Germinating Yucca elata and Yucca faxoniana.

Last year I sprouted several Yucca baccata (Banana Yucca) and Yucca elata (Soaptree Yucca), and while I grew out many Yucca baccata, I only managed to grow out a single Yucca elata. Despite our brutally frigid temperatures, this single Soaptree Yucca handled the Colorado winter seemingly with ease. This is encouraging - the tree yuccas fascinate me, which naturally means I plan on growing some more.

Today I began cold-stratifying eight Yucca elata seeds and eight Yucca faxoniana (Texas Tree Yucca) seeds. Yucca elata is probably the hardiest of the tree yuccas, but success with yuccas, agaves, and their kin is determined by many factors, and USDA hardiness zone is only one of these. Siting (soil structure, exposure, microclimate, etc.) seems to be as critical in determining success with members of the family Agavaceae, particularly when planting them in areas pushing the limits of their cold-tolerance. My Yucca elata is growing on an exposed bed of decomposed granite, granite chunks, pumice, and loam-amended clay (the native soil). Since our regionally native Yucca glauca (Soapweed or Great Plains Yucca, a short colony-forming species) thrives in our local soil, I'm not surprised that its relatives have done well either. Still, since many reputable sources cite Yucca elata as hardy to "about zone 6a", I am very pleased that our less-than-a-year-old seedling survived the winter. Yucca elata has a reliable branching habit, adding to its "tree" image.

Yucca elata blooming. Photo by Stan Shebs.

Yucca faxoniana - the Texas Tree Yucca - is the other tree yucca that I am attempting to propagate. This hardy tree yucca has one of the thickest, stoutest trunks (a foot in diameter) that supposedly forms a single stem reaching a mature height of up to 15 feet. I am following a similar protocol for cold-stratifying the seeds (about a month), since it seems that yuccas benefit from cold-stratification even if they are native to a milder clime. Some sources claim that Yucca faxoniana is hardy to USDA Zone 5, while others are far more conservative (USDA Zone 8), so I will just have to grow them out and see. My source claimed it was hardy to USDA Zone 5. As I wrote earlier, it seems that Yucca faxoniana is similar to Agave parryi in that differing populations of the same species can have quite different hardiness limits depending upon the conditions of their native environment. Perhaps my seeds come from a hardier population. We'll see. I plan on growing these near my Chilopsis linearis (Desert Willow) in a protected, sunny location that warms to daytime temperatures quickly in the winter.

Update 04-23-2014: I was able to read the section in Mary and Gary Irish's excellent book Agaves, Yuccas, and Related Plants on Yucca faxoniana and they indicate that it can tolerate severe cold - a relative term here in Colorado - down to approximately 0 degrees Fahrenheit. We frequently get a tad colder, but I still think siting will play a factor in its success or failure.

Yucca elata with Yucca baccata in the upper right.
Photo by Bill Jury.

I also managed to get a photo of my Yucca elata yesterday. It is putting on new growth and has very little winter damage. Not too bad for a year-old seedling.

Grass trees and ginkgo.

A few days ago I received a grafted year-old Ginkgo biloba 'Mayfield' from Kigi Nursery. This happened to coincide with one of Colorado's late-Spring cold spells, so I simply potted the little tree up. Besides, the ginkgo hasn't broken dormancy yet, so forcing it to wait for warmer weather shouldn't really be an issue. For those unfamiliar with this particular cultivar of ginkgo, 'Mayfield' is an upright columnar form, and it is probably the narrowest in cultivation. Introduced from Ohio in the 1940's, Ginkgo biloba 'Mayfield' is not as well known as 'Princeton Sentry', but it is the perfect specimen tree for our small, commensurately narrow yard.

