A Gardening Journal

Cup and Saucer Vine

The broad trumpets of cup & saucer vine are an enduring curiousity Summer to frost. They all open creamy white, as here. Those of the straight species quickly darken to grape-purple, whereas these of the 'Alba' form stay bright.

 

Cobaea scandens flower fingers 081316 640

 

In the picture below, see the full cycle of the flowers of Cobaea scandens 'Alba': bud (3 o'clock), just opening (1 o'clock), fully open (11 o'clock), younger bud (7 o'clock), and completed-flowering-but-not-yet-set-seed (6:30 o'clock). Seed set is usually sparse-to-none north of the subtropics: The flowers are believed to be pollinated by bats, and none of the flower-pollinating bat species are hardy colder than Zone 7.

 

Cobaea scandens Alba flowers buds 082516 640

 

Cobaea scandens is a vigorous vine, evergreen and perennial only in the subtropics and warmer. Given that it can grow ten to twenty feet when grown from seed as an annual in the comparatively cool climates of temperate North America, I can only imagine its vigor when able to persist year by year.

 

I vote for 'Alba', because the blossoms are showy even from a distance. Thank goodness: Even when grown as an annual, this self-climbing vine can grow so large that it lends itself to large-scale effects that are also showy from clear across your garden. Long multi-branched tendrils emerge from the tips of the leaves, and can grasp almost anything. Below, one of the countless tendrils that have wrapped themselves securely around the lengths of rebar that I've fashioned into an enormous tripod as part of this year's Summer display.

 

Cobea scandens Alba tendrils on rebar 080116 640

 

Below, a tendril from a leaf of one of the vine's many side stems that have wandered away from the intended climbing structure—the rebar tripod—into the fronds of a quartet of potted Bulgarian palms that I grouped around the tripod. Soon after the tendril has made contact with, well, almost anything, its countless fingers tighten their grip by coiling and flexing. The large-scale and simple structure of the fronds is a strikingly visual host for the tendrils.

 

Cobea scandens Alba tendrils on palm frond 080116 cropped 640

 

Tendrils that aren't successful in making contact with anything show their frustration by coiling up in a ball.

 

Cobea scandens Alba frustrated tendrils 080116 640

 

Below, three of the seven enormous tripods I've created to host various annual vines. Nearest is the tower of Cobaea scandens 'Alba' surrounded by the quartet of Trachycarpus fortunei var. bulgaria. Next is a tower of Malabar spinach, Basella alba 'Rubra'.  In the distance, the tower of moon vine, Ipomoea alba. (Out of view in other locations in the gardens are towers of Ipomoea lobataIpomoea x multifida, Ipomoea quamoclit, and Merremia tuberosa.)

 

Cobaea scandens Alba West Alley 082516 640

 

These are fourteen-foot towers, but these three vines are capable of racing up to twenty feet by September. Another of my towers just happens to be twenty feet, and this year it's hosting Ipomoea quamoclit 'Alba'. I'll post on it and Ipomoea x multifida soon.

 

 

Here's how to grow Cobaea scandens:

 

 

Latin Name

Cobaea scandens 'Alba'

Common Name

Cup and saucer vine. Also known as cathedral bells. 

Family

Polemoniaceae, the Phlox family.

What kind of plant is it?

Evergreen perennial vine that succeeds as an annual in climates colder than Zone 9.

Hardiness

Zones 9 to 11.

Habit

Quickly branching scrambling vine that is as likely to sprawl and explore as it is to ascend even readily available supports.

Rate of Growth

Fast.

Size in six months

Ten to twenty feet tall and (only if you guide the side stems outward diligently) wide. Growing ad libitum, Cobaea scandens is more likely to form a central upward panel surrounded by a distinctly lower flouncing skirt of side growth. A consistently full screen of growth as dense side to side as it is top to bottom would be achieved only with your partnership. See the second "How to handle it" box below.

Texture

Dense.

