Tropical Flowering Tree Society

Summer 1999

TROPICALS AND SUBTROPICALS IN THE DESERT

Few people would consider Arizona a lush tropical paradise. After all, it's so hot and dry here that the Yuma weather stations were told years ago not to mention how long temperatures lingered in the 110+ mark after June because it would adversely affect both tourists and residents. For those of us who live here, however, summer heat and dryness are facts of life. Other critical problems for Arizona gardeners are desiccating winds, especially in the Spring during the interminable dry season, and "caliche" layers of salt deposits in the soil that cause very shallow rooting and/or rotting of roots. But the visitor flying into the "Valley of the Sun" (as it’s known), gazes down from the airplane at a vast expanse of green oasis. Despite the problems, this oasis holds great horticultural promise. A surprising number of lush tropical and subtropical plants, discussed below, thrive here if given enough water to survive the heat stress.

Metropolitan Phoenix has a frost-free period averaging 329-363 days (with all-time record lows of 18-25 degrees). It also suffers, however, through close to 100 days a year of 100 degrees plus weather (85-107 days depending on the local topography). Average annual precipitation is in the area of 8-10 inches, but variability is great. Night-time lows which hover around 80-90 during most of the summer tax plants' growth. Relative humidity, compounded by hot desert winds, is low, averaging well below 50% all day from about April to the end of June, and even when our "monsoon" kicks in in July, it averages below 40%.

Before I take you on a tour of our oasis, let me first mention three criteria that I have used in selecting our most significant flowering plants. The first is that this little article will not focus on the typical xeriscape (or desert) landscape plants that are recommended for southern Arizona according to the USDA Hardiness Zone Map. I focus on what many landscapers in the Southwest now call "oasis" gardening, creating microclimates by grouping plants for protection from hot afternoon sun, dry desert winds, and occasional winter cold snaps. Providing this kind of oasis location makes it possible to grow trees and shrubs that normally do not thrive outside Zone 10a or 10b (average lows of 30-40 F) by increasing ambient humidity as well as providing dappled shade. The second criterion is that I list only those plants successfully in cultivation as "traditionals" or "experimentals" in southern Arizona at the present time. Thirdly, fruiting trees and shrubs are excluded. Although Phoenicians are proud of the many beautiful palms in our valley, I will only mention here Arizona’s signature desert trees, the date palms which abound in old stands dating back to the date-producing years in the first half of the century, and the native palm (Washingtonia filifera and its Mexican cousin W. robusta), which sprout in every yard and ditch! I apologize for describing much more than just our tropical and subtropical flowering trees. But hopefully you will find the tour of our oasis justifies a bit of a horticultural ramble.

LARGE TREES

The always interesting Chorisia speciosa must be considered one of the queens of our valley, flowering better than in some coastal California areas. Although rarely seen in the city itself, there is a nice specimen of the fragrant-flowered Dalbergia sissoo (D. latifolia) in the Boyce Thomson Arboretum just east of Phoenix in the foothills. The giant Enterolobium cyclocarpum is an experimental which has made it through two years of Arizona summers and winters. It seems to love the heat if its roots do not dry out too much. Most Eucalyptus, of course, feel right at home in southern Arizona.

Although not spectacular (or even noticeable some say!) in bloom, fig trees provide a verdant backdrop in our gardens, but are unfortunately underplanted in Phoenix. Ficus microcarpa does fabulously, providing a wonderful green canopy that truly makes the desert look tropical. There is little to compare to the impression one gets from a heavy Arizona Monsoon rainfall in July, glistening on a fig’s green leaves and trailing bougainvillea after months of dusty and windy Spring weather. The fresh colors conjure up images of Romanticized colonial India.

Speaking of India, I have seen F. benghalensis, recently planted by the Phoenix Zoo in one of their exhibits, come through both winter frosts (down into the high 20s F) and scorching summer heat in areas exposed to full sun. Ficus elastica, including the dark-leaved variety, eventually grow quite large here, up to 30-40 feet, although only when protected from afternoon sun when young. Wonderful specimens can be seen on the Arizona State University campus, which incidentally has a very inviting arboretum for those interested in viewing mature specimens of many of the plants in this listing. Buddha’s tree, Ficus religiosa, deserves a chance in the valley, if for no other reason than to keep the India motif going.

