See Juniperus scopulorum and Juniperus deppeana  and  Juniperus osteosperma.

Semi-desert. Shrublands, woodlands. Spring.
Above: Cannonball Mesa, December 7, 2020.
Left: Painted Desert area of Petrified Forest National Park, Arizona. March 24, 2007.

In the Four Corners area, Juniperus monosperma is typically a small, symmetrical, shrubby tree to four meters tall, but in some areas south into New Mexico and Arizona it grows to seven meters and is recorded even to twelve meters. It typically has a number of upward curving branches emanating from the base or below ground. The photograph immediately above shows a very old Juniperus monosperma with an uncommon massive spread of about 30 feet. 

Juniperus osteosperma has a more open appearance with just a few stout branches from a massive central trunk and it grows typically to seven meters tall but can be as much as twelve meters tall.

When these two species are young (just a century or two) their growth pattern is often very similar, looking very much like the tree in the photograph at left.

The species are best distinguished from each other by their cone and leaf characteristics:

J. monosperma has relatively juicy seed cones about 6-8 mm in diameter; J. osteosperma has dry seed cones about 8-9 mm in diameter. Both typically have one seed in the cone. The "juicy" versus "dry" distinguishing characteristic doesn't work, of course, if you cannot find any cones, nor does it work if you are looking at the trees in the spring when last year's cones are all dried out. Cones of J. monosperma are said to be "reddish blue to brownish blue"; those of J. osteosperma "bluish brown, often almost tan". Both are glaucous.

With a hand lens or microscope you can find another distinguishing characteristic: the tiny leaves of J. monosperma most often have a minute dark spot (a gland) on the side of the leaf facing you. Also, on the younger leaves, you will sometimes find a white, crystalline exudate, sticky and very aromatic. These characteristics are not present on J. osteosperma leaves.

Leaves of the two are supposedly slightly different in their shades of green: J. monosperma is said to have leaves that are green to dark green; J. osteosperma has leaves that are light yellow-green. 

Juniperus monosperma is present in almost all counties of New Mexico and Arizona and in those counties of Utah that border Arizona. Its presence in Colorado is not agreed on:

The Flora of North America shows it present within a bell-shaped curve area downward from central Colorado to the New Mexico border, but probably not including the Four Corners area.

Weber indicates, "Reports of [Juniperus] monosperma on the Western Slope are incorrect". That is a strange statement since Weber collected it in Mesa County in 1951 and that specimen is in the herbarium of the University of Wyoming.

Kartesz (BONAP) and Ackerfield (Flora of Colorado) show it present in a number of western Colorado counties including the Four Corners counties of Montezuma and La Plata, but the authors of the Flora of the Four Corners Region missed it there.

As the photograph at the top of the page and the close-up leaf photographs below indicate, I have found J. monosperma in Utah within a mile of the Colorado border in lower Cross Canyon, and I have also found it in a number of locations in Colorado in Canyons of the Ancients National Monument, where it certainly is outnumbered by Juniperus osteosperma, but is still found in scattered numbers.

George Engelmann named this tree Juniperus occidentalis variety monosperma in 1878 from specimens collected (by Engelmann?) probably near Pike's Peak.  Charles Sargent named it Juniperus monosperma in 1896 and Per Axel Rydberg named it Sabina monosperma in 1905.

Semi-desert. Shrublands, woodlands. Spring.
Petrified Forest National Park, Arizona. March 24, 2007.

Juniperus monosperma is dioecious (male and female flowers on separate trees) whereas Juniperus osteosperma is usually monoecious.  In the photograph at left, male flowers are about to open and flood the air with yellow pollen.

Semi-desert. Shrublands, woodlands. Spring.
Left: De-Na-Zin/Bisti Wilderness, New Mexico, October 8, 2007.
Immediately below: Lower Cross Canyon, Utah, April 9, 2021.

When fruit is very young, as in the photograph below, it is fleshy and very light in color. The cone-like structures mature in one season to blue, as in the photograph at left.  The 6-8 mm cones house the seed and are full of liquid resin  --  in contrast to the 8-9 mm dry and mealy cones of Juniperus osteosperma.

Both Juniperus monosperma and Juniperus osteosperma almost always have just one seed within their blue cones.

Semi-desert. Shrublands, woodlands. Spring.
Lower Cross Canyon, Utah. May 2, 2016.

Arrows point to the dark gland areas that are present on J. monosperma but not on J. osteosperma. The crystalline white exudate is present on some J. monosperma but not on J. osteosperma.

Notice also the elongated, sharply pointed new leaves at the top of each photograph. This shape is common to new leaves of both species.

Range map © John Kartesz,
Floristic Synthesis of North America

State Color KeySpecies present in state and native
Species present in state and exotic
Species not present in state

County Color Key

Species present and not rare
Species present and rare
Species extirpated (historic)
Species extinct
Species noxious
Species exotic and present
Native species, but adventive in state
Eradicated
Questionable presence

Range map for Juniperus monosperma