This post explains how using the correct temperature regimes for your species can increase your success when germinating plants native to the American Midwest (and elsewhere too). Note that temperature is just one piece of the puzzle: growers must also manage moisture, exposure to light, physical and chemical scarification, substrate, exogenous hormones, etc. Future posts will cover those topics and will also discuss the different types of seed dormancy.

Temperature is critical when stratifying and sowing seeds, but information is scarce.

Until a few years ago, most of my plant propagation experience was with common agricultural crops: flowers, vegetables, fruits, herbs–basically anything you can buy from Johnny’s Seeds. And when you buy from Johnny’s, many varieties display a little graph showing relative germination success at various temperatures. So for example, the graph for eggplant shows optimum germination at soil temps of about 32° C (89.6° F), with germination decreasing as soil temperature is increased or decreased (see figure 1). This makes it very easy to propagate seeds from Johnny’s: all I have to do is put my seedling tray on my heating mat, stick my thermostat probe in the soil, and set my thermostat to 32° C.

But when you buy seeds from native plant nurseries like Prairie Moon or Wild Seed Project, they don’t give you species-specific information on germination or stratification temperatures. And for many people, this doesn’t matter. If you’re restoring dozens of acres to prairie, you’ll almost certainly be direct seeding rather than planting plugs. And if you’re spreading a seed mix, stratification and germination temperatures don’t matter. The seeds will experience the temperatures they need to experience, and eventually they will germinate (or not). Specific temperatures also don’t matter to the many hobbyist growers who use the milk jug winter sowing method, where seeds sit out all winter and spring in a milk jug, and simply germinate when ambient temperatures reach the correct temperature for the seeds. While I appreciate the simplicity of this method, I don’t use it. I like to sow my seeds early, as early as November for slow species like Gentiana andrewsii. My goal is to give my seedlings one full year’s worth of growth before planting them out each spring.¹ This ensures that most of my transplants flower their first year in my garden (such as Lupinus perennis subsp. perennis and Echinacea tennesseensis), and I also find that perennials survive their first year at a higher rate–and with less babying–when planted large and early in the season.

So for me, it’s important that I be able to reliably germinate seeds indoors. When I first started growing plants native to the midwestern United States, I would follow Prairie Moon’s recommended germination protocols to a tee, and then become incredibly frustrated when some of the species produced not a single cotyledon. Many of my seeds did germinate quickly when I sowed them at room temperature in my house after following pretreatment protocols from Prairie Moon’s website, but I was haunted by my failures: Gentiana andrewsii, Pedicularis canadensis, Angelica atropurpurea…the list goes on.

The problem, it turned out, was temperature. Sometimes the stratification temperature(s), sometimes the germination temperature, and sometimes both. I don’t want anyone else to fail in their native seed propagation, so I’ll use the remainder of this post to summarize all the most important information you need to know about temperature regimes for stratifying and sowing native seeds.

If you want the definitive source of seed germination information, I’d suggest Carol C. Baskin and Jerry M. Baskin’s Seeds: Ecology, Biogeography, and Evolution of Dormancy and Germination, Second Edition. This is legitimately one of my top ten favorite books, but be warned: it is by no means a how-to guide. So for everyone that wants the quick(ish) answer rather than a year of reading, I’m endeavoring to synthesize some of the information contained within its 1500+ pages into a series of blog post that resembles something of a practical guide to starting less-commonly-cultivated plants.

Four temperature regimes to rule them all

I have a lot to say about stratification and germination temperatures, but I’m going to start with the most important information and explain the rest after. Long story short, I think a lot of people have an incomplete picture of what temperatures they should be using for stratifying and germinating seeds. I’m here to tell you that if you want to germinate less-commonly-cultivated plants, you need to know four temperature regimes for stratifying and sowing your seeds (see table below).

