Every year, the National Garden Bureau, a non-profit organization promoting the pleasures of home gardening, selects one perennial, one annual, one edible plant and one bulb to celebrate. It’s a great way to discover a new plant or to learn a bit more about a plant you may already be growing.
Here’s the second of 2018’s four plants, the annual called calibrachoa or million bells (Calibrachoa × hybrida). Go to 2018: The Year of the Coreopsis to learn more about this year’s perennial honoree.
From Obscurity to the Limelight
Few plants have become so popular so quickly as the calibrachoa. Who had ever heard of the genus Calibrachoa until the Japanese company Suntory launched the Million Bells® series of hybrids in 1992? I well remember that my local garden center didn’t even know what to call this new plant and sold it at first under the name mini-petunia! The new plant caught on immediately and the calibrachoa, still called million bells by the public even when the plant belongs to some other series, is now among the most popular annuals in the world!
The Right Name
If the name million bells stuck as a common name for this plant, I think it’s because people were afraid to try pronouncing the botanical name Calibrachoa. It just seems so complicated, but it’s not. Try it and you’ll see. Just remember that the “ch” is pronounced “k” and give it a try.
When I pronounce it, it sounds likes ka-li-bra-KO-ah, if that’s any help.
Still having a hard time remembering the name? Sometimes knowing its origin can help a lot. If that’s useful to you, know that the genus Calibrachoa was named for the Mexican botanist and apothecary, Antonio de la Cal y Bracho (1764–1833). Cal y Bracho = calibrachoa. Simple!
Petunias and Calibrachoas: Kissing Cousins
For almost two centuries, calibrachoas were placed in the genus Petunia, as both plants belong to the tomato family, the Solanaceae, and share many characteristics, including and most especially trumpet-shaped flowers.
However, as early as the beginning of the 20th century, taxonomists knew something was wrong: the small-flowered species (the ones now called Calibrachoa) were not interfertile with the large-flowered species (Petunia). However, it was easy to cross any small-flowered “petunia” with another small-flowered petunia and any large-flowered petunia with another large-flowered petunia. In other words, the two groups, although physically similar, were genetically different … and sexually incompatible.
It wasn’t until 1985 that botanists D.O. Wijnands and J.J. Bos formally proposed splitting the genus Petunia in two based on the plant’s chromosome number. Today the genus Petunia now includes the 21 species whose haploid chromosome number (n) is 7, while a new genus, Calibrachoa, was created to incorporate the 28 species whose haploid number (n) is 9. The difference in chromosome numbers is at the heart of the two genera’s inability to cross: normally, plants (and animals) must share the same number of chromosomes in order to produce offspring.
Telling the Two Apart
Visually, calibrachoas and petunias can be readily distinguished by the size of their flowers. Those of calibrachoas are only about 2.5 to 3 cm in diameter (about 1 to 1¼ inches) whereas petunia flowers typically measure two or even three times more.
However, there are other differences:
- Petunia stems and leaves are covered with sticky hairs, as you’ll quickly discover when you prune or deadhead them. Hybrid calibrachoas are simply not sticky.
- Petunia flowers, especially the larger ones, are often damaged by rain, while calibrachoa flowers usually come through even major downpours intact.
- Petunias are, for the most part, true annuals and die at the end of the season. Calibrachoas are perennials by nature (at least in mild climates), sometimes even shrubby. Their stems even become woody over time.
- The natural range of petunia flower colors is quite limited (white, purple and red), although, thanks to 150 years of hybridization, it’s much now much more extensive than it was originally. The 28 wild Calibrachoa species, though, come in every color of the rainbow. Through hybridization, their color range has expanded even further. You’ll never see a pure orange or deep yellow petunia, for example, but both shades—and many more!—are readily available in calibrachoas.
- Calibrachoas are more cold-tolerant than petunias and can bloom well into fall, even coming through light frosts unscathed.
- There are other, less visible morphological differences. For example, calibrachoa seed capsules are ribbed while petunia seed capsules aren’t, but I suggest leaving the more obscure details to botanists!
