Don’t Make This Mistake When You Plant Milkweed (A How-To Guide)

This post is in partnership with Greenwood Nursery. All thoughts and words are my own.

It’s one of the greatest migrations of the animal kingdom: Every spring, hundreds of thousands of monarch butterflies sweep across the continent from Mexico to Canada and then back again in fall, a journey spanning nearly 3,000 miles and multiple generations.

In fact, recent studies show that it takes up to four generations for monarchs to make it north out of Mexico and into Southern Canada! Along the way, the butterflies feed on a variety of nectar-rich flowers but breed on only type of plant: milkweed.

There are 140 species of milkweed, but only a quarter of them are known to be important host plants for monarchs and even less are available commercially for planting.

How can you help these butterflies along the way? By growing your own milkweed—but more importantly, the right kind of milkweed.

Why milkweed plants are so important for monarchs

When you think about it, it’s rather remarkable how a butterfly can spot a single milkweed from the sky. She spots the milkweed’s characteristic cluster of petite, star-shaped flowers, lands on it, scratches it with her front legs, tastes it with her feet, and confirms that it’s indeed milkweed. She then proceeds to lay her egg on a leaf—and only on the leaf of a milkweed plant.

This is because monarch caterpillars depend on the plant for food, eating every leaf in their path until they complete their metamorphosis (a cycle that takes about four weeks). Unlike adult monarchs (butterflies), caterpillars cannot simply feed on any plant they please.

And that might lead you to wonder: Why milkweed?

Perhaps it’s by natural design: The chemical compounds in the plant make the caterpillars toxic to potential predators (such as birds), with the sap of certain species of milkweed being more toxic than others. (The toxicity effect—a built-in defense mechanism—is also found in other striped caterpillars.)

The compounds, however, have no adverse effect on the caterpillars themselves. They spend up to two weeks eating, growing, and molting on the same milkweed plant before they pupate.

The striking appearance of the butterflies and caterpillars also serves as a warning to predators that they are toxic. Predators learn to associate the colors and patterns with a bad taste and generally avoid preying on monarchs altogether.

Read next: How to identify the green caterpillars in your garden

Unfortunately for the monarchs, milkweed populations have dwindled due to the use of herbicides in crop fields, where milkweed pops up abundantly between rows of corn and soybeans in the Midwest. It’s no accident that common milkweed is called as such because of its invasiveness as a weed! (Albeit an edible weed for us humans.)

But loss of habitat is only one part of the problem. Also to blame are climate change, drought, and disease—the culprits responsible for not only the decimation of milkweed, but also other nectar sources for adult monarchs (and all types of butterflies, for that matter).

The key to supporting monarch butterflies

Growing milkweed is just one piece of the puzzle. To support the monarch butterfly population (as well as other pollinators), it’s important to plant other nectar-rich flowers in the garden that bloom at different times so adult monarchs have continuous food sources.

After all, they’re not likely to stay and lay eggs on your milkweed plants if they themselves don’t have flowers to feed on.

Colorful annuals like zinnias and phlox are attractive to butterflies as well as different types of bees and even hummingbirds, who love loosely clustered flowers. Growing a good variety of annuals also helps sustain all the beneficial insects, adds beauty to your yard, and contributes to the overall health of the local ecosystem.

Read more: Make this easy hummingbird food recipe when no flowers are in bloom

Equally important are perennials, especially native perennials like echinacea (coneflowers) and coreopsis, that can provide a constant and reliable source of food.

For perennials that do double duty as food for butterflies and food for you, consider planting flowering herbs like fennel, rosemary, sage, and oregano to round out your butterfly garden.

Related: These are the best types of flowers to grow for bees

Choosing the right species for your garden

The next step is to plant milkweed in your garden. More importantly, strive to plant native milkweed for the monarchs.

Planting non-native milkweed is a common mistake that many people make.

