Milkweed Seed Germination
To get a jumpstart on summer, we like to germinate our milkweed seeds indoors before planting them outside after the last frost date. Why start so early? Because some varieties of milkweed, like A. physocarpa, can take four to five months to reach maturity. By starting seed indoors, we avoid having to buy expensive, nursery grown plants with limited selection and possible exposure to pesticides. Today’s blog covers simple techniques you can use to quickly and easily germinate Common Milkweed (Asclepias syriaca), Tropical Milkweed (Asclepias curassavica) and Hairy Balls Milkweed (Asclepias physocarpa). Visit our store to see the types of seeds we have in stock.
Tropical milkweed growing in a large container.
For timing our plantings, we plan back from our zone’s last frost date, allowing for twelve weeks total, which breaks down as follows: two weeks for stratification (detailed below), two weeks for germination, and 6-8 weeks of growing time before transplanting out. We are in zone 7 with a typical last frost date of May 1st, so we start the indoor planting process in February.
Fresh cut Hairy Balls milkweed being displayed in a container.
When you search online and in seed catalogs, there is a huge range of advice on how to germinate milkweed seeds, and it can all be a bit overwhelming. To save you time and effort, we’ve dug through a lot of material to find a simple, successful planting technique so you achieve the results you desire.
Common milkweed being grown in Suzanne’s “Monarch Waystation” meadow.
We start by using Cold Moist Stratification, or CMS for short. This technique involves placing seeds in a moist medium and placing them in your refrigerator for anywhere from 2-8 weeks. The purpose of CMS is to mimic winter and breakdown the seed coat for germination. Some of your seed will likely germinate without CMS, but if you want germination rates >80%, don’t skip this important step. Here are the steps we recommend:
Step 1: Lay out your seed on a section of paper towel. We recommend you plant 2-3 seeds per pot or cell to account for lower germination rates. In the photo below, we are planning to fill three 6-cell packs, so we are starting with 12 seeds of each variety. Label your bag, moisten and fold the paper towel over the seed, insert the wet towel into the plastic bag, and place the bag in the refrigerator for at least two weeks. Longer periods of CMS may be necessary if you want higher germination rates or the source of your seed came from a very cold part of the country. You DO NOT need to use CMS on tropical milkweed or Hairy Balls milkweed seed.
We are preparing two seeds for every desired plant to account for less than 100% germination.
Moistened and folded paper towels have been placed in plastic bags and are ready for two weeks in the refrigerator.
Step 2: Plant the seeds in small pots or planting containers, using the moistened seed starting mix of your choice. We like to use small peat moss pots as they make transplanting simple by avoiding damage to the young plant’s roots. Do not fully cover the seed as they need light to germinate. We use plastic wrap or a humidity dome to keep the seeds moist.
Milkweed seeds sown on seed starting mix in a plastic seed tray. If desired, they can be covered with a very light sprinkling of vermiculite to help retain moisture. Cover seeds with a humidity dome to maintain moisture during germination.
Step 3: Place your newly planted seeds under a grow light (preferred) or in a sunny windowsill (optional). If you are placing the seeds where they will get direct sunlight, be sure to add a few ventilation holes in your humidity cover so the seeds don’t get cooked by the sun. Your seeds will germinate in 7-14 days, usually without having to add additional water. However, the seeds must remain moist during germination, so bottom water or mist the seeds as necessary.
Step 4: Remove the plastic wrap or humidity dome when the majority of your seeds have sprouted or show signs of sprouting. Bottom water the seeds for 2-3 weeks. When your plants have 2 sets of true leaves, proceed to the transplanting step below.
Hairy Balls milkweed seed germinating in a peat pellet.
Step 5: Transplant your milkweed seedlings to a larger, deeper pot. Milkweed plants produce large taproots, and if the plants become rootbound in a small pot it may permanently stunt their growth. For common milkweed, a 6-12 inch pot with average soil is recommended. For tropical or Hairy Balls milkweed, 4-6” pots will suffice until you can get them planted in their final location.
Step 6: Harden off your plants and plant them in large pots, or directly into your garden or wildflower meadow, after all danger of frost has passed.
A newly hatched male Monarch butterfly.
That’s it! Monarch butterfly populations have declined by 90% over the past 15 years, so thank you for doing your part to help Monarch butterflies thrive where you live. Want even more Monarchs? Plant pollinator flowers somewhere near your milkweed plants to attract more butterflies. Comment below and let us know how the germination process worked for you.
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