Pinus strobiformis / southwestern white pine

subgenus Strobus (Lemmon), section Quinquefoliae (Duhamel), subsection Strobus (Loudon).

Pinus strobiformis, as described in 1848 by Georg Engelmann (1809–1884), in Memoir of a Tour to Northern Mexico: connected with Col. Doniphan's Expedition in 1846 and 1847, is commonly known as Mexican white pine, Arizona white pine, Chihuahuan white pine, and southwestern white pine; as well as pino blanco, pinabete, and pino enano in the Spanish language. The species name describes this tree's general resemblance all members of subsection Strobus.

Ethnobotany. The seeds were eaten by the indigenous people of the southwestern U.S. It is locally (in Mexico) used for cabinetry, doors and window frames.

<em>Pinus strobiformis </em>— closeup of mature seed cone.
Pinus strobiformis — closeup of mature seed cone.

Description. Southwestern white pine is an evergreen coniferous species of tree that grows to mature heights of 50 to 80 feet (15 - 24 m) with a slender, straight trunk up to 20 to 36 inches (50 - 90 cm) in diameter, measuring at breast height; and a crown conic, becoming rounded to irregular with age.

  • Bark is smooth and silvery gray on young trees, aging to a dark grayish brown, furrowed and divided into rough rectangular plates.
  • Branches grow spreading-ascending;
  • Twigs are slender, colored pale red-brown, with a puberulous or glabrous texture, sometimes glaucous, aging gray or gray-brown, and smooth.
  • Foliar buds are ellipsoid shaped, resinous, red-brown in color, measuring circa 0.4 inch (1 cm).
  • Leaves (needles) are borne 5 per fascicle, growing spreading to ascending-upcurved, persisting 3 to 5 years on the tree. Individual needles measure 1.6 to 4 inches (4 - 10 cm) long by 0.024 to 0.04 inch (0.6 - 1 mm) thick. They are straight to slightly twisted and pliant, colored dark green to Blue-green, without evident stomatal lines on abaxial surfaces; adaxial surfaces are conspicuously whitened by narrow lines of stomata. Needle margins are sharp, razorlike and entire to finely serrulate, with narrowly acute to short-subulated apices.
  • Foliar sheaths measure 0.6 to 0.8 inch (1.5 - 2 cm), and shed early.
  • Pollen cones are cylindrically shaped, pale yellow-brown in color, and circa 0.24 to 0.4 inch (6 - 10 mm) long.
  • Seed cones mature 2 years after pollination, shedding seeds and falling soon thereafter, Individual cones are pendent, symmetrically lance-cylindric before opening, broadly lance-cylindric when open, creamy brown to light yellow-brown in color. measuring 6 to 10 inches (15 - 25 cm) long, with 2.4 inch (6 cm) long peduncles.
  • Cone scales number around 100 per cone, bearing apophyses that are somewhat thickened, and strongly cross-keeled, with a reflexed tip. Umbos are terminal and low.
  • Seeds are ovoid shaped, with a 0.4 to 0.52 inch (10 - 13 mm) long body, colored red-brown, and essentially wingless.
natural range of <em>Pinus strobiformis </em>
natural range of Pinus strobiformis

Distribution. This species is native to USA — Arizona, New Mexico, and Texas; in Mexico — Chihuahua, Coahuila, Durango, Nuevo León, Sinaloa, Sonora (type subspecies); subsp. veitchii is found in Mexico — Guanajuato, Hidalgo, México, Michoacán, Morelos, Puebla, Querétaro, Tlaxcala, and Veracruz. Populations in the U.S. and northernmost Mexico are of the reflexa type, showing characters intermediate between typical P. strobiformis and P. flexilis. Grows at elevations of 6,500 to 10,000 feet (1,900 - 3,000 m) above sea level. Habitat dry rocky slopes in high mountains, or as a minor component in mixed conifer forests. Within habitat, it mostly grows on moist, cool sites with associates such as P. hartwegii, and P. culminicola.

Hardy to USDA Zone 8 — cold hardiness limit between 10 to 20ºF (-12.1°C to -6.7°C).

Attribution from: Jesse P. Perry; The pines of Mexico and Central America; ©1991 Timber Press, Portland, Oregon.

