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.