It’s not by any fault of their own, however: One of the most popular (and non-native) milkweed species, Asclepias curassavica or tropical milkweed, is widely sold in nurseries. It’s also been found to do more harm than good.

When grown in warm climates where the plants don’t die back, the year-round tropical milkweed encourages monarch butterflies to overwinter in the Southern United States rather than continue their migration to Mexico.

By hatching and feeding on tropical milkweed, caterpillars have an increased chance of being infected with a protozoan parasite called Ophryocystis elektroscirrha (OE), which weakens them as adults.

OE is found in all species of milkweed, but isn’t a problem with natives because the leaves are only desirable to caterpillars in the early stages of growth. Once the plant goes dormant in winter and loses its leaves, OE is no longer a threat.

But with tropical milkweed, the leaves stay viable all winter long and make it more likely that a caterpillar will become infected.

A report by the Monarch Joint Venture found that 49 percent of winter-breeding monarchs had OE infections, compared to only 9 percent of migratory monarchs sampled in Mexico and 15 percent of migratory monarchs sampled in the northern US and Canada.

This is not to say that gardeners in California and the coastal Southern states (or zones 8 through 11) who grow tropical milkweed should immediately rip it out—just remember to cut the plants back in late fall to reduce the spread of OE, promote new growth of fresh, healthy leaves, and encourage monarchs to finish their migration.

If you want to plant new milkweed, there are several species of native milkweed in the US. One of the most common, Asclepias tuberosa, is also known as butterflyweed. It’s native to most parts of the country (except the Northwest) and blooms in a brilliant orange or yellow.

Not to worry, Northwesterners: You can still plant showy milkweed (Asclepias speciosa) or Mexican whorled milkweed (Asclepias fascicularis).

Butterflyweed does best in dry, sandy soil with little nutrition. It’s often seen growing in fields and along the sides of the road, which means it’s drought-tolerant and a good choice for low-maintenance perennial gardens.

I’ve grown a few species of milkweed over the years, including butterflyweed, and found them to be impressively resilient—surviving not only major heat spells, but also oleander aphids, milkweed bugs, and many, many cycles of hungry striped caterpillars.

It takes only a few caterpillars to defoliate an entire plant in a couple of weeks, but sure enough, the milkweed comes back every time.

Disclosure: If you shop from my article or make a purchase through one of my links, I may receive commissions on some of the products I recommend.

If butterflyweed is not native where you are, or you want to diversify the milkweeds in your garden, I encourage you to visit a native plant nursery in your area. You can also source seeds from a local vendor on the Xerces Society’s Milkweed Seed Finder or online from Botanical Interests.

So, how many milkweed plants do you need to support the caterpillars?

Unfortunately there’s no exact answer to this, as it’s unknown how many milkweed plants are needed to attract monarch butterflies as they move through your yard.

It’s generally estimated that one milkweed plant feeds one monarch caterpillar, but since each species varies in size, leaf shape, and growth rate—and monarchs sometimes lay multiple eggs on a single plant—it’s a good idea to grow at least six plants to make sure you always have a steady supply of milkweed throughout the season. If you have space for 10 plants or more, even better!

If you can’t start out with that many plants, not to worry—remember that it’s called milkweed for a reason! The plants reseed freely and you’ll likely have more milkweed popping up in new spots in your yard if you let them go to seed every year.

When and where to plant milkweed starts

Milkweed plants can be planted in spring or fall.

In spring, your milkweed plants will likely be dormant with no green leaves. This is when the plant is focusing all its energy into developing a strong root system, and it won’t “wake up” until the soil warms. Oftentimes, milkweed is one of the last perennials to break dormancy in your garden.

The key thing to remember is to not overwater your milkweed while it’s dormant.

It’s easy to water too much in spring, leading to root rot in your plants. (Swamp milkweed is the only species that can handle overwatering, making it perfect for wet climates and rain gardens.)

If you plant milkweed in fall, your plants will be able to establish themselves before winter. In cold climates, milkweed will die back and return in spring with new growth.