Ginkgo, of course, is precisely the sort of tree one would expect to find in Sir Arthur Conan Doyle's "Maple White Land" from The Lost World. It is a survivor, a relic from the early Jurassic; other ginkgo-like fossils date to an even earlier era. Ginkgo biloba was discovered by naturalist Engelbert Kaempfer in China, as so many of our "living fossils" have been, in 1691; over 400 years later, this sole-surviving ginkgo species now has a cosmopolitan dispersal as an ornamental tree with an ancient lineage. Botanist Peter Crane has written an excellent, comprehensive natural history of the ginkgo tree in Ginkgo: The Tree That Time Forgot (Yale University Press, copyright 2013). If you're interested in the story of this incredible survivor, then this is the book for you.

Just a couple days ago, after almost two months, the first Xanthorrhoea australis seedlings poked their pointy leaves out of the soil. As of this moment, six of the eight seeds appear to have germinated, which is a better ratio than I anticipated. Actually, I had started to wonder if the seeds were viable, or if I'd used an appropriate soil blend in which to germinate them. I guess I should have known that some seeds just take longer to germinate. As their name implies, these seedlings look for all the world like a tiny sprout of grass...perhaps with blades a bit more stiff and thick, but grass nonetheless. It will take as long as twenty years before Xanthorrhoea australis develops its characteristic trunk. I will try to post a picture of the seedlings in the near future. But, as I tell myself, this is what these will look like one day:

Xanthorrhoea australis. Photo courtesy of Australian Seed.

I borrowed this photograph from Australian Seed. They do ship internationally, according to their website, so if you can't source this amazing plant domestically, perhaps give them a try. It looks like Australian Seed carries the seeds of many other native (and unusual) Australian plants, so keep them in mind for these as well (I know I will). And, while the Ginkgo is a far better-known Mesozoic survivor, the family Xanthorrhea is ancient as well - it is possible that the Cretaceous saw the first Grass Trees growing in the soil of a fractured Gondwana.

Ephedra nevadensis and cold-stratification.

Last night I began cold-stratifying a small flat of six to eight Ephedra nevadensis. I have promised myself I will leave them in the refrigerator for the requisite 30 days. Unfortunately, the single seedling I managed to propagate last year appears to have succumbed to the elements (namely, my own distraction - it was not with my other seedlings and I missed watering it). A small pot in an arid climate can kill a seedling of even very xeric-adapted plants. It's still possible that the seedling is simply dormant, since they grow very slowly, but maybe not. Gardening is a learning experience, even if the learning curve is sometimes steep.

I also ordered packets of Ephedra intermedia and Ephedra equisetina (Bluestem Joint Fir), and I look forward to propagating these as well.

A reliably hardy palm for Colorado!

Yucca rostrata.

Okay, not really. But the tree yuccas are really cool, and Yucca rostrata 'Sapphire Skies' has a very tropical feel. Its upright habit and rigid leaves make it a striking specimen in the garden. 'Sapphire Skies' might be slightly hardier than the species. Its leaves are stiff, shorter than the species, and, according to Monrovia, "form a dense crown atop an elegant single trunk." Like most yuccas, Yucca rostrata is slow-growing, but it is likely one of the hardiest tree-like yucca species...and it is certainly one of the most beautiful. On the issue of hardiness, siting seems to be a significant contributor to success or failure. Depending on the source, Yucca rostrata 'Sapphire Skies' is hardy to USDA Zone 5b, while the species seems to be hardy to USDA Zone 6. Again, proper siting is key. Its soil should drain well and it should be in a location with full sun. In Colorado's dry climate, this would be an excellent candidate for the tropical garden.

Yucca rostrata 'Sapphire Skies'.
Courtesy of Monrovia.

Other hardy tree-like yuccas includeYucca elata, commonly called the Soaptree Yucca (hardy to USDA Zone 6a, though mine sailed through temperatures that dropped below -10 Fahrenheit this past winter), and the compact form of the famous Joshua Tree, Yucca brevifolia v. jaegeriana (reportedly hardy to USDA Zone 5). Even the standard form of Yucca brevifolia is hardy to USDA Zone 6a. Yucca torreyi (reportedly hardy to USDA Zone 5), Yucca faxoniana 'Giant Faxon' (from the northern population of the species, reportedly hardy to USDA Zone 5, though other sources indicate the species is only hardy to Zone 8; perhaps Yucca faxoniana is similar to Agave parryi, where different populations and subspecies can vary wildly in hardiness, but I digress), and Yucca schottii, the Mountain Yucca (hardy to USDA Zone 6a or maybe Zone 5b) are a few others that might prove marginal in Colorado but would work great in the right microclimate. Many of these species appear far more at home in an arid desert landscape, but Yucca rostrata lends itself to either the tropical or the desert garden, as would a mature grove of Yucca schottii. With marginal yuccas, perhaps the only way to know if a species is hardy in your garden is to try growing it. It is definitely worth it - these are amazing plants!

