Growing milkweed in containers
Milkweed in a pot is entirely possible if you respect one thing: these are deep-rooted plants, and depth matters more than width.
Depth is the whole problem
Most milkweeds send down a taproot fast. A butterfly weed seedling six weeks old may have a root eight inches long under two inches of top growth. Put that in a shallow pot and the root circles, the plant stalls, and it never thrives.
Minimum depth: 12 inches. Better: 14 to 18 inches. Width matters much less. A tall narrow pot beats a wide shallow one every time.
Best species for containers
| Species | Why it works |
|---|---|
| Swamp milkweed | Fibrous roots, clump-forming, tolerates regular watering. The easiest container milkweed |
| Butterfly weed | Compact and drought-tolerant, but needs a deep pot and gritty mix |
| Whorled milkweed | Small, fine-textured, thrives on neglect |
| Tropical milkweed | Excellent in pots, but only as an annual in zone 8 and colder |
| Aquatic milkweed | Ideal for a pot standing in a saucer of water, in the Southeast |
| Rush milkweed | Architectural and drought-proof in Southwest gardens |
Avoid common milkweed and showy milkweed, whose rhizomes fill a pot and strangle themselves within two seasons.
Compost and drainage
Use a free-draining mix, not a moisture-retentive potting compost. A good general recipe is two parts peat-free multipurpose to one part horticultural grit or perlite. For butterfly weed and desert species, go to a fifty-fifty mix with grit.
Drainage holes must be generous. Do not use a cache pot without emptying it, because waterlogged roots kill more container milkweed than anything else.
Do not feed heavily. A single application of slow-release fertiliser in spring at half the recommended rate is plenty. Rich feeding produces soft, floppy, aphid-prone plants.
Watering
Container plants dry out much faster than ground plants, so the standard advice about drought tolerance does not fully apply.
- Swamp and aquatic milkweed: keep consistently moist.
- Butterfly weed, whorled, desert species: water thoroughly, then let the top two inches dry before watering again.
- All species: reduce watering sharply once dormant.
Overwintering
Roots in a pot experience much colder temperatures than roots in the ground. As a rule, a containerised perennial survives winter about two USDA zones less hardy than its rated range, so a zone 5 plant in a pot needs zone 7 conditions.
Options:
- Sink the pot into a garden bed or a pile of leaves for the winter and lift it in spring.
- Move it to an unheated garage or shed after the foliage dies back. Water lightly once a month so the roots do not desiccate completely.
- Group pots together against a house wall and wrap the outsides with hessian or bubble wrap.
- Grow tropical milkweed as an annual and start again from seed, which takes about twelve weeks to flower.
Cut back dead top growth in late winter, not fall.
Raising monarchs on a balcony
It works, with realistic expectations. A group of three to five pots of swamp milkweed on a sunny balcony will attract monarchs in the breeding range, especially above the second or third floor where females are already flying at that height.
- Group the pots so they read as a patch.
- Add a nectar pot of pentas, zinnias, lantana or a native aster beside the milkweed.
- Expect to need more plants than you think; a few caterpillars strip a potted milkweed in days. Keep a spare plant in reserve.
- Do not spray anything, including on nearby ornamental plants.
Frequently asked questions
Can you grow milkweed in a pot?
Yes, provided the pot is at least 12 inches deep, because most milkweeds are taprooted and stall in shallow containers. Swamp milkweed is the easiest, and butterfly weed works well in a deep pot with a gritty mix. Avoid common and showy milkweed, whose rhizomes quickly fill a container.
How big a pot does milkweed need?
At least 12 inches deep, ideally 14 to 18. Depth matters far more than width because of the taproot. A tall narrow pot outperforms a wide shallow one.
Will milkweed survive winter in a pot?
Only if you protect it. Roots in containers experience temperatures roughly two hardiness zones colder than roots in the ground. Sink the pot in the garden, move it into an unheated garage, or group and insulate pots against a wall.
References
- Luna, T. and Dumroese, R. K. (2013). Monarchs (Danaus plexippus) and milkweeds (Asclepias species): the current situation and methods for propagating milkweeds. Native Plants Journal 14(1): 5–15. Practical propagation protocols, including stratification requirements. View
- Borders, B. and Lee-Mäder, E. (2014). Milkweeds: A Conservation Practitioner’s Guide. The Xerces Society for Invertebrate Conservation, Portland, Oregon. Seed production, establishment and habitat management at field scale. View
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