Grevillea robusta grows and flowers well here, although much like Casuarina trees they tend to become straggly. The queen among all our large flowering tropicals is, of course, the Jacaranda mimosifolia (or obtusifolia) that adorn every mall and large house in the Valley of the Sun. Frequently we are treated to both spring and fall flowering. Among more common sights are the old Melia azedarach and Melaleuca leucadendron plantings. Rhus lancea, known locally as African Sumac, is an old stand-by that grows relatively large and perfumes the spring air with its flowers' pungent aroma. Uncommon but distinctive are the Schefflera actinophylla (Brassaia) trees, which can grow quite large in Phoenix. There is one beautiful specimen I know well that dwarfs the four-story condo complex I used to live in. Finally, Schinus molle was one of the first trees planted in the valley and specimens have graced lawns for almost a century now, and Sch. Terebinthifolius has now been added to the gardener's palette. The curious South American tree Tipuana tipu, is also quite hardy here, as is Sesbania tripetii. These are our limited supply of large tropical and subtropical flowering trees.

SMALL TREES

"Small is beautiful" should be our motto. Detail in the garden is where Arizona’s landscape is particularly beautiful. Acacias do wonderfully in the desert, although the lushest-looking ones, like A. abyssinica and A. farnesiana are not as frequently planted as they ought to be. In older neighborhoods, one will come across nice specimens of Albizia julibrissin, but nurseries seem not carry them anymore. Widely used and immediately striking, Bauhinia trees are the pride of Phoenix's smaller yards and perfume the air lightly when blooming. Bauhinia purpurea, B. x. blakeana and B. galpinii love Phoenix heat, although they look unkempt when not watered sufficiently. Bauhinia variegata 'Candida' looks good too here. The common California road tree, Callistemon citrinus, is a standard commercial landscaping tree, doing as well in the desert as in California. A nice weeping effect is achieved in some of our plantings by using Callistemon viminalis, the "Weeping Bottlebrush" tree. Among other fairly common trees, Cassia artemisioides does well in southern Arizona. I have seen mature Cassia fistula trees here but they do not seem to be currently planted. All of these flower profusely in the spring or early summer with splashes of yellow and red.

Two recent additions to the landscaping palette are Clusia rosea and Coccoloba uvifera, which the fantastic Phoenix Zoo have added to their Colombian Bear exhibit. So far, after a year and a half, they are doing fine. The near absence of Delonix regia in southern Arizona is a mystery to me. A huge specimen grows on the south side of the Arizona Legislature building and flowers well. Although it hasn’t developed the thick lateral buttress roots I have seen on some Hawaiian flamboyant trees, growth is impressively tall and healthy. Why Delonix has not been planted more frequently in Arizona will surprise our Floridian and Hawaiian fellow plant-lovers who enjoy their distinctive outline and flowers almost everywhere. Another conspicuously absent family of small trees is the Erythrina genus. I have been told that Phoenicians used to grow Erythrina trees, probably the common California E. crista-galli, but I have not seen any in the five years I have traveled around the state. On the other hand, every neighborhood has bananas, Musa acuminata and its look-alike Ensete ventricosum, growing in some yard. More than any other plant, these give Arizona gardens a lush tropical look. Unfortunately, I have never seen it fruit here and occasional winter frosts and dry winds do take their toll on the large leaves.

Other common plantings include Ficus lyrata and Ficus benjamina, which do well with a bit of shade. Noble and delicate Lagerstroemia indica bloom during the heat of summer, gracing avenues on the Arizona State University campus. Two other small tropical trees, Ricinus communis, and Tecoma stans, will be found in both new developments and older neighborhoods, although Thevetia trees, with their yellow bell-shaped blossoms are more prolific here.