Simulated Season(s)	Daytime Temperature	Nighttime Temperature	Abbreviated as	Typically Used for
Winter	5° C (41° F)	5° C (41° F)	5° C	Stratifying
Early spring / Late fall	15° C (59° F)	6° C (42.8° F)	15/6° C	Sowing
Spring / Fall	20° C (68° F)	10° C (50° F)	20/10° C	Stratifying or Sowing
Summer	30° C (86°F)	15° C (59° F)	30/15° C	Stratifying or Sowing

Seeds of nearly any midwestern native plant can be germinated using some combination of these four regimes. But you may be asking yourself two questions:

1. How do I achieve these temperature regimes in my house?
2. How do I know which temperature regimes to use on my seeds–and for how long should I incubate my seeds at these temperatures?

Achieving a constant 5° C regime is easy–just stick your seeds in the fridge. But the other regimes are harder. I’ll describe my setup in more detail in a future post, but basically I do all my seed starting in my very cold basement, which ranges from 5° C to 10° C in the winter. I have a 1020 tray for each regime, covered with a humidity dome. Under each tray is a heat mat, and I use a Bluetooth-connected outlet thermostat to program the heat mat to one temperature at night and one temperature during the day (for what it’s worth, I use the “CONTROLLER 79 PRO” from AC Infinity). If you don’t have a cold basement, you’ll have to get creative–but that’s a topic for another post.

As for which regimes to use and for how long–that’s the tricky part. Although I love Prairie Moon, I have found that Prairie Moon’s pretreatment protocols are not always reliable, especially for plants they don’t sell in plugs. Sometimes they list only the germination code “M,” indicating that plants are best started outdoors in the fall, but some of these seeds can easily be started indoors with the proper temperature regimes. The Native Plant Network keeps a database of germination protocols, but I often find myself unsatisfied with them, due to a lack of information on temperature regimes, as well as a lack of information on how successful the protocols were.

Over the coming months, I intend to compile a database of protocols for germinating midwestern native seeds based on my own personal experiments as well as my review of protocols published in academic journals. But this database that I’m creating is guaranteed to be highly incomplete. Baskin & Baskin’s Seeds² is a great reference, and often has at least some information for thousands of species. Google Scholar is a great search engine for finding journal articles, which you can leverage to find germination studies. If you can’t find anything, Baskin & Baskin suggest a “move along” study, but that’s a topic for another blog.

So that’s my explanation for what I think you should be doing. For those who want a little bit more of the why, keep reading.

A more complete understanding of stratification temperatures

When you search “stratification,” most of the webpages Google churns out will give information on cold stratification–usually a few weeks or months of moist storage in the fridge (~5° C, 41° F). If you find a really good resource, they’ll also provide information on warm stratification, which is often presented as a few weeks or months of moist storage at a constant temperature of 25° C (77° F) or so. This is a very simplified version of stratification, and this simplicity can be beneficial for first-time growers. A huge number of midwestern native plants can be successfully germinated with either no stratification or a simple cold stratification in the fridge. But as growers start exploring less-commonly-cultivated species, they may find that the simple stratification protocols fail them.

The constant 5° C temperature regime is a very good regime for seeds that need to be incubated at winter temperatures to germinate. It does a great job of approximating the temperatures that seeds experience in the winter when they’re in snow-covered (therefore snow-insulated) soil. But warm stratification requires more nuance. I have found that keeping seeds at a constant 25° C is often not effective for breaking dormancy.

First, and most importantly, many seeds require (or benefit from) alternating temperatures during warm stratification³ (i.e., one temperature for day and one temperature for night). When the sun comes out during the day, the temperature of the upper few millimeters of soil increases by 10° to 15° C. Many seeds need to experience these same temperature fluctuations during warm stratification to effectively break dormancy. It is very rare for alternating temperatures to inhibit germination; the only species I know of that experiences substantial germination inhibition from alternating temperature is the New Zealand native Arthropodium cirratum.⁴ So considering that it is common for alternating temperatures to promote germination and rare for them to inhibit germination, I hedge my bets and always use alternating temperatures.

