Asclepias Fascicularis: The Narrowleaf Milkweed

Monarch butterflies are some of the simply recognizable butterflies and are vital pollinators. They’re vulnerable to endangerment, so many organizations work exhausting to maintain them thriving and off the endangered record. Slender-leaf milkweed, Asclepias fascicularis, is a key a part of monarch conservation as a result of monarch caterpillars are restricted to a strict eating regimen of milkweed.

Asclepias fascicularis is a beautiful plant that boasts clusters of small flowers. Whether or not you select to develop them for the monarch butterflies or on your personal enjoyment, you’ll respect this addition to the backyard. You should watch out when rising narrow-leaf milkweed as a result of it’s poisonous to people and livestock and, subsequently, deer-resistant. By no means eat it, and all the time put on gloves when dealing with it. Save the munching for the caterpillars!

Let’s have a look at how one can develop narrow-leaf milkweed – also called Mexican whorled milkweed – so you possibly can add this sun-loving flower to your butterfly gardens. The monarchs and different critters will thanks for it.

Fast Care Information

Narrowleaf milkweed flowers
Narrowleaf milkweed flowers. Supply: David A. Hofmann
Frequent Title Narrowleaf milkweed, Mexican whorled milkweed, narrow-leaved milkweed
Scientific Title Asclepias fascicularis (aka Asclepias mexicana)
Household Apocynaceae
Top & Unfold 1-3 ft tall, 1 foot vast
Mild Full solar
Soil Effectively-draining, tolerant of poor soils
Water 1 inch per week, drought-tolerant
Pests & Illnesses Aphids, blue milkweed beetles, milkweed bugs, fungal ailments

All About Slender-Leaf Milkweed

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

The narrowleaf milkweed (Asclepias fascicularis, aka its earlier scientific title, Asclepias mexicana), also called Mexican whorled milkweed, is a perennial herb native to North America and Central America within the yellow pine forest, reasonable slopes, valleys, and grasslands. Like different milkweeds, it is a vital host plant for monarch butterflies, useful bugs, and wildlife and is important for monarch caterpillars to feed on. 

Asclepias fascicularis can naturally be present in northeast Washington all the way down to Baja California, the place it’s thought-about native California milkweed. It can be discovered naturally within the disturbed grounds of some components of Idaho, northern Arizona, and Colorado. The plant loves full solar and may stand up to some drought, nevertheless it doesn’t develop effectively in deserts.

As a result of it’s a perennial herb, the narrowleaf milkweed is a plant that you could count on to return again. It additionally self-seeds, so as soon as you determine them in your backyard, they need to cut up open, spill seeds, and reliably come again annually. Look out for the erect slender leaves of those milkweeds and distinctive flower to differentiate it from close by grass vegetation. 

The plant has erect stems that may develop as much as 3 ft tall. Surrounding the erect stems are distinctive, lengthy pointed leaves which can be skinny and develop as much as 6 inches lengthy. The umbels stand erect on the stem as effectively and are made up of a number of small flowers that may be barely inexperienced, white, purple, or pale pink. These a number of umbels bloom all through summer season and fall, and you may harvest the seed pods in summer season.

After the pale pink or white flowers of this perennial herb bloom and fade, giant seed pods that seem like horns protrude from the distinctive lengthy pointed leaves. Inside these seed pods are plentiful silky hairs that help the plant in propagation. The seed shade is gentle brown, and their form is disc-like and connected to the plentiful silky hairs. Because the pods cut up open to spill seeds on the earth, they’re generally swept up within the wind to germinate elsewhere.  

Mexican whorled milkweed (Asclepias fascicularis) isn’t the one type of milkweed on the market, so you possibly can plant it with different Asclepias species or with different vegetation that appeal to monarch butterflies just like the California aster, Indian mallow, or herbs like mint, lavender, and completely different sorts of sage. These different vegetation will deliver monarchs to your backyard, however keep in mind, it’s solely the milkweed the caterpillars will eat.

The milkweed household, and all species throughout the household, are poisonous to people and animals, as they include cardiac glycosides. So it’s finest to plant them away out of your edible backyard or fields the place livestock graze. When you’ve got curious children and pets, contemplate planting them in a gated space. It’s good follow to all the time put on gloves whenever you deal with milkweed and wash your palms completely after dealing with them.

Mexican Whorled Milkweed Care

A. fascicularis foliage
A. fascicularis foliage with out flowers. Supply: chuck b.

As soon as Mexican whorled milkweed (Asclepias fascicularis) is established in your backyard, you received’t should assume too exhausting about preserving it alive! It’s low upkeep and has few pest and illness points. It’s even deer-resistant!

