Growing milkweed for butterflies

Butterfly milkweed with Monarch caterpillar

Sunday, November 18, 2023

Milkweed is a favorite of pollinators for pollen and the food source for monarch butterfly caterpillars.  Milkweed's botanical name is asclepias and is native to much of North America.  Tropical milkweed (asclepias curassavica) is not recommended to be planted as it's bloom time is out of sync with monarch's life cycle and migration.

Look for native milkweed varieties to plant in your pollinator garden.  Varieties like common milkweed (asclepias syriaca), rose or swamp milkweed (a. incarnata) and butterfly or butterflyweed milkweed (a. tuberosa) are all readily available and good choices in the Midwest.  In our area, additional native varieties are purple milkweed (a. purpurascens, state special concern), white milkweed (a. variegata), whorled milkweed (a. verticillata), and green milkweed (a. viridis) are showy.  Additional natives in our area are clasping milkweed (a. amplexicaulis), poke milkweed (a. exaltata), prairie milkweed (a. hirtella, state threatened), aquatic milkweed (a. perennis), four-leave milkweed (a. quadrifolia), redring milkweed (a. variegata), and spider milkweed (a. viridis).

Showy milkweed and common milkweed love to self-sow so put them where you are okay with them filling in the area.  The other option is to pick the seed pods before they open.  Once they have dried and after the first frost of the fall season, go pick all the pods.  You can also leave them on the plant and put a string around the pod so that it cannot open to release the seeds to stratify them in place.   You'll have plenty of seeds to give to friends and family.

Most milkweed varieties need lots of sun to flourish.  Be sure to give it a sunny spot in the garden.  When you see sacks (chrysalis) and caterpillars covering the plant starting in June, don't freak out, these are just monarch butterflies.  Monarchs arrive in our area as early as April and most head back south in September.

Some milkweed varieties will flower the first year  they are propagated like Rose milkweed.  Some will not flower until the second year like Butterfly milkweed.  Check the variety you are starting from seeds to know if you can expect flowers in their first growing season.  

All native milkweed has a milky sap that can be irritating to the skin and eyes if touched.  The exception is butterfly milkweed; it does not exude a milky sap when a stem or leave is broken.  All parts of milkweed plants are toxic as they contain cardiac glycosides.  If pulling plants or trimming, gloves are recommended.  Be sure to wash them after handing milkweed plants. 

Milkweed can be hard to start from seed if you are planting in the warmer months.  The easiest way is to sow the seed in the fall.  Milkweed requires a "moist stratification" period of several weeks.  What this means is that the seed needs to be exposed to moisture and cold temperatures.  If you are planning on starting seeds indoors or outdoors in the spring, you can mimic these conditions by putting the seed in a moist paper towel in a ziplock in the refrigerator for 30 days.

You can then sow the seeds indoors or outdoors.  Ideal germination temperature for seeds are 75F.  Seeds will germinate in about 7-10 days at this temperature.  Soaking seeds in warm water for 24 hours after removing from the refrigerator and then planting improves germination rates.  If starting indoors, sow seeds 6-8 weeks before last frost.  If starting outdoors, sow after the last frost.  You will have to stratify to get good germination rates.

For starting indoors, use seed starting mix, fill the peat pots or seed flats, water well and let drain.  Then sow the seeds and cover with 1/4" of additional seed starting mix.  Mist the top of the soil to get this upper layer moist and cover so soil stays moist until the seeds sprout.  Remove cover as soon as seedlings emerge.  A fan on low directed over the seedlings can help the stems, making for sturdier seedlings.

Seedlings will be ready to transplant outdoors when they have 3-4 true leaves and danger of frost has passed.  

You can use a greenhouse to be able to get an even earlier start.  You can move your seedlings to the greenhouse when they have 2 true leaves and 6 weeks or so before your last frost date.  In my area, the last frost is around the first of April.  If I back up to when I should start stratifying my seeds, this is what it would look like.  

1 month stratifying + 1 week germinating + 3 weeks to get to 3-4 leaves + 6 weeks in greenhouse

So for our last frost date of April 1, I would start stratifying in mid-December, have seedlings by end of January, seedlings with 3-4 true leaves by mid-February that are ready to put in the greenhouse for 6 weeks, and transplant out to the garden in early April.

When you go to transplant butterfly milkweed, it has a long taproot.  If you break the taproot, the plant will not survive.  Be careful when removing this plant from a flat or you can start in peat or coir pots that you can plant in entirety in the garden.