Second, warm stratification is not one-size fits all. On the one hand, Erythronium albidum germinates well after six weeks incubated at summer temperatures (30/15° C), followed by cold stratification.⁵ But on the other hand, in order to achieve good germination, Jeffersonia diphylla requires incubation at autumn temperatures (20/10° C) prior to cold stratification. Incubation at autumn temperatures can either be in addition to summer temperatures (incubation first at 30/15° for 12 weeks, then incubation at 20/10° C for 6 weeks), or else in lieu of summer temperatures (24 weeks of just the 20/10° C regime).⁶ Either way, without a period at 20/10° C, the seeds will have very poor germination. So when warm stratifying seeds, you must be careful to use the temperature regime appropriate for the seed, and not assume that all seeds need the same regime for warm stratification.

Sowing temperature regimes are also critical to germination success

Just as when stratifying seeds, when sowing seeds, many species require (or benefit from) alternating temperature regimes. Additionally, while some seeds can easily be germinated at a wide range of temperatures, others are very picky. Seed sellers don’t list optimum germination temperature regimes for native plants, so you’ll have to do your best to look up the optimum sowing temperature regime of your seeds. To keep things simple, I utilize just three temperature regimes for all my sowing:

• Early spring / Late fall: 15/6° C (59/43° F)
• Spring/Fall: 20/10° C (68/50° F)
• Summer: 30/15° C (86/59° F)

Note that, although I personally only use three regimes for sowing, seed germination researchers use more. Baskin & Baskin, for instance, often use a 25/15° C and a 35/20° C regime in addition to the three I mentioned above. If you’re propagating plants that are native to the southern states, you may need to use the warmer 35/20° C regime.

After pretreatment, seeds typically germinate best at one specific regime, but still germinate at other regimes (see figure 3). Gentianella quinquefolia favors cold regimes, with very high germination at 15/6° C, some germination at 20/10° C, and no germination at 30/15° C.⁷  Callicarpa americana, for its part, favors warm regimes with highest germination at 30/15° C, and no germination at 15/6° C.⁸ And others, like Oligoneuron album (formerly Aster ptarmicoides) favor the not-so-cold and not-so-hot 20/10°C regime, but shows some germination at all of the other temperature regimes.⁹

Although alternating temperatures often help–and seldom hurt–they’re not always necessary. If you don’t have a setup that allows you to alternate the temperature, using a constant (or near-constant) temperature often suffices. I’ve successfully germinated many seeds at my house’s ambient temperature, which is about 20° C during the day and 17° C at night. But if your seeds aren’t germinating, there’s a very good chance that you need to change your temperature regime. ■

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

Want to support this work? Buy me a cup of coffee (or a bag of potting medium) with the donation button at the bottom of this page. Or better yet, send me your germination success stories.

---

Notes

1. For a seed that germinates in summer and senesces in the fall, a year of growth is only a few months. ↩︎
2. Carol C. Baskin and Jerry M. Baskin, Seeds: Ecology, Biogeography, and, Evolution of Dormancy and Germination, Second Edition (San Diego, CA: Academic Press, 2014). ↩︎
3. For a robust list of citations of studies showing that seeds require–or benefit from–alternating  temperatures, see pages 19-20 of Baskin & Baskin’s Seeds (full citation above). ↩︎
4. A. J. Conner and L. N. Conner, “Germination and Dormancy of Arthropodium Cirratum Seeds,” New Zealand Natural Sciences 15 (1988): 3–10, https://doi.org/10.26021/525. ↩︎
5. Jerry M. Baskin and Carol C. Baskin, “Seed Germination Ecophysiology of the Woodland Spring Geophyte Erythronium Albidum,” Botanical Gazette 146, no. 1 (March 1985): 130–36, https://doi.org/10.1086/337507. ↩︎
6. Jerry M. Baskin and Carol C. Baskin, “Seed Germination Ecophysiology of Jeffersonia Diphylla, a Perennial Herb of Mesic Deciduous Forests,” American Journal of Botany 76, no. 7 (1989): 1073–80, https://doi.org/10.2307/2444529. ↩︎
7. C. C. Baskin and J. M. Baskin, “Seed Dormancy in Wild Flowers.,” in Flower Seeds: Biology and Technology, CABI Books, 2005, 163–85, https://doi.org/10.1079/9780851999067.0163. ↩︎
8. Same as above. ↩︎
9. Same as above. ↩︎