Asclepias Fascicularis: The Narrowleaf Milkweed

Monarch butterflies are some of the simply recognizable butterflies and are essential pollinators. They’re liable to endangerment, so many organizations work onerous to maintain them thriving and off the endangered listing. Slim-leaf milkweed, Asclepias fascicularis, is a key a part of monarch conservation as a result of monarch caterpillars are restricted to a strict weight loss program 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 to your personal enjoyment, you’ll admire this addition to the backyard. You’ll want to 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 take a look at find out how to develop narrow-leaf milkweed – also referred to as Mexican whorled milkweed – so you’ll be able to add this sun-loving flower to your butterfly gardens. The monarchs and different critters will thanks for it.

Fast Care Information

Narrowleaf milkweed flowers. Supply: David A. Hofmann
Widespread 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 broad
Gentle Full solar
Soil Effectively-draining, tolerant of poor soils
Water 1 inch per week, drought-tolerant
Pests & Ailments Aphids, blue milkweed beetles, milkweed bugs, fungal ailments

All About Slim-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 referred to as 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 critical 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 of native California milkweed. It may also be discovered naturally within the disturbed grounds of some elements of Idaho, northern Arizona, and Colorado. The plant loves full solar and might face up to some drought, but it surely doesn’t develop effectively in deserts.

As a result of it’s a perennial herb, the narrowleaf milkweed is a plant you can count on to come back again. It additionally self-seeds, so as soon as you identify them in your backyard, they need to break up open, spill seeds, and reliably come again annually. Look out for the erect slim leaves of those milkweeds and distinctive flower to tell apart it from close by grass crops. 

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 time and fall, and you’ll harvest the seed pods in summer time.

After the pale pink or white flowers of this perennial herb bloom and fade, massive 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 colour is mild brown, and their form is disc-like and hooked up to the plentiful silky hairs. Because the pods break 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 sort of milkweed on the market, so you’ll be able to plant it with different Asclepias species or with different crops that appeal to monarch butterflies just like the California aster, Indian mallow, or herbs like mint, lavender, and various kinds of sage. These different crops will carry 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. If in case you have curious children and pets, take into account planting them in a gated space. It’s good follow to all the time put on gloves while you deal with milkweed and wash your arms 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 onerous about maintaining 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 crops are finest fitted to hardiness zones 6-10, the place they are going to die again in winter however will come again as soon as the temperatures get heat once more. You might be able to develop it as an annual in colder climates, however don’t count on them to come back again after a protracted winter of freezing temperatures.

Water and Humidity

Narrowleaf milkweed crops are drought-tolerant and don’t require a lot water. It would solely want about 1 inch of water per week, however it may possibly 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.

If you water your crops, 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’ll contract can hurt monarchs. Due to this fact, in the summertime, water with soaker hoses or drip irrigation. Cautious hand irrigation can also be acceptable. 


The Asclepias fascicularis plant wants well-draining soil to forestall standing water. These Asclepias crops are tolerant of poor soil, like clay soils or silty ones, making them simple to look after even in uncared for flower beds. These crops develop effectively in clay soil however can tolerate different kinds, 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’ll tolerate barely acidic or alkaline clay soil, as effectively.


Milkweeds usually don’t want fertilizing since they’re native to North America and might tolerate poor soils. If in case you have 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 slim milkweeds will produce extra whorled skinny leaf development on their stem quite than pretty clusters of blooms. 


Pruning of the slim leaves or lavender flowers isn’t vital 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 development within the spring.

Narrowleaf Milkweed Propagation

Narrowleaf milkweed crops are finest propagated by seed. The seed of those Asclepias crops are simple 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 light amidst the slim whorled leaf sample. Make sure to put on gloves while 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, strive storing the slim 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 it’s worthwhile to 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 the explanation why your narrowleaf milkweed crops received’t germinate from seed. It’s doable 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 development. 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 doubtless might be subsequent 12 months.

When 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 development and might inhibit the expansion of flowers on crops. Milkweed species want little to no fertilizer, so make certain you don’t unintentionally overfertilize them.


Monarch caterpillars aren’t the one critter that likes to munch on narrowleaf milkweed. Aphids wish to suck the sap out of the plant, which might kill the milkweed in massive numbers. You possibly can wash off aphids with water or appeal to ladybugs to the realm to allow them to eat them. Aphids love milkweed, so you might need 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 only a few days. Crimson and black milkweed bugs, Lygaeus kalmii and Oncopeltus fasciatus, are two forms of pests that eat milkweed seed. They could not trigger your whole milkweed backyard to die, however they’ll severely have an effect on the unfold by limiting how a lot seed you have got.

One of the best ways to take away these pests is by hand. We don’t suggest utilizing any chemical substances like pesticides since they might additionally hurt the monarch butterfly. Insecticidal soaps are one other technique, however make certain they received’t damage the butterflies earlier than you employ them.

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

Ailments 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 might infect its eggs and larvae. The contaminated bugs will doubtless have deformed wings and received’t have the ability to fly usually. Some monarchs might seem wholesome however nonetheless be contaminated. 

Chopping again milkweed within the winter can stop the unfold of this parasite to the beloved butterfly. You can even 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 more likely 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 also can 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 crops 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 doable. It could be finest to take away your complete 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 risk to butterfly species, so watch out should you determine to go this route.

Regularly 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 of invasive. Nonetheless, it may possibly turn into weedy and develop aggressively if the circumstances are proper. If in case you have a chosen area for them in a butterfly backyard, this isn’t essentially a foul factor!

Q: Is Asclepias fascicularis native to California?

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

Q: How do you handle Asclepias fascicularis?

A: Asclepias fascicularis wants full solar and about an inch of water every week. It’s drought-tolerant and might largely be left alone when grown in the best 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 mild to germinate, so don’t cowl them up with soil. Seeds might proceed germinating for as much as two months after planting, so hold the realm well-watered till the crops are established.

Q: The place do you have to 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 house gardeners who don’t dwell close to these websites, you might need 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 chosen spot for milkweed so it may possibly 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 realm 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 might be prepared for larvae to eat about one month after planting.

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