In the Footsteps of a Giant: The Hybrid Junipers of Southern Wisconsin

23 December, 2025

My body has a way of giving out when granted an extended break in the school year - always a not-so-subtle reminder that working with kids is hard. As expected, this winter break has been no exception, bringing me body aches and a persistent cough. However, the warm sunny weather on the fourth day of break was simply too pleasant to not get outside. I could not and did not resist, selecting a dry prairie a short walk from a 19th century cemetery in Primrose, Wisconsin. The plan was to mosey up a small hill and then accommodate my beleaguered lungs by doing what I do best: moving about three meters a minute and zeroing in on minutiae. My sitting and obsessing would target two plants: a small population of rare ferns and some unusual junipers I'd seen on iNaturalist.

I admired the headstone of a Mrs. J. M. Grimes (1798-1857), then walked through a small oak savanna and up to the summit of a wonderful exposure, perched 70 feet above the surrounding farmlands. A large face of exposed St. Peter Sandstone was characteristically yellow and visibly deteriorating, hinting at its origin as sand at the edge of a 460-million-year-old sea. Formed into dunes by maritime winds, it was polished into rounded grains and ultimately blown into the water. Then it settled to the sea floor and solidified with essentially no extra minerals to cement it together.

Above the ancient sea floor was a carpet of high-quality dry prairie, featuring winter husks of uncommon species such as Aromatic Aster (Symphyotrichum oblongifolium) and Side-oats Grama (Bouteloua curtipendula). At the interface between the prairie and exposed rock, I could immediately see some of my woody targets. Ignoring them for a moment, I quickly found the "rare" ferns and found them to actually be a much more common species. I then returned to give the junipers their deserved attention.

They were - in a word - bizarre. Three or four plants in particular were trying to be upright trees, like Eastern Red Cedar (Juniperus virginiana). They were doing a poor job at it, only reaching a meter or two tall, but their lower branches were elongated and cascading down the sandstone in spectacular manner. These prostrate lower branches looked much like Creeping Juniper (J. horizontalis), a plant of Great Lakes shorelines. They looked like both species for a reason, because they were the hybrid between the two: Juniperus × ambigens. This hybrid was first described by Norman C. Fassett, a giant of Wisconsin botany active in the 1920's-40's. Little did I know, as I sat there staring at these strange plants, how closely I was following his footsteps.

Aside from the obvious and very showy mixture of growth forms, there were a few other intermediate traits (skip the next paragraph if you're not a big plant nerd):

First, J. virginiana has small female cones with 2-3 seeds and J. horizontalis has large female cones with 3-5 seeds. These plants had large cones with 3-4 seeds. Second, the tiny leaf-scales on J. virginiana taper evenly to an acute tip while those on J. horizontalis terminate in a little apiculus (a tiny tooth- or needle-like tip). These plants have acute tips and sometimes a very tiny apiculus. Third, the pedicels holding the ripe fruits of J. virginiana are mostly stiff, while those of J. horizontalis are mostly arched downward. These plants had an even mix of both. Finally, the color appeared intermediate between the bright green or glaucous blue of J. horizontalis and the reddish-orange young growth of J. virginiana. These traits are shown, in order from left to right, in the pictures below. For comparison purposes, the second to last picture is of pure Eastern Red Cedar at the site. The very last picture is of pure Juniperus horizontalis from Lake Michigan. (A note about the growth habit: Hybrids can apparently be fully creeping, with no upright branches at all, but these partially tree-like individuals are typical as well.)

Although I did not know much about them prior to my visit, these junipers are quite special to this region, occurring in about 20 scattered populations. These have all been confirmed to be hybrids with cytological, morphological and chemical studies, and mostly occur on cliffs of sandstone much like this one. The overall shape of the small hybrid zone closely follows the edge of the Driftless Area, occupying an area of interplay between glaciated and unglaciated regions. A majority of these populations are shown as purple pins below, and the population I saw in Primrose is near the center of the southern cluster. The edge of the Driftless Area shown in light blue.

It's hard to know exactly what to make of this, but I think I can start to make sense of it. In the Great Lakes Region, Juniperus horizontalis is most commonly seen far to the north or on dunes of the Great Lakes, where its low spreading growth makes it a very effective colonizer. Many of the hybrid populations grow quite close to the old shorelines of a glacial lake called Lake Yahara, which once occupied the basins of all four major lakes in the modern-day Madison area. In the frigid conditions of the last glacial advance, these delicate sandstone exposures would have been extra prone to falling apart, creating a loose sandy substrate perfect for Creeping Juniper.

With this in mind, it is possible to imagine J. horizontalis making the jump from the shorelines of an ancient glacial lake to a string of crumbling sandstone summits - possibly assisted by certain fruit-eating birds, which love eating and pooping juniper fruits. From there, it is also easy to imagine how J. virginiana would have moved in following the retreat of the ice sheet and swamped out the "pure" J. horizontalis genetics. Regardless of whether this is accurate (it probably isn't... this is just guesswork), Creeping Juniper did end up in these locations and its genetics did get swamped, or we wouldn't see hybrids today. As an aside, I have to say that I love the symmetry of a species which thrives on sand dunes ending up on rock formations which ultimately trace their origins back to ancient sand dunes.

Upon digging into some literature that my friend James Riser (who originally identified these plants) sent me, I learned that the population I visited was slightly unusual for having some pure Eastern Red Cedar (J. virginiana) present. Apparently, many populations have only hybrids, which implies that these habitats may select for hybrid genetics. This is fascinating and could be explained by the fragile yet harsh nature of these slopes. The constantly deteriorating sandstone lends an unstable aspect to the habitat, which J. horizontalis is well-equipped to handle. However, the hot and dry bedrock habitat is more suited to J. virginiana, leading to a situation where genetics from both species may be favored. I will repeat, though, that this is all just conjecture.

I learned a lot more from the literature and could drone on for many more paragraphs, but I will end this post with a bit of happenstance that left me stunned. As I read Norman C. Fassett's paper about this hybrid, I came to where he formally described it in Latin and proceeded to cite a few specimens. My eyes caught on this collection: "Dry crumbling sandstone bluffs, Primrose Twp., Dane County, June 17, 1976."

Could he have collected specimens from another sandstone bluff near Primrose, now long gone? Possibly. But I think it's very reasonable to think that nearly a century before my first visit, this giant of Wisconsin botany walked past the same headstones I saw, then visited the same plants I did and used them to give these special junipers a name. What an incredible thought that is. Every student of botany follows in the footsteps of giants, but this experience made that feel so much more real to me. I think I know where I'll be going on June 17, 2026... hopefully with less beleaguered lungs.

One more chance to admire my favorite hybrid of the bunch. The form of this individual is really something special, truly the best of both parent species. Long may it live.