SHRUBS

Because creating microclimates for them is not difficult, shrubs are some of the valley's most versatile plants. As a substitute for the lovely tropical odiaeums, we often plant Aucuba japonica, which imitates the Codiaeum's yellow colors but withstands our climatic extremes. Allamanda cathartica, Alocasia macrorrhiza, Ardisia crenata, and Asparagus densiflorus are common. Bouvardia longiflora is a delightful tropical from Mexico. I have had some success with it in an Eastern exposure but regular water is a must. Brugmansia suaveola and Brugmansia S. 'Plena' are grown, but they do poorly during mid to late summer after flowering in the late winter and spring. Nevertheless, the exotic blooms are worth the extra effort. The same could be said of Brunfelsia pauciflora, which is not a great performer here, but rewards the patient gardener. (Continued on Page 6)

After two years of service as President of our Society, there are many mixed emotions as I step aside for Lewis Eastlick to lead our organization.

Pride - that our Show and Sale was very successful this year, we undertook a TFTS planting project at MetroZoo¼hopefully the first of many; and we hosted the Menninger Color Conference!

Appreciation - that the Post Office issued a Royal Poinciana stamp, but we have to buy those other three to get it! I’m sure Helen (Miss Poinciana) Eidson still smiled and felt all her efforts were finally rewarded.

by Dimitris T. Petropoulos

While collecting flowering trees, we were introduced to another sense of beauty—perfume! Many we were introduced to are rarely cultivated. Some are spectacular in bloom, while others are hardly noticeable except for their extraordinary scent.

Dais cotinifolia (Thymelaeceae)

Common name: PomPom tree

In its native home of South Africa, there are reports of 20-foot specimens, but in cultivation we expect smaller trees.

Flower clusters in dense umbels, 3 inches across, appear at the tip of the branches for several weeks into September. They are pale lilac to a delicate pink color, and their fragrance is Daphne-like. In our late Winter to early Spring, they are briefly deciduous.

Fagrea species. "Arbora" (Loganiaceae)

A 20-foot tall by 25 foot-wide tree, planted on the slope of an active volcano in Hawaii, provided us with seeds. We were told the tree was collected in the South Pacific.

Around mid-March, flower spikes begin developing at the apex of each branch. By the end of April, the panicles are 8 inches tall by inches wide. Soon thereafter, 3-inch long by 2½ inch wide trumpet-shaped flowers begin opening—a pale cream the first day, maturing a few days later into a rich ochre color. The whole tree canopy is a mass of sweetly scented flowers into mid-June. Spectacular!

Canaga kirkii, sny. Desmos chinensis (Anacadiaceae)

Common name: Dwarf Ylang-ylang

When fragrance is the prime objective, this tree deserves just consideration for background planting. A fully grown specimen can grow to 15 feet. Our tree is about 8 feet and has been blooming, on and off, since it was less than one foot tall. It has been evergreen and ever-blooming for 8 years! In the evening, the perfume is wonderfully sweet, during the day the scent is diminished and altered—the smell is similar to fruit peelings.

Rauvolfia caffra (Apocynaceae)

Common name: Quinine tree

White-scented flowers in dense heads cover this 18 foot tall evergreen tree during South Africa’s winter into spring season. The tree grows fast and is considered showy.

Our seedlings are about 3 feet tall. The leaves are large, a foot long and half as much wide, and are distinguished by a prominent mid-rib of striking white.

Portlandia grandiflora (Rubiaceae)

Common name: Glorias floridas de Cuba—

Mature trees have been reported to grow 15 feet. It has elliptic, leathery leaves and blooms from late Spring into Fall. The flowers are 12 cm long, white tinged with pink and trumpet-shaped. The air is filled with their luscious perfume, reminiscent of warm, creamy chocolate! It is most fragrant in the evening and early morning, but can be appreciated most any time.

Nuxia floribunda (Loganiaceae)

Common name: Kite tree

Another South African native that can mature to 30 feet is expected to be smaller in our region. This evergreen likes rich soil (fertilizers!) and plenty of water to perform at its best. Small, scented, fluted flowers in 8-inch foamy, cream-colored masses envelope the whole tree for much of the winter.

Dimitris Petropoulos is at Ernesto’s Good Earth Nursery, 6901 S. W. 97^th Avenue, South Miami, FL 33173, Telephone (305) 279-4665. Ed.