Yucca schottii. Photograph by Georges Jansoone.
This specimen looks for all the world like some species of Pandanus, minus the prop roots.

Conifers that evoke the tropics.

Conifers are iconic trees, though they are not necessarily what comes to mind when you picture a lost world. Of course, if you've seen BBC's Walking With Dinosaurs series, you've seen a tropical conifer island: New Caledonia. Podocarpaceae, Araucariaceae, and Cupressaceae make up a significant part of the flora on this relatively small splinter of ancient Gondwana. According to The Gymnosperm Database, New Caledonia holds 3 endemic conifer genera and 43 endemic conifer species. Actually, New Caledonia has no non-endemic conifers, and its conifer biodiversity is rivaled only by New Zealand (103,738 square miles), Sichuan (187,259 square miles), and California (163,696 square miles)...and, at a mere 7,172 square miles, New Caledonia is much smaller.

Brachiosaurus and Araucaria. From Walking with Dinosaurs miniseries.
Filmed in Patagonia (these trees are Araucaria araucana), as well as
Tasmania, California, and New Zealand.

So, how does a gardener capture the look of these tropical conifers in a temperate zone? Well, if you live in a more moderate climate (say, USDA zone 7-10 with sufficient rain), then you could always plant the hardier Auracaria araucana, or Monkey Puzzle Tree. They grow well in the boreal rainforest of the Pacific Northwest. However, in Colorado's USDA zone 5 the winters are simply too brutal. This is a land of extremes, and though I haven't tried killing any podocarps or araucaria yet, I doubt they would survive the frigid stretches of our average winter temperatures.

Back to the question. Does this mean there are no tropical-like conifers for Colorado? Well, with the right siting there might be a few...

Cryptomeria japonica 'Araucarioides'

Cryptomeria japonica 'Araucarioides'.

A cultivar of Japanese Cedar, the name says it all. Also known as the Whipcord Japanese Cedar, the needles are held tightly to the branches, giving it a very Araucaria-like look. Hardy to USDA Zones 5/6 through 9, the Whipcord Japanese Cedar is a perfect candidate for a pseudo-tropical garden. It should be situated away from drying winter winds, an important consideration in Colorado. Cryptomeria prefers moist, well-draining soil, and its growth is fastest in humus-rich soils. Not the conifer to plant if drought is a consideration, an equally important consideration in Colorado. Still, if these environmental conditions can be met, there is really no reason why Cryptomeria japonica 'Araucarioides' could not be attempted. Purported to remain much smaller than the species, the Whipcord Japanese Cedar should reach a height of 15 feet at maturity, its foliage increasing in fullness as it grows. Again, success with this conifer will undoubtedly be determined by microclimate and siting. Still, some sources do indicate that Cryptomeria can be drought-tolerant once established.

Pinus x schwerinii 'Wiethorst'.
Photograph by Sean Callahan,
American Conifer Society.

Pinus x schwerinii 'Wiethorst'
A cultivar originating from a witches' broom found on a hybrid between Pinus strobus (Eastern White Pine) and Pinus wallichiana (Bhutan Pine). Pinus strobus contributed an increased hardiness, Pinus wallichiana contributed long needles, and the witches' broom contributed a manageable size for smaller gardens. The longer, graceful needles are reminiscent of the tropical Central American pines and sub-tropical Mexican pines, so Pinus x schwerinni 'Wiethorst' definitely evokes the tropics. With space, the original Pinus x schwerinii could offer, albeit on a grander scale, the same effect as its dwarf clone. Pinus x schwerinii 'Weithorst' was one of two conifers chosen by the American Conifer Society as their 2014 Collectors Conifer of the Year.