Grown for

its unusual flowers: The large green calyx flares back while the flower's broad trumpet is still a pointed marble-sized ball. When open, trumpets of the straight species emerge greenish white; the trumpet soon darkens to grape-purple whereas those of 'Alba' remain greenish white.  At the bud stage, Cobaea flowers could have been the inspiration for the voracious man-eater Audrey II in "Little Shop of Horrors." When open, the trumpet is much broader, as well as shorter, than those of, say, trumpet vine. The most widely-used common name, cup and saucer vine, isn't very apt the more you think about it: The calyx makes a very wavy saucer, and the outward-downward angle of the flowers would mean that anything contained in the cup would immediately be spilled. The other common name, cathedral bells, is more accurately evocative. The flowers' broad trumpets, their outward-and-down orientation, and their profusion all suggest a carillon of church bells in action, where any given bell faces outward at just such an angle as it swings back and forth.

 

its speed of growth and quick size: Cobaea scandens can grow ten to twenty feet by August, and its prolific side stems seem to keep growing even after the very highest and longest stems have reached their ultimate extent. A vine that is a foot tall in May, then, could provide full coverage of even a large support by August. See "Where to use it," below, for scenarios that take advantage of this vigor.

Flowering season

Mid-August to frost. Here in southern New England, that could be two months or even a bit longer.

Color combinations

The greenish white and palest yellow of the flowers wouldn't clash with anything,  But active harmony would be with partners that also in themselves bring white and pale yellow to the party, not just other colors that are in pleasing contrast. Pastels of any shade would also work. See "Plant partners," below.

Partner plants

Cobaea scandens can be so vigorous, fast-growing, and multistemmed that nearby plants are at risk of being overrun if you're not committed to a weekly review to trim out unwanted growth that scrambles ever outward. Thanks to the vine's long tendrils, stems can climb as well as hold securely onto underlying growth that might otherwise sway in the breeze: Almost nothing, then, would be safe from encroachment.

 

Practical partner plants tolerate the encroachment, or even welcome it.  Especially in Zone 7 and warmer, Liriope spicata is used as a large-scale groundcover. It tolerates both shade and sun, so will thrive before, during, and after Cobaea sidestems are surfing outward atop it during the hottest months.

 

Because Cobaea needs plenty of sun, it can interplay with much taller plants only if the vine is planted on their sunny sides. There, some of its stems can find their way back to and up into the shins of these taller hosts, but without danger of swamping them outright. What about planting Cobaea scandens 'Alba' six or even eight feet away from a clump of the 'Peppermint Stick' form of the giant grass Arundo donax? Its stalks grow as quickly as the stems of Cobaea, and can soar to twelve feet by the time the Cobaea would have reached its base. Miscanthus x giganteus 'Gilded Tower' would be a similarly-scaled partner.

 

Note that all three of these potential partners provide a grassy-leaved contrast to the Cobaea growth, so that there's textural interest long before the vine flowers.

Where to use it in your garden

Cobaea scandens will usually produce rampant growth capable of extending many yards in all directions; this is not a species for compact sites, or where a fastidious and controlled look is the priority. Instead, grow Cobaea in large-scale settings where its vigor and rambunctiousness can be celebrated.

 

Also take into account the likely need to control the vine's prolific and far-ranging side shoots: Site the vine where you'll have season-long access, so you can follow errant stems back toward the mother plant as far as you want, and then cut them off. The vine's eventual height and bulk can well suggest that it be sited at the very back of deep beds, to grow up walls, fences, or lattice. Be sure that you still have access to it even—especially—from August through frost, as everything else in the bed is also billowing and growing into one another. Otherwise, Cobaea will only be helping to shift the look from full and exuberant to swamped and defeated. 

 

Growing Cobaea up an enormous tripod is perhaps the easiest and most economical option, but other scenarios are as striking. Grow Cobaea up mesh or wires attached to the pillars of a pergola, and then—taking advantage of the vines' tendency to scramble and ramble rather than climb—let it explore more wires or mesh strung atop the pergola's canopy. Be prepared to see flowers borne by only the portions of the stems ascending the pillars, not the portions atop the canopy: The blooms are borne outward of and, in the latter case, atop, the prevailing bulk of growth, so as to be able to attract pollinators—especially, as hypothesized, bats.