Pinus strobiformis — a mature tree in habitat, south of El Salto, Durango, Mexico.
Photo by C.J. Earle
A stand of Pinus strobiformis in habitat.
Photo by Chris Schnepf, University of Idaho
Pinus strobiformis — a 2004 accession at the New York Botanical Garden,The Bronx, NY (USDA Hardiness Zone 7a); photo from 2020.
Photo by Katherine Wagner-Reiss
Pinus strobiformis — bark detail.
Photo by C.J. Earle
Pinus strobiformis — foliage and seed cones in situ.
Photo by C.J. Earle

Comments

Fred Cain

I'm not quite sure about the zone 8 on this one. I recently bought some seedlings that had been raised in a tree nursery in Vermont. That's hardly in zone 8. Also, based on my own memory and another website as well, this tree can be found at elevations between 8,000 and 10,000 feet in Arizona. As a former Arizona resident, I once saw a reported temperature of -40°F at Hawley Lake in the White Mountains. Zone 8? Well, I'm just not sure how those hardiness zones are arrived at.
Regards,
Fred M. Cain,
Topeka, IN

David Olszyk

Hi Fred, nearly everybody (including us) generally accept this reference:

<em>Frost resistance and the distribution of conifers</em> by P. Bannister, P. and G. Neuner. pages 3 to 22 in <em>Conifer cold hardiness</em> by F.J. Bigras and S.J. Colombo (editors); ©2001: Kluwer Academic Publishers, Dordrecht, The Netherlands.

That nursery in Vermont? What's the provenance of their seed? Is it possible they're working with a <em>strobiformis/strobus</em> hybrid? So many questions.

This is a great reason why we have the comments section on these records - to report on individual results. Remember, it's always safest to think in generalities. Individual results vary greatly depending on seed provenance.

In the case of<em> Pinus strobiformis</em>, we really can't grow it in the Pacific NW (USDA Zone 8). Too cold and wet in spring. So ... I usually don't pay much attention at all to published zonal hardiness. I look around and see what grows well in the area. I lack the time, patience, and money to push zones.

Fred Cain

Dear Conifer Editor,
This website states it's hardy to zone 4<strong>:</strong>
<a href="https://landscapeplants.oregonstate.edu/plants/pinus-strobiformis" rel="nofollow">https://landscapeplants.oregonstate.edu/plants/pinus-strobiformis</a>

(I hope it's O.K. to quote that here).

This species of pine is native to relatively harsh, cold but relatively dry mountain conditions so I could well believe that it might be just a bit too wet in the Pacific Northwest. What I'm wondering is whether we might've gotten this pine confused with another Mexican White Pine that is almost certainly hardy only to zone 8. It's all kind of a mystery and confusing to me. At any rate, I will try to keep everyone posted on how mine are doing. So far they look good and the weather has been cooperating with me.

Regards,
Fred M. Cain

Kevin J

My family has a house that is in the Sacramento Mountains of southern New Mexico. We are around 7600 ft in zone 6 a-b. The weather can get down to -10 at times and has been as low as -20 a few years back. Normal lows get to about 0 and highs can get up to around 85 to 90 right before monsoon rains begin in July. We are considered an arid mountain range with monsoon moisture in the summer and some snow in the winter with spring and fall with limited to no rain. These pines grow well among the Ponderosa pines and Douglas firs. There are not many around the neighborhood but those here are hardy and strong.

Fred Cain

Kevin,

Thanks for your response. You have helped confirm my suspicion that the zone 8 on our website is inaccurate. Most nurseries that sell pinus strobiformis claim they are hardy in zones 5 and 6 and MAYBE zone 7. If anything, I believe that zone 8 would be too hot for this tree.

I live in northern Indiana in zone 5a. Two years ago I bought some pinus strobiformis plugs from a nursery in Vermont (hardly zone 8). Some of them died but it was not on account of the weather but rather they were killed by moles. The two specimens I have left are doing fine and look good and hopefully their roots go down deep enough now so that the moles cannot desiccate the roots or force them out of the ground. The moles actually forced one seedling clear out of the ground.

I have never been to the Sacramento Mountains but I grew up in Arizona so I am very familiar with the mountaintop forests in the Southwest. There is pinus strobiformis on Mount Lemmon just north of Tucson. Some of the largest trees almost look superficially like sugar pines.