Caesalpinia pulcherrima, on the other hand, is a common highway plant that reaches heights of ten feet in protected areas. It can be trained as a standard and delights the eye with brilliant reddish and orange yellow blooms that make up for the lack of flamboyant trees in Arizona. C. gillesii is a slower grower, although signs are it likes Arizona heat and resists the drought. Calliandra californica, red fairy duster, Calliandra haematocephala, and Carissa grandiflora are all common parking lot plants. Ironically, the sun-loving Carissa seems to flower better if given some mid-day or afternoon shade here. One of my favorite plants is Cestrum nocturnum. Granted, not a impressive plant in itself, but it is one of the few truly fragrant shrubs that turns the desert into an enchanting tropical oasis with its nocturnal perfume. I have seen Gardenias here, including the Tahitian gardenias made into "standards," but they resent Arizona’s demanding climate. Occasionally one will come across Clivia miniata and Cyperus alternifolius - amazingly planted in full desert sun in some places! Euphorbia milii, Fatsia japonica, and Guaiacum sanchum or lignum vitae are about as frequently used as the previous two plants. Guaiacum can be seen in the Phoenix Botanical Garden, and is currently offered in Garden stores valley-wide. Euphorbia pulcherrima is a challenge in the valley of the sun.

All subtropical and tropical Hibiscus species are currently commonly available from nurseries and can be grown here as shrubs, hedges, or accent plants (many in "standard" form) in shade to full sun. There is enough light here to provide good flowering even on northern exposures. For some reason the common red rosa-sinensis clearly out-flowers the rest. Specimens of Ixora coccinea and Jatropha integerrima that I have seen in desert gardens survive, but suffer from heat and water stress which induce whitefly infestations in spring and summer. About as common as hibiscus, I list jasmines with the shrubs because they rarely act like vines here, tending to ramble on the ground instead of climbing and exposing themselves to desert winds. Jasmines do very well in Phoenix, but unfortunately, some of the intensity of their fragrance is lost in the dry air. Especially good are Jasminum polyanthum, J. nitidum, and J. sambac. Justicia brandegeana never seems to get out of its infancy, hugging the ground to about 1 or 2 feet maximum, although otherwise it seems to be quite resilient. On the other hand, Lantana camara and montevidensis can become huge, although most people trim them severely to keep them as groundcovers. Monstera deliciosa and Philodendron selloum grow as shrubs in Phoenix, or at least I have never seen one here exposed to the elements on a tree's branches. Nevertheless, both grow quite well as understory accent plants.

One of my favorite shrubs is Murraya paniculata, which manages to perfume the dry desert air here unlike most fragrant plants. Growth is fast and flowering assured. Despite this, it is rare in desert gardens. The hardy Saharan native Nerium oleander, on the other hand, abounds in the Valley. Except for a short summer respite, it flowers continuously. Pittosporum tobira grows well except where afternoon sun scorches its leaves. The variegated form's colors show through particularly well in the bright light of the desert. Plumbago auriculata or Cape Plumbago is a standard spreading shrub of Phoenix gardens, doing very well in full sun, although it languishes without sufficient water while flowering.

VINES

Our walls and fences are adorned by colorful vines throughout the Fall to Spring growing season. Very rare but worth the effort are Antigonon leptopus and Beaumontia grandiflora. All types of Bougainvillea, on the other hand, thrive in Phoenix and adjacent areas and are the commonest flower seen here. Most of them flower almost continuously except for short periods of excessive summer heat. Single-stemmed bougainvillea can be trained into small accent trees in desert conditions. Campsis radicans grows well against northern or eastern walls and under eaves (ironically for such a strong grower, because of the shade provided). The jury is still out on Clerodendrum thomsoniae, although nurseries here are pushing this little vine. Bugs and heat seem to destroy it easily and lack of humidity, I think, restricts its flowering. Campsis radicans, Distictis buccinatoria, Dischidia nummularia, Ficus pumila, Gelsemium sempervirens, and Hardenbergia comptoniana are all frequently seen adorning sunny or shady walls. The first two and mature species of F. pumila with their large leathery leaves and fruit are particularly common. Gloriosa rothschildiana makes an interesting addition to the garden while growing and remains dormant both during the winter cold season and during extreme summer drought.