Solar and Temperature

Asclepias fascicularis requires solar and heat climate to thrive. It wants not less than 6 hours of direct solar and however is considerably shade tolerant. Narrowleaf milkweed vegetation are finest suited to hardiness zones 6-10, the place they may die again in winter however will come again as soon as the temperatures get heat once more. You could possibly develop it as an annual in colder climates, however don’t count on them to return again after a protracted winter of freezing temperatures.

Water and Humidity

Narrowleaf milkweed vegetation are drought-tolerant and don’t require a lot water. It’s going to solely want about 1 inch of water per week, however it could actually deal with some dry climates. Nonetheless, it doesn’t develop effectively in deserts, so attempt to provide your milkweed with water every week to maintain them completely satisfied. Moist soils are finest.

Whenever you water your vegetation, water on the base to forestall the leaves and lavender or purple flowers from getting moist. Moist leaves can enhance the possibility of fungal infections or different ailments, and a few of the ailments they will contract can hurt monarchs. Due to this fact, in the summertime, water with soaker hoses or drip irrigation. Cautious hand irrigation can be acceptable. 


The Asclepias fascicularis plant wants well-draining soil to forestall standing water. These Asclepias vegetation are tolerant of poor soil, like clay soils or silty ones, making them straightforward to look after even in uncared for flower beds. These vegetation develop effectively in clay soil however can tolerate other forms, so long as water can drain. They develop effectively on slopes and valleys since water can simply drain away from the roots. A impartial pH degree is finest, however they will tolerate barely acidic or alkaline clay soil, as effectively.


Milkweeds usually don’t want fertilizing since they’re native to North America and may tolerate poor soils. When you’ve got a flower mattress that’s fully devoid of vitamins, including compost to the soil could also be useful. Remember the fact that too many vitamins, particularly nitrogen, can stop the flowers’ lavender or purple clusters from blooming. Nitrogen-fertilized slender milkweeds will produce extra whorled skinny leaf progress on their stem slightly than beautiful clusters of blooms. 


Pruning of the slender leaves or lavender flowers isn’t needed for this Asclepias to thrive, however doing so within the winter can stop the event of a dangerous parasite known as Ophryocystis elektroscirrha, or OE, that infects the monarch butterfly and causes deformed wings. You possibly can lower the skinny stem of the plant again to six inches within the winter. This may also assist promote new progress within the spring.

Narrowleaf Milkweed Propagation

Narrowleaf milkweed vegetation are finest propagated by seed. The seed of those Asclepias vegetation are straightforward to gather as a result of they develop in seed pods that hold off the plant, which could be picked late July after the flower has bloomed and pale amidst the slender whorled leaf sample. Be sure you put on gloves whenever you deal with any a part of the Asclepias plant.

Like all milkweeds, seed germination will solely happen if the seed has been uncovered to chilly. If it doesn’t get chilly in your area, attempt storing the slender Asclepias fascicularis seed in sand in a plastic bag within the freezer for 2 months, after which unfold them in your backyard in early spring. 

Troubleshooting Asclepias fascicularis

Asclepias fascicularis
Asclepias fascicularis. Supply: Mathesont

Asclepias fascicularis has only a few pests and illness issues, however there are some you could look out for that may be detrimental to the monarch butterfly. Let’s check out what these issues are and how one can repair or stop them.

Rising Issues

There are a few explanation why your narrowleaf milkweed vegetation received’t germinate from seed. It’s potential that they weren’t uncovered to chilly sufficient climate or that it’s too sizzling. Seeds want some publicity to chilly temperatures to allow them to germinate within the spring, they usually can’t germinate if the temperature is above 85°F.

If any species of milkweeds you plant aren’t rising any flowers the primary 12 months, it’s most likely as a result of they’re centered on root progress. They’re a perennial plant that may come again 12 months after 12 months, they usually want robust roots to take action. In the event that they’re not blooming flowers the 12 months you planted them, they possible will probably be subsequent 12 months.

In the event you didn’t plant milkweed seeds this 12 months and also you don’t see any blooms, they might have an excessive amount of nitrogen. Nitrogen encourages leaf progress and may inhibit the expansion of flowers on vegetation. Milkweed species want little to no fertilizer, so be certain you don’t unintentionally overfertilize them.


