The structure of the garden: a look at tropicalesque trees for Colorado. Part one: Family Bignoniaceae.

Trees are natural garden structures. They provide shade, texture, depth, scale, dimension...the list goes on. Not every style of garden requires a forest, but any garden attempting to evoke the tropics must, by its nature, include trees. To begin, the tropics include flowering trees ranging from red (Delonix regia and Brachychiton acerifolius) to orange (Colevillea racemosa) to pink (Cassia javanica) to yellow (Handroanthus chrysotrichus) to purple (Jacaranda mimosifolia) to white...you get the idea. Our northern palate sometimes seems quite bland by comparison, limited to mostly pinks, roses, and whites, along with an occasional yellow. And most northern flowering trees are decidedly not tropical in appearance. Second, the tropics -- at least in our imagination -- are evergreen year-round. This, of course, is only true in tropical rain forest habitats, as many trees from dry tropical forests drop their foliage in response to drought in order to limit water-loss. Third, tropical trees, we tell ourselves, have bold leaves -- big and bright, like Ficus auriculata, or graceful and ferny, like Delonix regia or Jacaranda mimosifolia. Finally, tropical trees are lush. So, how does one create a tropicalesque look along Colorado's Front Range? Here are some ideas.

BIGNONIACEAE

Catalpa speciosa and other Catalpa species and cultivars
Indian Bean Tree, Catalpa
This family is well-known to gardeners throughout the world, from temperate to tropical, and it provides gardeners in Colorado (or similar climates) perhaps the most tropical-looking and easiest-care tree for structure in the tropical garden, Catalpa speciosa (Northern Catalpa). The large, bold, lime-green leaves that drape from its branches make it look utterly out of place, yet it tolerates the extremes of seasonal drought, heat, and cold that define Eastern Colorado and its Intermountain West neighbors. Sometime between May and June, long racemes of orchid-shaped white flowers grace its branches, occasionally obscuring the leaves. Hail can, of course, shred these trees bare, but give it a month and you wouldn't know it (assuming there are no successive hail storms). It grows quickly, reaching an average height of 40 - 70 feet tall (occasionally taller). It doesn't suffer from any major pests aside from the larvae of the Catalpa Sphinx Moth, which can cause significant defoliation. While this has not been a problem in my garden, the Catalpa Sphinx Moth is found in Colorado, so I would not rule it out. As implied earlier, summer hail seems to be the number one defoliator of Catalpa in my region.

Catalpa speciosa flowers and leaves.
Photo by Epibase
CC BY 3.0 (https://creativecommons.org/licenses/by/3.0)

In addition to Catalpa speciosa, Catalpa ovata (Chinese or Yellow Catalpa) and Catalpa x erubescens 'Purpurea' (Purple or Hybrid Catalpa) are excellent candidates for the tropical garden. Chinese Catalpa is similar to Northern Catalpa in many respects, but it is a smaller tree, typically reaching heights of  20 - 30 feet. Also, its leaves are lobed, similar to Paulownia tomentosa. This is a trait it shares with the Purple or Hybrid Catalpa, which, as its name implies, is a hybrid between the Chinese Catalpa and Catalpa bignonioides (Southern Catalpa). Both of these trees are considered hardy to at least USDA zone 5. Finally, Catalpa bungei (Manchurian Catalpa) is another small tree (20-30 feet) with similar growth habit and attributes. Like C. ovata, C. bungei is hardy to USDA zone 5. Some of these are only consistently available as seeds, but they grow quickly.

Chilopsis linearis
Desert Willow
Close relative to the catalpas is Chilopsis linearis, or Desert Willow, so-named because of its long, willow-like leaves. While the Desert Willow is not reliably hardy in northeastern Colorado, both the Denver Botanic Gardens and the Denver Zoo have grown several cultivars for a number of years. Many of these have visible evidence of past winter-kill, but they are definitely worth growing for a couple reasons. First, they are incredibly drought-tolerant, coming from the American Southwest, and can thrive on only 10-20 inches of rain annually (though they will appreciate a little more). Second, the blooms, which can range from pink to magenta to white depending on the cultivar, are larger than its catalpa relatives and every bit as showy. They tend to be large shrubs rather than stately trees in the north, but in the right microclimate there are few flowering trees or shrubs that quite like it. For USDA zone 5 regions, Chilopsis linearis 'Conchas Dam Pink' (a High Country Gardens exclusive) has proven extremely hardy, surviving -19F in my garden with minimal winter die-back. There are other zone-pushing cultivars available, but I can vouch for this one's hardiness. Thankfully, more and more cultivars are being tested that push this beautiful tree's range further north!

