslands. It 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.

Characteristics

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 return each year. Look out for the erect narrow leaves of these milkweeds and distinctive flowers to distinguish it from nearby grass plants. 

The plant has erect stems that can grow up to three feet tall. Surrounding the stems are distinctive, long, pointed thin leaves that grow up to six 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. They bloom throughout summer and fall.

After the flowers fade, large seed pods that look like horns. 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.  

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.

Planting

Press Asclepias fascicularis 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.

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. 

Milkweed grows fairly quickly. It should germinate in seven to ten days and will be ready for larvae to eat about one month after planting.

How to Grow

Once 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!

Light

Asclepias fascicularis thrives in full sun and warm weather. It needs at least six hours of direct sun but is also somewhat shade-tolerant. In most areas, it does what it can to survive, so it is not very fussy about the conditions.

Water

Narrowleaf milkweed plants are drought-tolerant and don’t require much water. It will only need about one 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

Asclepias fascicularis needs well-draining soil to prevent standing water. These Asclepias plants are tolerant of poor soil, like clay or silty ones, making them easy to care for even in neglected flower beds. They 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 also tolerate slightly acidic or alkaline clay soil.

Temperature

They are best suited for hardiness zones 6-10, where they will die back in winter but return once the temperatures get warm. 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.

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. 

Maintenance

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 six inches in the winter. This will also help promote new growth in the spring.

Propagation

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

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. 

Common Problems

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 (29°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.

Diseases

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. Many fungal diseases can’t be cured, so all you can do is prevent the spread.

Frequently Asked Questions