Monarch caterpillars aren’t the one critter that likes to munch on narrowleaf milkweed. Aphids prefer to suck the sap out of the plant, which might kill the milkweed in giant numbers. You possibly can wash off aphids with water or appeal to ladybugs to the world to allow them to eat them. Aphids love milkweed, so you could wish to plant milkweed away from the remainder of your backyard to forestall outbreaks from spreading.

Blue milkweed beetles are shiny blue-green beetles that may decimate a milkweed backyard in just some days. Purple and black milkweed bugs, Lygaeus kalmii and Oncopeltus fasciatus, are two sorts of pests that eat milkweed seed. They could not trigger your total milkweed backyard to die, however they will severely have an effect on the unfold by limiting how a lot seed you might have.

The easiest way to take away these pests is by hand. We don’t advocate utilizing any chemical substances like pesticides since they might additionally hurt the monarch butterfly. Insecticidal soaps are one other technique, however be certain they received’t damage the butterflies earlier than you utilize them.

Milkweed muncher
A milkweed muncher exhausting at work! Supply: TJ Gehling

Illnesses Of Asclepias Fascicularis

The parasite talked about earlier, Ophryocystis elektroscirrha (OE), is a microscopic parasite that lives on milkweed within the type of spores. They connect to the monarch butterfly and may infect its eggs and larvae. The contaminated bugs will possible have deformed wings and received’t be capable of fly usually. Some monarchs might seem wholesome however nonetheless be contaminated. 

Reducing again milkweed within the winter can stop the unfold of this parasite to the beloved butterfly. It’s also possible to stop the unfold by solely planting milkweed species which can be native to the place you reside. The tropical milkweed plant (Asclepias curassavica) is a beautiful selection however isn’t native to the US and is extra prone to carry OE.

A narrowleaf milkweed plant might develop fungal infections if their leaves get moist when watered or in the event that they obtain an excessive amount of water. An excessive amount of rain may also trigger leaf infections, which is why well-draining soil is a should. You possibly can stop fungal infections by solely watering when the soil is dry and solely watering on the base of the plant. If vegetation are overcrowded, take away some to extend airflow so ailments can’t unfold as rapidly.

Take away any diseased leaf matter should you discover a diseased plant as quickly as potential. It could be finest to take away your entire plant. Fungal ailments can’t be cured, so all you are able to do is stop the unfold. You possibly can stop fungal ailments with fungal sprays, however these might pose a menace to butterfly species, so watch out should you determine to go this route.

Steadily Requested Questions

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

Q: Is Asclepias fascicularis invasive?

A: The Asclepias fascicularis plant is native to the US and often isn’t thought-about invasive. Nonetheless, it could actually change into weedy and develop aggressively if the circumstances are proper. When you’ve got a delegated area for them in a butterfly backyard, this isn’t essentially a nasty factor!

Q: Is Asclepias fascicularis native to California?

A: The species Asclepias fascicularis is native to most of California in addition to components of Washington, Idaho, and Arizona.

Q: How do you deal with Asclepias fascicularis?

A: Asclepias fascicularis wants full solar and about an inch of water every week. It’s drought-tolerant and may largely be left alone when grown in the appropriate circumstances, making it a straightforward plant to look after.

Q: How do you plant Asclepias fascicularis?

A: Press the seeds into the soil and water them generously. The seeds want gentle to germinate, so don’t cowl them up with soil. Seeds might proceed germinating for as much as two months after planting, so preserve the world well-watered till the vegetation are established.

Q: The place must you not plant milkweed?

A: You shouldn’t plant milkweed close to monarch butterfly overwintering websites. The presence of milkweed might encourage them to proceed mating, which might disrupt their lifecycle. For dwelling gardeners who don’t stay close to these websites, you could wish to keep away from planting them close to your vegetable beds since they usually appeal to aphids. Additionally, don’t plant them the place pets or babies can attain them as they include poisonous compounds. 

Q: Will milkweed take over my backyard?

A: In some circumstances, milkweed would possibly take over your backyard. It’s finest to decide on a delegated spot for milkweed so it could actually develop freely.

Q: Is Asclepias fascicularis good for monarch butterflies?

A: Asclepias fascicularis is nice for monarchs! The caterpillars solely eat milkweed, and Asclepias fascicularis is native to the US, so it’s a secure choice to develop.

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

A: Asclepias fascicularis is finest for monarchs in Southern California as a result of it’s native to the world and grows effectively within the local weather.

Q: How briskly does milkweed develop?

A: Milkweed grows pretty rapidly. It ought to germinate in 7-10 days and will probably be prepared for larvae to eat about one month after planting.

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