Chilopsis linearis 'Conchas Dam Pink'.
Photo from High Country Gardens.

x Chitalpa tashkentensis
Chitalpa
The third member of this triad is actually an intergeneric hybrid between the two: x Chitalpa tashkentensis. This tree was originally hybridized in Soviet-era Tashkent, the capitol of Uzbekistan, and is a cross between Desert Willow and Southern Catalpa. It is not quite as hardy as Northern Catalpa, but hardier than many cultivars of Desert Willow -- USDA zone 6. A couple common cultivars include 'Morning Cloud' (pale pink to white flowers with a more upright growth habit) and 'Pink Dawn' (light pink flowers with more spreading growth habit). Chitalpa leaves are an intergrade between its two parents: lanceolate, like Desert Willow, but wider at the center. How it will perform in my garden remains to be seen, but we rarely see temperatures that qualify us as USDA zone 5b, so I have decided to trial x Chitalpa tashkentensis 'Pink Dawn'.

x Chitalpa tashkentensis 'Pink Dawn'.
Photo by By Frau Siebenschläfer - Own work, CC BY-SA 3.0,
https://commons.wikimedia.org/w/index.php?curid=15726811

Subtropical species
Now, if we consider extreme protection techniques, we can look at some of the subtropical members of this family. This moves from zone-pushing to outright zone-denial. Perhaps the best candidate is the Southwest native Tecoma stans var. angustata (Arizona Yellow Bells), a shorter, more drought-tolerant form of  the more tropical Yellow Bells commonly sold. This is a species that spans the Western Hemisphere, with a range that includes Florida, the Southwest, the West Indies, and Central and South America. With such a wide distribution, it is also known as Trumpet Bush, Yellow Elder, and Esperanza. Understand that this is a tropical plant! But Tecoma stans var. angustata is purportedly one of the hardiest forms, tolerating temperatures dipping to 20F. Dieback occurs at 10F, but branches are quickly regrown once warm weather returns. This is the growth habit seen at the northernmost part of its range. If you are able to protect and maintain Arizona Yellow Bells as a tree, it will likely reach a maximum height of 10 feet, though some indicate it could grow taller with adequate water. Not towering, but figuring out a winter protection plan is very possible. And the yellow flowers are stunning! For more information and pics, check out the Arizona-Sonora Desert Museum here.

Tecoma stans var. angustata, Arizona Yellow Bells.
Photo by Mark Dimmitt, courtesy of Arizona-Sonora Desert Museum.

Tecoma stans var. angustata flowers.
Photo by Mark Dimmett, courtesy of Arizona-Sonora Desert Museam.

A second zone-denial candidate is actually a cultivar of a tree I mentioned at the beginning of this post, Jacaranda mimosifolia 'Sakai01', a dwarf jacaranda sold under the trade name Bonsai Blue Jacaranda. Hardy into USDA zone 9, this jacaranda is probably hardy to occasional brief dips just below freezing. I include it here because of its dwarf nature (10-12 feet tall), its ferny foliage, and its emblematic tropical flowering habit in a hue not found in more northern trees' color palette: purple-blue. Like Arizona Yellow Bells, protecting Bonsai Blue Jacaranda during winter is an attainable goal. For more information on the Bonsai Blue Jacaranda, visit Monrovia's page here.

Bonsai Blue Jacaranda. Photo courtesy of Monrovia.

So, are these last two appropriate for a Colorado garden? Without extreme measures, neither of these would survive the long, cold months of Colorado's winters, not even in Grand Junction or south of Pueblo. It's doubtful that they would even make it through an average October. But with the right protection and supplemental heating, it can be done. In a future post I will explore various winter protection techniques, but that is for a later series.

Not a palm tree, but close.

Yucca rostrata.

Okay, not really. Nevertheless, 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 include Yucca 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), 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 reminds me of a tropical Pandanus species, minus the prop roots.

Hardy conifers that evoke the tropics.

Conifers are iconic trees, though they are not necessarily what comes to mind when you picture an exotic 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.

Araucaria columnaris on the Isle of Pines, New Caledonia.
Photo by Rene Moutouh.

Araucaria bidwilli. Photo by S. Rae.

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 Araucaria 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, in spite of the fact that we seem to be edging toward USDA Zone 6. 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 coldest potential winter temperatures, not to mention Colorado's tendency to warm up early in the spring only to be followed by a limb-shattering cold snap the following month.

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'
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 (hence its cultivar name). 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 an issue, 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.

Cryptomeria japonica 'Araucarioides'.

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.

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

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 (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. Still, Sciadopitys is an amazing tree that never fails to garner attention in the garden.