Interestingly, one of the most obvious differences between the two genera has been largely eliminated over just the last few years. Many gardeners will recall, with a shudder, the need to deadhead petunias and remove all those sticky faded flowers in order to keep the plants blooming, whereas calibrachoas needed no deadheading to flower right through the season, an obvious plus. Well, there is no longer any need to deadhead petunias, at least, not recent cultivars, as self-cleaning, continuous blooming varieties are now widely available. Read more about how this came about in the article Non-Stop Bloom with No Effort!
The Origin of × Petchoa
As mentioned above, when two genera have different numbers of chromosomes, that makes hybridizing impossible… at least in theory. In practice, though, there are ways around this, notably through embryo rescue (saving embryos that would normally have aborted and growing them on to maturity in an artificial ovary). That’s how Japanese hybridizers managed to create an intergeneric cross between petunias and calibrachoas: a triploid plant called × Petchoa. Petchoas are sterile (like most hybrids between two different genera) and can only be multiplied by cuttings or tissue culture. They are commercially available and resemble calibrachoas in their habit and cultural needs, but bear flowers closer in size to petunia blooms.
Well-Adapted to Containers
Calibrachoas are native to tropical regions of South America, particularly Brazil, where they grow mainly on cliff faces and on scree slopes. This ability to grow in an environment where the soil drains very quickly means they are naturally well adapted to container culture, especially hanging baskets.
In fact, calibrachoas usually do better in containers than in the ground, as, unlike petunias, they don’t adapt that well to clay soils or soils with a high pH (alkaline soils). Today’s calibrachoas are better to adapted clay and alkaline soils than the original ones, but still are not really happy there. Fortunately, potting soils are light and slightly acid, exactly what they prefer.
In most respects, calibrachoas are everyday annuals and are just as easy to grow as any other.
They like full sun (at least six hours of sunshine a day) and well-drained soil. For best bloom, choose a fairly rich soil; if not, fertilize regularly. As mentioned above, calibrachoas bloom non-stop and are “self-cleaning” (faded flowers drop off all on their own). Thus, there is no need to deadhead to ensure bloom all the summer and rarely any need to prune them.
Although calibrachoas are somewhat drought-tolerant, letting them dry to the point of wilting is harmful, especially if this happens repeatedly. A wilted plant can take weeks to fully recover and indeed, may never do so completely. That’s why it’s best to water regularly so that the soil remains slightly moist at all times.
As mentioned, calibrachoas are perennials … in areas free from severe frost (zone 9 and above). Elsewhere, you’ll have to take cuttings in early fall to ensure their long-term survival. Grow the baby plants indoors over the winter, near a sunny window or under a grow light.
You can also harvest seeds in the fall and sow them indoors in the spring, but it’s important to note that all commonly available calibrachoas are complex hybrids and, therefore, the seeds will not be true to type, giving you a mixed bag of plants.
Until recently, only varieties grow vegetatively, that is, from cuttings or tissue culture, were available to gardeners, but since 2014, you can find packs of hybrid calibrachoa seeds in the Kabloom® series in seed catalogs and better garden centers.
Sow the seeds indoors about eight weeks before the planned planting-out date, just pressing the seed lightly into the potting mix without covering it. Place the seedling container in the sun or under a grow light, as the seeds need light to germinate. Keep the seed trays at a temperature of 70 to 75 ° F (21 to 24 ° C). Germination will occur in about 10–14 days. From then on, just grow seedlings on like those of any other annual.
A Wide Choice
There are more than 300 varieties of calibrachoas currently available in several series. The plants vary in size and habit (some types are trailing, others have a bushier habit), flower type (single or double) and size, and especially in color: white, pink, red, orange, yellow, blue, purple, green and more, sometimes with a contrasting eye, a star blazoned over the base color, streaks or splotches, etc. Almost no color or combination of colors is impossible with this amazing little plant.
In addition to the Million Bells series, launched over 25 years ago, there are now many other series, each including a range of calibrachoa forms and colors. The series include Aloha Kona®, Cabaret®, Callie®, MiniFamous®, Superbells® and several more.
This spring, when the time comes to fill your flower boxes and baskets with pretty flowering plants for the summer, help celebrate the Year of the Calibrachoa by including some of these stunning and easy plants in your arrangements!