 

Despite its tendency to sprawl, Cobaea is not likely to be successful as a groundcover: Its flowers tend to angle downward, so could be only marginally visible. The floral display might succeed despite those flowers' orientation if the groundcovered area were atop a high wall: Stray side stems would wind up cascading as they explored outward. Late in the the season, those "escapee" stems would finally be old enough to form blossoms; see "Quirks."

Culture

Full sun and any reasonably moist and nutrient-rich soil. Because Cobaea scandens is grown as an annual in climates colder than Zone 9, the usual priority for Winter drainage isn't relevant.

How to handle it: The Basics

Plant outside in Spring only after the danger of frost has passed. Cobaea scandens grows quickly—potentially to twenty feet by late Summer—and branches freely, so right from the beginning provide a sufficiently sturdy and extensive structure for it to scramble up and through. Although the vine's growth itself isn't noticeably heavy, it becomes dense and, therefore, creates greater and greater resistance to the wind throughout the growing season. The supporting structure will need, then, not so much to support the dead weight of the vines but, rather, be able to remain upright and in place even as the vines' ever-increasing bulk transforms it into a very large sail.  The danger is that the structure could be blown over in a storm just when the vines are in their massive and floriferous late-Summer peak, not that the structure would collapse simply from the vine's weight.

 

I grow large-scale annual vines such as Cobaea up huge metal tripods that are anchored to the ground.  This year, many of them are formed of a vertical fourteen-foot pole of rebar with three ten-foot bracing poles. All the poles are pounded a foot into the ground; the ten-footers are tilted inward to meet the center pole, and held in place either by multiple wrappings with tightly tied clothesline (the quick and dirty way) or by securing with a worm gear clamp (the elegant way).

 

Cobaea scandens truly is scandant—scrambling and wandering opportunistically—more than climbing. Side stems are produced even at a young age, and their priority seems to be lateral growth more than vertical. And even the vertical stems need reminding every foot or so, because their leading tips are as likely to wave about in space in search, presumably, of still more enticing support, as continue to grasp the current support that has been working well all season.

 

There seems to be a connection between the means of support—impressively long and digitated tendrils—and the lack of dedication to verticality. Cobaea scandens displays none of the race-to-the-top-of-the-flagpole focus of, say, the many species of morning glory. While these also produce easily-distractible side stems, their main stems still continue upward on their own recognizance and in all haste. For me, Argyreia nervosa, Ipomoea alba, Ipomoea lobata, and Ipomoea quamoclit are all capable of twining up twenty-foot poles by September; Argyreia tops out a pole that high by the first week in August! 

 

Cobaea, though, needs encouragement to climb. After stems have lengthened a foot or two, lead them back to your preferred support and, gently, wrap one or two of the long tendrils around and around the support. Try to have the "finger" tips of the tendril find the fingers and "wrists" of that or other tendrils as they return around the support: As is typical of so many climbing vines (twining or tendrilling), the growth has a slight tactility that enhances its ability to cling—especially to itself. If you're luckier, you'll have a couple of stem tips to lead to the same support, so that there will be multiple tendrils that can be led around the support—and, in the process, around each other in a singular kind of embrace. Either way, by the next day, the individual fingers of tendrils that have achieved (on their own or with your help) a satisfying grip will have coiled or become wavy so as to become shorter and, so, tighten their embrace of the support. Those that are unsuccessful in finding something to embrace coil up into a small frustrated tangle jutting out into free space.

How to handle it: Another option—or two?

If you have a south- or west-facing wall that is truly high and wide, and is also accessible by a ladder, you could attach mesh or a grid of wires over the entire expanse. Then train Cobaea over it, preparing for a September sheet of blossoms that would be far more reliable a display in the high heat of Summers typical of North American than, say, the wall of  'Perle d'Azur' clematis that is so thrillingly iconic at Sissinghurst Castle.