Sciadopitys verticillata 'Sternschnuppe'
This is a narrow, upright cultivar of the Japanese Umbrella Pine, a monotypic genus whose fossil record dates back about 230 million years. Any Sciadopitys cultivar would make a stunning specimen conifer in the pseudo-tropical garden. Despite its exotic look, the Japanese Umbrella Pine is definitely not tropical. Hardy to USDA Zones 5 through 8, Sciadopitys verticillata, like Cryptomeria japonica, prefers moist, rich, well-drained soil in full sun, though it does prefer some afternoon shade in hot climates (during the summer, that'd be you, Colorado). It is a slow grower. The Missouri Botanical Garden cautions that it might "not be reliably winter hardy throughout USDA Zone 5", and suggest that it should be planted in a site protected from winter winds. Sciadopitys verticillata 'Sternschnuppe', commonly known as the Shooting Star Japanese Umbrella Pine, has thicker, broader, and shorter needles than the species, creating "a uniquely appealing texture not found in other conifers", according to Iseli Nursery. On the downside, this species can be subject to snow and ice damage during the winter, so it does require extra attention regarding snow loads. If snow is an issue (as it occasionally is here in Colorado), consider Sciadopitys verticillata 'Joe Kozey', another narrow cultivar purported to handle snow loads better due to its branches being sturdier and held closer to the trunk. Regardless of the cultivar, Sciadopitys is an amazing, ancient tree that never fails to garner attention in the garden.

Update on germinating Leucospermum cordifolium and Metasequoia glyptostroboides.

As of this evening, two of the Leucospermum cordifolium (Nodding Pincushion) seeds have germinated. What remains of their seed's outer shell is very hard, which makes me appreciate the hydrogen peroxide solution that they soaked in prior to planting. This solution, only about 1%, breaks down a clear (and presumably tough) outer coat on the already tough shell. The two seedlings are robust, though slow-growing.

Metasequoia forest in Cretaceous Montana. Artwork by Julius T. Csotonyi.

Metasequoia glyptostroboides (Dawn Redwood), on the other hand, appears to a far more delicate, tender seedling, though looks can be deceiving. The tallest of these seedlings is about a centimeter tall. As of this morning, only two of the seedlings have uncurled into an upright, vertical position; the remainder are still curled toward the soil, slowly unfurling upward. The protocol for germinating Dawn Redwood seeds is various and, occasionally, contradictory. I used an amalgamation of the commonest advice from reputable online sources, mostly adhering to the protocol published by Forest Research, a division of the Forestry Commission UK titled "Seed storage and pretreatment for Metasequoia glyptostroboides" (I've included the link here; also note their downloadable Raising Trees and Shrubs from Seed at the bottom of the page). I started by using a damp soil blend of 1:1:1:3 peat moss, perlite, sand, and potting soil in plastic pots, planting the seeds on the surface and covering with a dusting of fine potting soil. After planting the seeds, I placed each of the pots in ziploc bags in the refrigerator. These remained in the refrigerator for approximately five weeks, a shorter time than Forest Research recommends. Once I removed the pots from the ziploc bags, I placed them in a sunny window facing to the west. I've kept the soil damp (water from beneath) and misted the soil surface one to two times daily. Our house (as well as our climate) is far from humid, which is why I've suspected that misting the soil surface might be necessary. Thankfully, mold has not been an issue for me with this method, but I think that has more to do with clean seeds and soil, a shorter period of cold-stratification in the ziploc bags, and Colorado's typically low humidity.

What remains to be seen is how well Metasequoia glyptostroboides tolerates our seasonally xeric climate...and if I have room for such a potentially large tree in our narrow yard!

Finally, if you like Julius T. Csotonyi's paleoart in this post, you can check out his many works and purchase prints through his website here. He has some amazing artwork!