Rick Shory

Offering a little something you might not otherwise have

milkweed flowers closeup

Germinating milkweed seeds

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Is there any other flower people plant in their garden hoping the bugs will eat it? To encourage monarch butterflies, many people want to grow the caterpillar host plant, milkweed.

milkweed flower heads

milkweed stalks

I never thought there was anything to it. When I lived in Colorado, a milkweed seed floated into my garden one day, germinated, and set about its usual program of taking over the world.

milkweed spreading through a garden

Yet, here in Portland, many people have told me of failure. “I got the seeds, but they never grew!” So, when some milkweed seeds came into my hands last fall, I decided to see what I could figure out.

ripe milkweed pod, just opening

I was on a garden tour of a native plant nursery. There was a just-ripened pod of showy milkweed (Asclepias speciosa) on the drying stalk. The owner said I could have it. People on the tour were asking how to germinate the seed, and others were clambering “Cold stratify! Cold stratify!”

Now, I have been “cold stratify”-ing seeds since I was a teenager, and think nothing of it. But I have come to realize that for a lot of people, it’s beyond their ken. “Cold stratify” is technically redundant. Stratify means cold, as see this blog.

I had my doubts about milkweed. It’s typically a summer-ripened seed. A lot of summer annuals have a simple germination requirement of dry storage. If the seeds sprouted as soon as they got ripe they wouldn’t have much growing season left, plus, lots of competition from adult plants. So, the seeds have an internal inhibition of germination when they are fresh, which fades over time.

Even though milkweed is perennial rather than annual, I wondered if it might be the same. It sure would be easier if people could forget about stratification! So, this was my hypothesis. That milkweed seeds just needed a period of dry storage to grow. To test it, I would do a series of germination assays, over a period of months, and see what changed.

Cut to the chase. I was wrong. The seeds germinated right away. There was some variation over the months, but probably random. I just put the seeds at about 85°F, and voilà! Milkweed seedlings.

The 85°F temperature was my standard growth chamber, where I was already growing other things. Maybe it was luck.

A good germination test, to be statistically valid, ought to use 100 seeds. My milkweed pod only held about 150 seeds total. So I compromised, and did smaller tests of ten seeds each month (I skipped February). This means the results are more prone to random variation. Still, we learned something.

I germinated the seeds in what I call a “standard germination test setup”, a paper towel rolled around a chopstick, as in this blog. I checked the setup every night, and as soon as a seed put out a root, I planted it in potting soil. Then I later gave the seedlings away.

milkweed seed with root coming out

root can emerge, up to the length of the seed, in one day

milkweed seedling in plug tray

seedling planted

Incidentally, I found I could keep the seedlings in “suspended animation” by putting them in a sunny window in my unheated garage through the winter. They seemed to remain healthy, but stopped their pesky growth! The picture is of three batches started a month apart. Not growing, but unfazed by the cold. What a magnificent weed!

seedlings inside garage window

seedlings in unheated garage, in winter

Below is a chart of the total germination by month. If I’d had enough seeds for good tests, the counts might not have shown any variation at all. Still, germination was never less than 50%.

chart of germination percent by month

total germination by month

The chart of germination speed is even less instructive. Sometimes they started soon, sometimes after a delay. Sometimes they came up quick, sometimes drawn out. But not really over that much variation. Germination usually started the 4th day (rarely 3rd). No more seeds ever germinated after the 12th day.

chart of germination by elapsed days

rates of germination

All too much science never reports negative results. So here I admit, my hypothesis did not pan out. My experiment did not show anything about how to enhance milkweed seed germination. However, it clearly disproves “Cold stratify!”

Now, it is just possible this particular milkweed lost its chill requirement from being in cultivation. This is how it could work. There is always variation in plants. Suppose there were a big batch of wild-collected milkweed seeds, brought into a nursery to grow. Suppose, within those thousands, there were a few freak seeds that did not need any cold period to grow, while all the others did. Those few would be first to sprout. If the nursery person pounced on those, grew them out, and further propagated them, he/she might soon have a strain of milkweed that, genetically, did not need “cold stratify” while wild milkweed does. This is how a lot of crop seeds, such as beans and corn, have evidently lost any wild-type germination inhibition.

If you ever find wild milkweed with enough seeds, you can do your own tests. Cold treat one batch, don’t treat another, and then write your own blog!

Meanwhile, if you just want to get some milkweed growing, try planting your seeds at warm temperatures, and see what you get.

Asclepias speciosa

Here’s my original data, in case you want to check if it was the phase of the moon, or something!

start date count date count
10/6/2019 10/9/2019 1
10/6/2019 10/12/2019 3
10/6/2019 10/13/2019 4
10/6/2019 10/14/2019 5
11/6/2019 11/13/2019 3
11/6/2019 11/14/2019 4
11/6/2019 11/16/2019 6
11/6/2019 11/17/2019 8
12/6/2019 12/11/2019 1
12/6/2019 12/12/2019 3
12/6/2019 12/13/2019 6
1/6/2020 1/10/2020 3
1/6/2020 1/11/2020 6
1/6/2020 1/12/2020 7
1/6/2020 1/13/2020 8
1/6/2020 1/14/2020 9
3/6/2020 3/10/2020 2
3/6/2020 3/11/2020 6
3/6/2020 3/12/2020 7
3/6/2020 3/13/2020 8
4/6/2020 4/10/2020 5
4/6/2020 4/11/2020 9
5/6/2020 5/10/2020 1
5/6/2020 5/11/2020 4
5/6/2020 5/12/2020 5
5/6/2020 5/13/2020 6
5/6/2020 5/14/2020 7
5/6/2020 5/16/2020 9
6/6/2020 6/11/2020 1
6/6/2020 6/12/2020 4
6/6/2020 6/13/2020 6

 

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