 

Because quick coverage is key—and Cobaea is a perennial only in the subtropics and warmer—it's a phyrric victory to achieve uniform coverage of a temperate-climate wall by planting just one vine and patiently training its stems upward and outward: The vine might not reach the desired size until just weeks before frost. Instead, plant Cobaea in multiples—every four feet wouldn't be too close—so that side-to-side fullness is achieved almost by default as the vines scramble upward and outward. Inevitably, some stems will grow outward in their attempts to discover yet more support that, alas, is not just one more foot away at the front of the wall. Especially earlier in the season, when all possible coverage is eagerly awaited, guide these escapee stems back to the structure to assist with side-to-side coverage. Later in the season, just cut them off.

Quirks and special cases

Despite this species' enduring popularity, the preferred pollinator of Cobaea scandens is still in question. This highly respected reference only speculates that the flowers "are reportedly pollinated by bats." Page three of this resource details typical floral morphology favored by bats. Among the qualities listed, these seem quite well represented by the blooms of Cobaea: wide bell-shaped flowers that project beyond the foliage, with prominent stamens bearing plentiful pollen.

 

Beginning in mid-to-late Summer, stems that will bear flowers emerge from axillary buds—meaning a bud between the base of each leaf and the stem—but only from growth that is comparatively old. My tripod of Cobaea is fourteen feet tall, and yet the top few feet of growth has yet to produce any buds.

 

Given the speed of growth, that top portion would usually be just weeks old—until the overall growth rate slows down with the shortening days and gradually cooling temperatures typical of the flowering season. Then, the current top portion is less and less superseded by new growth that is higher still and, so, when it's finally old enough to begin flowering, there is less and less still-juvenile growth above it. When flowering first started, vegetative growth was accumulating so quickly that the top eight feet didn't yet display flowers. Now, just the top two feet has yet to bear them.

 

If flowering is the icing on the cake, with the cake being the extent of overall growth, then who cares? But if your priority is for flowers to be distributed uniformly over the expanse of growth—or you'd like to grow Cobaea where it isn't desirable for the vines to lengthen ad libitum, you might be able to trim away the still-juvenile tips of stems without reducing the amount of flowering below. Or would such tipping-back encourage some of those axillary buds to produce more side stems—more juvenile vegetative growth—instead of flowers? Perhaps next year I'll grow Cobaea and limit its growth, say, to merely ten feet by just such tip-pruning. Then, I might be able to assess if flower production on the remaining growth is affected.

Downsides

When Cobaea is grown as an annual, it's a long wait until flowering finally ramps up in August. Viewers whose worldview is too narrowly floral-centric could become impatient with the ever-increasing bulk of vegetative growth. A more satisfying strategy is to see the vegetative phase—May through July—as the opportunity to appreciate the details of the foliage and tendrils. Annual vines that flower when much younger and, therefore, smaller, include Basella alba, Ipomoea lobata, and Ipomoea quamoclit.

 

In the subtropics and tropics where it's hardy, conversely, the Summer-into-Fall flowering season of Cobaea is welcome. There will always be plenty of other plants in flower in other seasons, so that no one need be impatient in, say, June that Cobaea isn't yet in flower.

 

The dark coloring of the flowers of the straight species makes them less showy at any distance; those of 'Alba' can be seen from across the garden.

Variants

Only a few variants of Cobaea scandens are readily available; besides 'Alba', there's 'Royal Plum', whose flowers emerge white but soon darken to plum-purple. 'Royal Plum' may be the same as 'Purple'. The straight species is also garden-worthy: Its flower emerge white, and change to grape-purple.

 

The Cobaea genus is reported as comprising eighteen species, but only C. scandens is currently available for ornamental horticulture in the United States. Flowers of C. lutea are greenish-yellow; if it is as easy to grow as C. scandens, it would definitely be worth trialing.

Availability

Seeds are readily available online. Young plants are sometimes available at plant sales or at specialty nurseries. Possibly because they grow so quickly, and their tendrils can become tangled in adjacent plants overnight, you're unlikely to find anything other than starter plants. It would be a high-maintenance nightmare to grow for sale "specimen" Cobaea, say, trained up five-foot bamboo peastakes. No problem: Those starter plants could be twenty feet tall by September.

Propagation

By seed. 

Native habitat

Cobaea scandens is native to Mexico and tropical South America. The genus name honors a 17th-Century Jesuit, Bernardo Cobo, who was a missionary and naturalist in Mexico and Peru.

 
 
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