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Swamp Milkweed Grow-Along, Chapter 1: An intro to winter sowing for Monarchs

By Samuel LeGresley, Communications Coordinator at Nature NB

A hummingbid on Swamp Milkweed
A hummingbird on Swamp Milkweed. Photo: Pierre Janin

A lot of our native plants need a little special care to germinate, but it’s nothing too complicated if you have the space and time to experiment. Embark on this grow-along journey with Nature NB by ordering your milkweed seeds and sowing along with us! For a few years now, Nature NB has been sending packets of Swamp Milkweed seeds for a small donation. If you don’t have yours yet, it is not too late to order and start the journey!

Swamp Milkweed is a great way to attract many pollinators, including the Monarch Butterfly, from which the caterpillar feeds on its leaves. The Monarch’s recent Endangered IUCN status has sparked great awareness of this species and its plight.

The habitat for this butterfly must include plants from the milkweed genus (Asclepias sp.). Following a million-year old exposure to this plant, the butterfly’s digestive system has not only developed enzymes ready to digest the milkweed’s toxic latex, but its behavior includes biting the leaf at the base, in order to prevent the flow of huge amounts of latex. This specialization comes at a cost though, for it is the only genus of plants capable of feeding the larvae.

Our local milkweeds are the Common Milkweed (Asclepias syriaca) and the Swamp Milkweed (Asclepias incarnata). The Swamp Milkweed is better suited to gardens as it does not spread by underground stems (rhizomes), unlike its Common cousin. Both are native to this region, and support a host of other insects with food as well as nectar for our only hummingbird (Ruby-throated).

But there is a caveat: as the plant’s flower only lasts during the very warm summer months, late-blooming plants are needed to complement the diet of the Monarchs and give them enough food for their migration to Mexico. We can think of Asters (Symphyotrichum spp., for example) and Goldenrods (Solidago spp.). This means we need to incorporate plants that bloom in many seasons in our gardens, and leave portions of larger yards as unmowed meadows that can naturally feature those species. 

However, those plants are not as easy to obtain in nurseries, and require a special treatment to germinate. Let’s explore this together.

Winter sowing 

Now, growing this plant from seed is not as easy as simply sprinkling the seeds on the ground come spring. You might get a few plants that way, but the germination rates will be dismal and it’s harder to keep track of, even when done in the fall where the process might occur naturally. That is why, on the packets sent by Nature NB prepared by the Maritime Monarch Watchers, we recommend a fridge stratification method of 6 to 9 weeks, in which seeds are placed in a moist growing medium in a plastic bag and transplanted to pots once they germinate.

This year, we’re testing out something different. To mimic the cycles of nature, a lot of people have tried the time-tested method of winter sowing, including the Ottawa Wildflower Seed Library, the founder of whom we featured in our last 5 Questions blog. Keep reading to find a link to this seed library’s guide.

This method has its advantages, as it only requires an outdoor location where sun is partial (not facing south) and where the seeds can experience a full range of weather conditions (i.e. not under an overhang). It is also a good practice to place a screen on top of the containers so that your precious seeds don’t get stolen by birds or rodents!

With the arrival of snow in December and January, it’s time to check the weather forecast in your region. If it doesn’t show rain or warm periods for the week and it’s later than mid- to late December, it might be time to start winter sowing!

A way to break dormancy

Dormancy in plants means – depending on the species – a seed coating may be too thick for the embryo to grow through, or an embryo may be impeded by chemical processes. Reasons for this in nature include the fact that the seed might die if it started growing right after it fell off the plant (source). Regardless of the type of dormancy though, we need to help plants break from it. A way to tell whether a plant with large seeds might be alive or not is to do a cut test with an X-Acto knife.

A cut test of Swamp Milkweed shows the whitish endosperm.
The X-Ray machine at the NTSC shows swamp milkweed seeds and their endosperm in white.

Alternatively, I went to the National Tree Seed Centre to do a special X-Ray test conducted in a machine designed for medical uses, but purchased to check whether seeds have a live endosperm, the part that gives food to an embryo. In this image, we can see a batch of Swamp Milkweed seeds taken from the ones we are sending out, and they all have a white endosperm! This means they seem to be very healthy and ready to take on the dormancy-breaking process.

A cut test will lead to similar results by showing a white endosperm, but it kills the seed. A way to manage this is to take a small sample from your seeds, and cut into them, calculating the percentage of seeds that are full (i.e. white inside) vs. empty. 

Grow-along step one: Order and give’em a sow!

Here are some resources to help you on the way to growing your plants.

  • This series of videos by Ottawa Wildflower Seed Library explains to you in a nutshell this easy process. 
  • If you like having a printed list, click here; the Ottawa Wildflower Seed Library has a list of resources that is very useful to start planting for our general climate.

Next up we’ll see you in January, when the Nature NB staff will have started to grow Milkweed and other native plants outside! Consider joining us by growing along at home before or during the next installment of our Swamp Milkweed grow-along.

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