Gardening With Hardy Tropicals

by Tony Avent Written for the News and Observer, Raleigh NCMay 24, 1997
As gardeners get more and more adventurous, the temptation increases to try and grow plants that aren't supposed to grow in your area. Whether it's a type of horticultural rebellion or just curiosity, I'm not going to guess. As you might guess, the original concept of greenhouses was to allow gardeners to grow new plants that shouldn't grow in their climate. Part of this attempt to extend hardiness zones derives from our desires to go where we aren't supposed to grow.
The concept of growing tropicals outdoors never grew beyond a handful of collectors, until the realization was made that references to hardiness that were made in England had no relevance to hardiness in the southeastern United States. As it turned out, hardiness was greatly increased with sugar production inside the plant, which in turn correlated to high summer temperatures. In England, the sun and high temperatures required to produce sugars were virtually non existent. Many "hardy tropicals" would die at 25-30 degrees, while the same plant would survive sub 0 temperatures in the southeastern US.
The myth was perpetuated by gardeners in Florida, who confused ground hardiness temperatures with container hardiness temperatures. When these Florida growers would have a sudden temperature drop to 20 degrees, many of these vigorously growing tropical would die and hence the incorrect hardiness data. In our climate, where temperature cool slowly in the fall, many of these plants actually have a dormancy mechanism that can be induced with gradual temperature decreases.
As with all plants of marginal hardiness, these hardy tropical should be planted early in the season. This will allow the plants to become well established and build sugar storage for the upcoming winter dormant season. Tropical plants installed late in the season, will usually not survive.
Probably the most common hardy tropical in the area are canna lilies. If you read all of the English reference books, they indicate a total lack of hardiness, but as anyone riding down the road can attest, they return each spring with a vengeance. Most cannas are reliably hardy to 10 degrees, and should easily tolerate temperatures of 0F or slightly below with a good mulch.
With so many cannas to choose from, I have opted to grow primarily those sporting variegated foliage. There is an old canna, probably originating in India that is sold under a variety of names including C. Minerva', C. Nirvana', C. Striped Beauty' and probably a few other names that have cropped up recently. This canna has green leaves, highlighted with whitish yellow contrasting veins. After it reaches 4' in height, the top of the stalks are covered with beautiful slender reddish buds that open with large yellow flowers.
Other nice variegated foliage cannas are C. Pretoria' or C. Bengal Tiger' from South Africa. This 1963 import into the US has similar colored leaves to C. Minerva', but with a dramatic narrow red edge around the leaf. Also the flower color is bright orange.
The newest South African imports to the variegated foliage mix are C. Durban' a 4' tall clump with purple leaves with red veins and scarlet red flowers. The other is C. Pink Sunburst', a 2.5' vigorous dwarf with similar purple foliage with red veins, but topped with rich pink flowers, and C. Phaison' a 7' tall canna again with purple foliage and red veins, but topped with vivid bright orange flowers.
Probably the most unique of the new cannas is C. Stuttgart'. This foliage canna was found by Bob Hayes of the Brooklyn Botanic garden in a botanical garden in Stuttgart Germany. This canna has dusty green leaves with giant blotches of white in each leaf. Because this canna has so much white, it will only prosper in light shade. The small butterscotch flowers pale in comparison to the striking foliage.
Elephant ears are another plant that have just begun to be recognized as potentially hardy. The typical elephant ear with green foliage (Colocasia esculenta) has been hardy for me for many years with it giant grey green leaves to 4' in length. Botanically, some experts classify all of the colocasia that I mention under C. esculenta, but their habits are so different that I keep them under the more commonly recognized names.
My first realization that there was potentially more hardy elephant ears came on a visit to Jernigan's Gardens in Dunn, NC nearly 10 years ago, where I saw a lovely 2' tall elephant ear with black foliage and wide green veins (Colocasia antiquorum Illustris'). I was given a piece to try and found it to be extremely reliable, although very late to emerge (June). With a stoloniferous habit, C. Illustris' soon made a sizeable patch in a rich moist setting.
My next elephant ear was Colocasia fontanesii. This 7' giant held 3' long leaves atop wonderful jet black stems. Each leaf was a glossy metallic green black. Like C. Illustris', C. fontanesii spread on top of the ground thru thick rhizomes. For us, this has proved to be one the absolute hardiest and first emerging of the elephant ears.
My favorite of the elephant ear group has to be the new C. Black Magic'...imported into the US by California's Walter Pagels, who found it in a botanical garden in the Phillippines. This giant grower to 6' tall sports black stalks topped with 3' long powdery purple black leaves. This also has proved to be reliably hardy to 0F in our trials, although it can be slow to return from dormancy.
Hardy palms are another area that is exploding in popularity. Without a doubt, the hardiest of the palms available is the southeastern native needle palm, Rhapidophyllum hystrix. This clumping palm to 8' tall is best know for its sharp needles around the base of the trunk that protect the seed from predators.
Our NC native scrub palm, (Sabal minor) is another candidate for area gardens. Reliable in hardiness to -10F, this palm is even happy enough to reseed around the garden. While sabal minor is thought of as a non-trunked palm, this is not quite true. In actuality, the trunk grows downward instead of up. If you have ever tried to dig a scrub palm of any size, you will soon realize that it is nearly impossible due to the depth of the trunk (up to 5' deep).
The hardiest of the trunked palms is the windmill palm (Trachycarpus fortunei). This Chinese native can easily make a 30' specimen in this area. Hardiness is variable within seedlings, so be sure to ask if the seed came from a hardy specimen. My first dozen windmill palms died until I found a hardy seed source. The oldest trees around include 40+ year old specimens at Taylor's Nursery in Raleigh, two equally as old specimens in Charlotte, and a specimen at a church in Greensboro NC, and several nice clumps at the Norfolk, VA zoo.
Other "tropicals" that have proved hardy in our trials include: purple heart wandering jew (Setcresea pallida), dwarf chenille plant (Acalypha pumila), asparagus fern (Asparagus springeri), split leaf philodendron (Philodendron selloum), turks cap (Malvaviscus drummondii), most ginger lilies (Hedychium sp.), tongue fern (Pyrossia lingua), banana shrub (Michellia figo), cast iron plant (Aspidistra elatior) false dracena (Cordyline australis).