Sun coleus looks cool during hot Southwest Austin summers

March 27, 2012

Story and Photos by Bill Scheick

The annual Oak Hill Gazette Home & Garden Issue is now in stands

I first encountered coleus as a college botany student. Our textbook detailed classic experiments showing the dramatic effects of light and darkness on the plant. The course professor also required each class member to grow a coleus cutting in water at home and then to graph its rate of root development over several weeks.

This might seem a simple enough undertaking, but there were unnerving (if unconfirmed) rumors circulating in class about unlucky students who had failed to keep their coleus alive. They presumably would have no graph to submit to the course instructor by the due date.

Lucky for me, my coleus lived, demonstrating how easy it is to propagate some plants by simple cuttings. I confess, though, that the washed-out color of my student coleus was hardly impressive.

Which was a shame because, botany class assignments aside, coleus is all about brilliant colors.

Coleus popularity

By the end of the nineteenth century American gardeners were completely enchanted by colorful coleus hybrids. Coleus was so commonplace by the first decade of the twentieth century that landscape advisors no longer agreed on its merits. In The Flower Garden (1903) Ida Bennet still praised the plant as “indispensable for ornamental bedding,” while in A Woman’s Hardy Garden (1903) Helena Ely wrote: “Would that the Coleus might vanish from the land!” She particularly objected to its gaudiness.

The popularity of coleus eventually declined, doubtless to Helena Ely’s relief. Today, though, this short-lived, mint-family perennial is no longer just a granny-flower from a bygone time. Its boldly bright foliage is “cool” once again, and (to mix metaphors) “hotter” than ever.

It’s hotter now in two senses. Not only is there renewed interest in the old-fashioned shade lover, there are also new radiant coleus cultivars that sparkle when directly exposed to sunlight.

Sun coleus facts

These sun-loving cultivars reverse the standard coleus pattern. Instead of rapidly fading in exposed outdoor settings, sun coleuses fade in too much shade. The sheen of the purple leaf-centers of ‘Kiwi Fern,’ for example, becomes dull and muddy in too much shade. ‘Kiwi Fern’ and other sun coleuses dazzle in sunlight — preferably half-day exposure in Southwest Austin gardens, particularly during summer.

Such fantasy coleuses were first marketed in the mid-1990s. Ever since then, vibrant coleuses have transformed sun-drenched landscapes, including an entranceway located a little north of Barton Creek Square Mall on South Capital of Texas Highway (360).

Unfortunately, sun coleuses are not equal in their ability to look cool in direct sunlight. Thick-leafed and dark-colored solar coleuses, such as ‘Burgundy Sun,’ tend to perform best. The anthocyanins determining their coloration serve as a sunscreen protecting their foliage from ultraviolet damage.

The paler and variegated sun coleuses, such as ‘Aurora,’ are less resilient. Although variegation appeals to some gardeners, it is in fact a genetic or viral accident that puts plants at a disadvantage. Yellow leaf-segments lack the chlorophyll necessary for the photosynthetic conversion of water and carbon dioxide into plant-sustaining nutrients.

While these yellow leaf-parts still serve a plant, the white segments are devoid of both chlorophyll and yellow pigment. White leaf-tissue is nearly useless to a plant, and it burns easily when exposed to too much sunlight. So even though variegated sun varieties such as ‘Gay’s Delight’ are more solar tolerant than standard coleuses, they cannot take as much direct exposure to sunlight as can the dark and solid-hued types.

Sun coleus care

Sun coleuses are exceptionally easy to grow provided they receive adequate light, moisture and drainage. While sun coleuses are more compact than shade varieties, they are just as vulnerable to fatal damage from cold.

In my Southwest Austin experience, the biggest challenge with in-ground sun coleus is maintaining sufficient soil moisture. While coleus foliage is not especially large, it does discharge a considerable amount of water vapor. This is normal. The loss of water through leaves (transpiration) results in an upward pull of moisture through roots. This is how these plants obtain the soil nutrients enabling them to grow and to stay upright. Insufficient water results in damage to leaves and roots.

Compared to sun coleuses, the roots of the antique shade-loving coleuses are easier to keep from drying out. Sun coleuses may glow in sunlight, but their roots are still old-fashioned in their insistence on cool, moist conditions in draining soil. So several inches of bark mulch help, and watering as needed is crucial.

In general, coleuses last longer, maybe up to three years, as container plants provided they are continually pruned to prevent a leggy-look and also to curtail bolting (“fatal” flowering). Sun coleuses, however, can be challenging as potted plants. Their need for direct sunlight and, at the same time, their opposite insistence on cool roots and regular, evenly distributed soil moisture can be hard to accommodate in a container. In Southwest Austin, direct exposure to sunlight and wind quickly heats and dries potted roots.

If you plan to keep your coleuses for more than a year, it is possible to bury their pots temporarily in the ground. Plenty of pine-bark mulch is a must. This in-ground technique keeps potted coleus roots cooler, makes watering them easier and enables container removal before a freeze.

The next hardest problem, again in my experience, is monitoring coleus’s growth rate. During Southwest Austin spring, sun and heat can wildly accelerate coleus development. Since coleus tends to die after flowering, a fast-growing selection can seem to be here today and gone tomorrow — hardly ideal.

Luckily, most sun coleuses are slow to flower, if they bloom at all. Even so, periodically snipping their stem tips retards flowering and subsequent plant fatigue. Such pinching back also helps maintain both foliage compactness and hue intensity. Even so, coleuses tend to slow down when temperatures range above 90º F.

Sun coleus selections

Identifying coleuses with certainty can be difficult. Some bear multiple names — ‘Alabama Sunset,’ for example, is also marketed as ‘Texas Parking Lot,’ ‘Coppertone,’ ‘Bellingrath Pink,’ ‘Alabama,’ ‘Alabama Red’ and ‘Alabama Sun.’ There is, as well, no complete record of all coleus cultivars.

Because coleuses are easily propagated from cuttings, some growers market selections cultivated locally under unregistered names. So think caveat emptor (buyer beware). When purchasing any coleus read its label carefully, if one is available — it often isn’t. Assess how the plant is performing in light or shade at the retail outlet. Question informed nursery personnel at such places as the Natural Gardener. Retail and plant-swap information is also available at www.coleusfinder.org.

Southern bedding trials have identified several particularly successful sun coleus series, such as Ducksfoot (including ‘Indian Frills’), Solar (including ‘Morning Mist,’ ‘Flare’), SuperSun (including ‘Plum Parfait,’ ‘Burgundy Sun’) and Sunlover (including ‘Red Ruffles,’ ‘Rustic Orange’). ‘Gay’s Delight,’ in the Sunlover series, has been praised for its amazing ability to reach full size in less than a month, though in Southwest Austin this likely means plenty of pinching back.

Another lauded vigorous grower, ‘Alabama Sunset’ changes from yellow in sunlight to pink in shade. Dramatic ‘Kiwi Fern,’ in the well-named Stained Glassworks series, likewise has received excellent scores in bedding trials. Its series mate ‘Oompah’ is a luscious, hard-to-resist chocolate coleus that doesn’t melt in the sun. And heart-leafed ‘Burgundy Sun’ and ruffle-edged, lance-leafed ‘Plum Parfait’ both earned a Texas A&M Superstar rating for endurance and visual impact.

Bill Scheick has written gardening articles for the Oak Hill Gazette, Dallas Morning News, Austin-American Statesman, Tropical Treasures Magazine and Texas Gardener Magazine, where he currently serves as a contributing editor.