Growing Swan Plant (Milkweed) for Monarchs

Swan plant (milkweed) in flower with small monarch caterpillar on stem
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If you are keen to help monarch butterflies, why not add a butterfly-friendly buffet to your garden by growing swan plants for monarch caterpillars? Swan plant (milkweed) is a key part of a creating a monarch friendly garden.  It’s a must grow if you’re keen to have eggs and caterpillars.

What Makes Swan Plant (Milkweed) So Special?

Swan Plants for Monarch Eggs and Caterpillars

Monarch butterflies need milkweed for survival, and milkweed is often in short supply so things can get ugly rather quickly.  Monarch butterflies lay their eggs exclusively on milkweed and their caterpillars are milkweed dependant until they prepare for metamorphosis. With very few emergency food exceptions, the only thing that monarch caterpillars can willingly and safety eat. You can read more about caterpillar life, including some of their interesting dining habits in our post about hosting monarch caterpillars in the garden.

Swan Plants for Predator Protection

In addition to nourishing the caterpillars for their weeks between hatching and pupating, the chemical compounds in milkweed sap help to make the monarch caterpillars toxic to predators. An amazing little bit of monarch evolution.

Although many pests avoid monarch caterpillars because of the milkweed toxin, they are (in our region) vulnerable to a number of pests and predators, including wasps, mantids, and shield bugs in our local area. Yes, it is nature at work, but you probably won’t be happy about it.  You can use countermeasures such as netting to protect the plants or traps to reduce wasps, but some loss is almost inevitably part of the cycle.

Before You Plant

Once you invite them in, help them thrive. They’re hungry, messy, little critters. You will need more swan plants than you think. Of course, all that voracious eating means that they’ll also poop a heck of a lot more than you’d ever expect! Then there’re the risks of pests and disease. Our local news recently featured a hilariously written (although the subject matter itself is entirely unfunny) recount of a family’s attempts at planting milkweed for monarchs, only to end up explaining a lot more about the circle of life to their kids than intended. Nature is wonderful, but it can be rather terrible, too!

Paper wasp hunting monarch caterpillars

Varieties and Planting Options

When selecting milkweed seed or plants, a local native variety is the best option, especially if you are planting in an area where your transient monarchs should be moving on as seasons change.

In our part of the world, the swan plant is the popular species of milkweed to plant for monarchs (and other Danaus family butterflies). Monarchs aren’t native to New Zealand, but we love them all the same (here is a link to the Monarch Butterfly New Zealand Trust for fellow kiwis). 

Swan plants are also known as balloon plant and the not-so-subtle bishop’s balls for rather obvious reasons. Check out the seed pods! Oh my…

Swan plant (milkweed) seed pod

Growing Swan Plant (Milkweed) in the Garden

Sourcing Safe Swan Plants

When planting as a food source, it’s essential that you purchase your plants from a reputable source (or grow your own from seed) to avoid inadvertently planting milkweed that has been pre-treated with systemic pesticides. If you prefer starting your own plants from seed, see the tips at the end of the post on seed saving and germinating swan plants.

Swan Plant Growing Conditions

Swan plants prefers sandy well-drained soil and full sun. They can grow up to 2m tall, so they also benefit from wind shelter or support. Just like monarchs, the swan plant isn’t native to New Zealand, but it has been naturalised. It is a perennial shrub, but is frost tender so often treated as an annual. I’m still experimenting with overwintering, with mixed success in our garden conditions.

Planting Placements

Remember, you’re planting swan plants for your eating and pooping caterpillar pals.  Planning and planting can help create a garden that suits you and your monarch guests. You may want to plant extra for back-up food, disperse the plants, or have a mixture of in-ground and mobile potted plants. 

Look for suitable aspects in your where you can (if you wish) mask the milkweed with foreground plantings or position the plants discretely so that the caterpillar pillaged stems don’t ruin the look of your landscaping. Making accommodations for ugly spells in the growth/feeding cycle will help you resist the urge to ditch the dregs before regrowth and/or seed set.