Probably the commonest vine, although it does not flower as well as it should, is Macfadyena unguis-cati, which rambles over the hottest of walls and fences. Mandevilla x amabilis 'Alice du Pont', and Mandevilla splendens 'Dipladenia Rosea' are finicky in the desert but worthwhile for their show of flowers. I used to have a Pandorea jasminoides that cascaded over a balcony and flowered regularly provided the roots were kept shady. Some drought seemed actually to increase the spring show of flowers. Another pleasant surprise among the vines is Stephanotis floribunda. When I moved from the San Francisco Bay Area to Phoenix, it seemed to relish the change from humid and cool air to hot and dry. It went from giving us a lackluster performance to growing strong and flowering prolifically in the bright springtime weather of Arizona. This past year, after transplanting, it flowered again in late summer! Tecoma alata is occasionally seen. My Thunbergia grandiflora grew fairly well on an East-facing wall in Phoenix for me for years, although I suspect given sufficient room it could turn into a giant due to its quick growth. Finally, I should mention a very common vine, Trachelospermum jasminoides (which usually doesn't make it past the ground cover stage) and a more recent addition, Vigna caracalla.

Such is the variety of tropicals and subtropicals that you will see in the "valley of the sun oasis." When all is said and done, not a bad selection, if one can find the water! But then again, our warm-climate compatriots in Florida, coastal California, and Hawaii also have problems of their own. Hopefully this listing will help readers everywhere in their own experiments to beautify their surroundings by giving them a sense of which plants can withstand the extreme conditions of a desert.

Our unending quest is to "push" tropical plants as far north as possible, and subtropicals as far south as we can. Fabio writes about "oasis" gardening that creates micro-climates. What they grow in the relatively harsh climate in and around Phoenix will amaze you.

Fabio is now at the University of Calgary, Alberta. He took many of his tropical plants with him, and they are now comfortably ensconced in a greenhouse. Ed.

by Cathy Ryan

1. Chorisia speciosa—Floss Silk Tree

2. Lysoiloma latisiliquum—Wild Tamarind

3. Tecoma stans—Yellow elder

4. Koelreuteria formosana—Golden Rain Tree

This tree is native to China and grows rather wide. The leaves are compound and of soft texture. Flower clusters cover the crown and are followed by papery pods. Needs will drained soil and full sun for good flowering. This is a good landscape tree and the fruit can be used in dried flower arrangements.

5. Callistemon spp.—Bottlebrush

If used in the fight location the Bottlebrush tree can make a nice tree for landscaping. The tree is evergreen with aromatic leaves and red flowers in clusters that resemble a bottlebrush. Will grow in any soil and can take some shade. It’s growth rate is moderate and is some what salt tolerant.

6. Clusia rosea—Pitch Apple

A beautiful evergreen tree with large thick waxy leaves that are persistent. This native tree grows to 30' and spreads out. The flowers are 3 inches wide and pink and white followed by apple like fruit. Requires little maintenance and is salt tolerant. Also called the Autograph tree because messages can be inscribed on the leaves. Once I saw "Beam me Scotty" written on a leaf.

7. Kigelia pinnata—Sausage Tree

This tree native to Africa has fruit that looks like sausage and hangs under the foliage. The fruit can weigh up to fifteen pounds. Wine-colored flowers dangle from the branches. Can be found growing in many soil types and is not salt tolerant. A will known tree throughout out the tropics for the fruit and flowers.

8. Caesalpinia granadillo—Bridalveil

A medium sized tree with a medium growth rate and evergreen. Showy flowers are yellow and the fruit is a legume. Prefers full sun and any soil. One of the nice thinks about this tree is the frilly appearance to the foliage. In the last several years this tree is being used in landscapes more. Several of these trees have recently been planted in Miami Spring to see how they do as a tree. The bark is like Crape Myrtle in that it exfoliates from the trunk.

9. Covillea racemosa—Colville’s Glory

This is a large tree with a narrow crown to 60'. Flowers are in one-sided racemes and orange. Because the flowers are up in the tree canopy you will only know it is blooming when you look at the ground. The fruit is a pod 10" to 12" long and squirrels like the seeds. This tree has been compared to Delonix regia in how beautiful it is. Great fall bloomer as orange flower color goes great with the Halloween and Thanksgiving season.

10. Carmichaelia odorata—Scented Bloom