Light

Milkweed plants grow best in full sun locations that get six to eight hours of sunlight per day.

Soil

Each milkweed variety prefers a different type of soil.

• Butterflyweed and Mexican whorled milkweed grow best in dry conditions.
• Swamp milkweed thrives in a moist environment, as its name implies.
• Sandhill milkweed prefers dry, sandy soils.
• Showy milkweed and common milkweed grow well in average garden soil.
• Tropical milkweed prefers hot, humid conditions, but can be grown as annual in cooler climates.

Spacing

Milkweed plants establish large, deep root systems and prefer not to be transplanted once they’re in place. Most species of milkweed should be spaced about 18 inches apart.

Swamp milkweed, however, eventually matures to form clumps up to 36 inches wide, and should be spaced 30 to 36 inches apart. (You can also plant swamp milkweed and its cultivars closer together, and just thin the plants as they grow in.)

Watering

With the exception of swamp milkweed (which needs a naturally moist environment or regular watering), other species of milkweed have average to less than average water needs. It’s better to err on the side of underwatering a bit, as they tolerate drier conditions.

A good rule of thumb is to wait until the top 1 to 2 inches of soil feel dry before you water again. Then, water deeply and ensure the soil drains well.

Fertilizing

Native milkweed is a low-maintenance perennial plant that doesn’t require fertilizer. It does fine in average to poor soils, so you can fuss less over this plant and just enjoy it!

How to grow milkweed from seed

Unlike other flower seeds you may be used to, you can’t just toss out milkweed seeds in spring and wait for them to grow… because you’ll be waiting quite a while!

This is another common mistake people make: not cold-stratifying their milkweed seeds first.

If you want to start milkweed from seed, the easiest way is to mimic Mother Nature and sow your seeds in place in fall after the first killing frost (typically in November). This allows the seeds to go through a period of cold stratification over winter and then germinate in spring.

Read this next: Why your seeds aren’t sprouting—and how to get them to germinate, fast

But if you live in a warm zone without frost, or if you want to start milkweed seeds in spring, you can cold-stratify the seeds in the refrigerator.

Simply put the milkweed seeds in a damp paper towel or paper coffee filter, place it in a resealable zip-top bag, and refrigerate in an out-of-the-way spot for about 30 days before planting out.

Once the seeds have been cold-stratified, you can sow them directly outside or start the seeds indoors for transplanting later. Milkweed seeds may take up to two weeks to germinate, so be patient!

Will it come back every year?

Milkweed is a hardy perennial that can survive down to zone 3 and comes back every year in spring. While the foliage itself dies back and the plant goes dormant in the winter months, the rootstock is still very much alive and does not need protection.

The exception is tropical milkweed (Asclepias curassavica), which is only hardy to zone 8b, remains evergreen to zone 9b, and is often grown as a frost-sensitive annual in cooler climates.

To help prevent parasitic infection in monarch butterflies, it’s recommended that gardeners in warm climates cut back their tropical milkweed plants in winter to encourage the butterflies to continue their migration to Mexico each year (and not stick around in the southern United States during winter).

Growing tips from my own personal experience

• Purchase plants from a reputable nursery to avoid systemic pesticide use that could harm pollinators, or start your plants from seed.
• Offer a couple varieties of milkweed, as some monarchs may have a preference for one or the other, and plant a few patches throughout your yard for them to land on.
• Be aware that common milkweed (Asclepias syriaca) can spread aggressively by runners. Plant it in a suitable place in your garden where it won’t invade your lawn or overcrowd other plants.
• Plant low-growing perennials in front of the milkweed, as they’ll hide the spindly “skeleton” stems once the monarchs finish feasting. You’ll still be able to see them bloom, as the caterpillars don’t eat all the way up to the flowers.

This post is sponsored by Greenwood Nursery. View my full advertising disclosure.

This post updated from an article that originally appeared on February 17, 2017.