Anthoxanthum hirtum

In my garden, I sometimes lose track of where I’ve planted my sweet grass (Anthoxanthum hirtum, formerly Hierochloe odorata). It moves around a bit (slowly) via rhizomes, and to the untrained eye, it can be slightly difficult to distinguish from the various weedy grasses that plague my garden. Fortunately, when I accidentally tread upon my sweet grass, it makes itself known by releasing its intoxicating scent. So at least when I’m standing right on top of it, it’s pretty easy to find.

Sweet grass owes its vanilla-ish scent to its high concentration of coumarin. Coumarin is also the primary scent chemical of tonka bean, a common perfumery ingredient. In fact, the word “coumarin” comes from the French word for the tonka bean: coumarou (originally from the Old Tupi word kumarú). Although it smells sweet, the taste of coumarin is bitter (perhaps to deter herbivores).

In recent years, with the surging popularity of Robin Wall Kimmerer’s Braiding Sweetgrass, many people in the U.S. have become aware of sweet grass’s importance to various groups of Indigenous people in North America. Fewer people seem to realize that the plant is also native to northern Europe, and it has long been important in the spiritual lives and folk ways of people there. But that could be the topic of a whole different post.

The easiest way to propagate sweet grass is to divide it. If you have a 2.5-inch pot of it, you can split that nine ways, and in a few weeks you’ll have nine 2.5-inch pots of it. That only works if you have access to a pot of sweet grass, and many people don’t. Here in southern Wisconsin, Shady Acres Nursery sells 3.5-inch pots for $3.95 (as of this writing), and that’s the only nursery I know of in the area that sells this plant.

If you don’t have access to live plants, fear not: you can also grow sweet grass from seed. I prefer to grow from seed anyway, because it’s more fun, and I like supporting the genetic diversity of native plants. Sweet grass is not difficult to germinate, but in my experience, it will not germinate at room temperature. As far as I can tell, it germinates best at spring temperatures (20°C day/10°C night). You can read more about alternating temperature regimes here.

I knew from previous experience that sweet grass germinates following eight weeks cold stratification, and I knew that it performs very poorly at room temperature. Building on that experience, I sought to answer two questions this year:

1. Does increasing cold stratification from eight weeks to fifteen weeks improve germination?
2. Does sweet grass germinate at all at room temperature (20–24°C in my home)?

To answer these questions, I set up a tiny experiment. I split a packet of sweet grass seeds (from Prairie Moon) into three treatment groups:

• Eight weeks cold stratification, followed by germination at spring temperatures (alternating 20/10°C).
• Fifteen weeks cold stratification, followed by germination at spring temperatures (alternating 20/10°C).
• Fifteen weeks cold stratification, followed by germination at room temperature (constant 20–24°C).

You might wonder why I didn’t test eight weeks of cold stratification followed by germination at room temperature. The answer is simply that I didn’t have enough seeds. Maybe I’ll test that in future experiments! Unfortunately, things went a bit haywire in my germination chamber, and I ended up with temps of 25°C during the day and 15°C at night. Next year I’ll retest at 20/10°C.

After four weeks, I counted the number of seeds that visibly germinated. I tested the results for statistical significance (using a chi-square test), and indeed, my experiment yielded statistically significant results (p=0.021). For pairwise significance, I used Fisher’s exact test, with p-values adjusted for multiple testing via the Holm–Bonferroni procedure.

So the longer stratification did not change germination percentage, and germination after stratification was relatively low for both successful treatments. With that being said, I’m quite satisfied with 29% germination. Since its very easy to divide sweetgrass, you only need a few seedlings to establish a large population. But if you know a way to get higher germination than that, let me know!

As expected, germination at room temperature was zero. It’s interesting that there was such a difference between 25/15°C and constant 20–24°C. Either the seeds require alternating temperatures, or else they need to experience temperatures below 20°C. There are a variety of species that will germinate much more successfully under an alternating temperature regime, such as Eleusine indica, which experiences 99% germination under an alternating temperature regime (35°C/20°C) and less than 10% germination at constant temperatures of 20°C, 25°C, 30°C, or 35°C.¹ It’s worth noting that sweet grass and Eluesine indica are both in the Poaceae (grass) family.

Next year I’ll run more tests to determine if sweetgrass requires alternating temperatures, of if it just requires cool temperatures. But the takeaway for now is don’t try to germinate sweet grass at room temperature. ■

Have a question? If you have questions about Midwestern native plant propagation that you want answered in a future post, I’d love to hear from you! Shoot me an email at plantpropagationproject@gmail.com or leave a comment below!

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Notes

1. Roy K. Nishimoto, and Lambert B. McCarty. “Fluctuating Temperature and Light Influence Seed Germination of Goosegrass (Eleusine indica).” Weed Science 45, no. 3 (1997): 426–29. http://www.jstor.org/stable/4046043. ↩︎