Care Cautions

Avoid using pesticides on or around your swan plants as this will also harm your monarchs. Try companion planting for deterrence and/or deal with pests manually where possible. Encouraging ladybirds/bugs and other beneficial insects to help – aphids are the primary pest problem for swan plants around here. You may also need to protect plants from slugs and snails around the base with pet and wildlife safe countermeasures, especially during spells of cool wet weather.

Monarch caterpillars on swan plant with seed pods

Plant and Handle with Care

Always remember that swan plants are toxic if ingested and swan plant sap can cause serious topical reactions, so take care, especially when gardening with children and pets. Wear gloves, protect your skin and eyes from sap if pruning, and wash up well after any/all handling – see the NZ National Poisons Centre for more detailed guidance.

Monarch caterpillar eating swan plant (milkweed) leaf

Hosting Very Hungry Caterpillars

Stocking the Swan Plant Buffet

Make sure your buffet is well stocked!  Monarch caterpillars chew through swan plant like you wouldn’t believe! Plant multiples in separated groupings to aid with control, over egg-laying, and/or feeding. Plant multiple batches of seeds in succession every few weeks to help ensure that you have enough milkweed to sustain caterpillars through to autumn.  If space allows, you can isolate some plant groupings with protective netting for additional control.

Check out our post on planning and planting a monarch friendly garden for ideas, as well as details on our garden process.

Pruning Swan Plants

If they’re been thoroughly nibbled early in the season, you can prune the eaten swan plants back for another round of regrowth. They will usually regenerate if left to their own devices, but pruning can  help straggly plants bush out to increase the food supply. Even if you milkweed hasn’t been nibbled to a stub, periodic pruning also helps reduce the spread of disease building up over multiple generations. Nourish nibbled and/or pruned plants with seaweed solution and/or liquid fertilisers (root feeding is best to avoid residue on the leaves) to aid regrowth. 

Swan plant (milkweed) growing in pots

Propagating Swan Plants from Cuttings

Although seed is the usual method used to propagate swan plants, I’ve had good success with propagation through cuttings taken during pruning.  The first was an accidental rooting of stems that I had placed in water by some plants as extra food.  Having discovered that they root from cuttings, I now do it on the regular. It’s a great way to replace or increase supply for future caterpillars or to gift plants to friends and neighbours. After pruning, I root the healthiest swan plant cuttings in water and then transplant to pots (or direct to soil). They don’t always set roots, but when they do, free plants!

Propagating Swan Plants from Seed

I start my swan plant seeds in the glasshouse for a seasonal head start, but a suitable indoor space would also work. In my experience, seed grown plants have been slow in the early stages, so the earlier I can get them growing the bigger they’ll be when needed. I’m still experimenting with overwintering with mixed success.

In our climate, they’ll also germinate if direct sown outdoors in favourable conditions. Left to their own devices, they will also self-seed if the caterpillar don’t nibble the entire plant before it can be pollinated and produce seeds. If you don’t want swan plant seeds sailing on their white fluffy tails all over the garden, you can remove the seed pods before maturity. 

Collecting Swan Plant Seeds

If you want to save seeds for controlled planting, try to catch the pods when they naturally split, but before the seeds start to swan out. If that’s too risky for you, you can bag the pods on the plant and harvest the seeds after maturity.

First time growing? No luck on saving seeds? Can’t be bothered? No worries! Seeds are also readily available for purchase from retailers. Some even donate on a per-packet basis back to butterfly conservation projects so you can feel extra good about your purchase.

Germinating Swan Plants for Transplant

Swan plants can be propagated for transplant using a seed tray or punnet of starting mix. Place seed individually in position or lightly scatter and then top with a thin layer of additional mix or sand.  Keep the seeds and soil moist until germinated (approximately 2-3 weeks), and then continue water regularly until the seedlings are big enough and garden conditions allow for transplant. While young seedlings are developing indoors, to help encourage strong stems, run your hand gently over them periodically to simulate outdoor conditions.

If possible, start your planting stagger early to have sizeable swan plants by the time butterfly season begins. Hungry caterpillars can quickly mow down small plants. Start more than you need to account for failed plants or transplants, and grow extras to ensure you have an ample food supply. 

Growing Swan Plant (Milkweed) for Monarchs

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