Coleus: Origin, Varieties and Growing Tips

A classic for summer terraces, coleus can be grown both indoors and outdoors. Although sold as an annual, it does very well indoors, allowing fans of colorful foliage to keep it year-round or until the next flower box. Don’t be fooled by salespeople who want you to throw away your coleus, and learn how to keep it as a houseplant now.

Origin

It is not possible to determine the origin of coleus, as its cultivation as an ornamental plant is not new – the plant is found from Africa to Australia, via Asia. Some sources suggest that its true origin is Java, Indonesia, but this information has not been confirmed.

Coleus is a member of the Lamiaceae family, whose name is mostly used to refer to aromatic plants: basil, mint, rosemary, lavender, thyme and so on. Outdoors, we also grow sage (Salvia), catmint (Nepeta) and bugleweed (Ajuga), all belonging to this family, and indoors… pretty much only coleus, along with its very close cousins, the plectranthus, some of which act as green friends for bright interiors.

Endless Names

It was difficult to find the latest scientific name for Coleus, as it keeps changing. Let’s quote Larry Hodgson himself: “Coleus blumei… oops, I mean Solenostemon scutellarioides… sorry, more like Plectranthus scutellarioides.” Indeed, Coleus has gone through all these nouns. As of 2019, it looks like Coleus scutellarioides has won the race… and that’s until the taxonomists change their minds again!

The suffix blumei was named by Dutch botanist Karl Ludwig Blume, when he brought it back from Java in the mid-19th century. For its part, scutellaroides means “resembling a skullcap (but not being one), or false skullcap” – skullcap referring to the 300 to 470 plants of the eponymous genus, whose flowers do indeed resemble those of coleus.

As far as prefixes are concerned, coleus were first referred to as Coleus, until 2006, when they were renamed Solenostemon (for the sole purpose of annoying me, as I have great difficulty spelling Solenostemon). In 2012, with the advances in DNA studies, it was decided that they were plectranthus instead, and the two genera were grouped together under this emblem… until 2019, when coleus were again distinguished from other plectranthus by being given the Coleus prefix. Yes, so much for that!

Description

Coleus are short-lived perennials (not strictly annuals, although they are sold under that label). Fast-growing, they generally reach heights of between 30 and 60 cm (1 to 2 feet); their stems eventually become covered with a light bark – they are then said to be semi-woody.

Wild coleus are very different from modern cultivars, the result of a long history of hybridization to preserve their best characteristics. In the wild, they are sparsely branched plants with quadrangular stems and oval, serrated leaves (reminiscent of mint). The leaves grow at the end of opposite petioles on the stem.

When in bloom, coleus produce spikes covered with small, relatively insignificant blue or lilac flowers. They usually bloom in autumn, crowning themselves with tubular flowers with fused petals.

Varieties

Obviously, cultivars carefully selected for their ornamental capabilities are very different from those found in the wild! The new ones are often highly branched, forming a small bush without having to pinch them back as we had to do with the old ones. Flowering is sometimes delayed, if not completely absent.

However, it’s really the foliage that gives this prized plant its charm! Of the colors that coleus can take on, only blue is absent; we then see traditional leaves with green to neon hues, as well as pink, red, purple (even almost black) and orange due to the same pigments found, for example, in plums or raspberries (anthocyanin). These same leaves can then be variegated with white or creamy patches, to varying degrees (but never totally: enough green pigment must remain in the leaf, hidden or not by anthocyanin, to produce energy via chlorophyll).

Leaf Shape

The shape of the leaves is relatively variable: some are more lanceolate or indented, and their serrated edges can also be more pronounced. We also see coleus with a drooping habit. Finally, the size of coleus can change: some now reach heights of around a meter (3 feet), or have dense, narrow leaves.

Leaf Color

A particularity of coleus is that the color of the leaves varies greatly according to the amount of light obtained: two specimens from the same basic plant can have quite different appearances, depending on the growing conditions.

Growing Tips

Light

There’s a certain amount of confusion about how to light coleus. Many people refer to it as a shade plant, and you might want to give it medium or low light indoors. However, this is not the case.

In fact, coleus can best be described as shade plants, as they thrive outdoors. If they receive a little direct sunlight, their colors can be even more vivid.

However, since shade outdoors is already brighter than the bright light in our homes, it’s important to understand that coleus is not a low-light plant at all. Indoors, it prefers bright light with a little direct sunlight.

Arrosage

Watering coleus is generally quite frequent. Like other thirsty plants, it requires evenly moist soil at all times. Since it doesn’t really have a resting period, you need to check frequently that it doesn’t need more water, even in winter.

It’s easy to spot a water-stressed coleus, as its leaves droop sadly like old handkerchiefs when it’s time to water it. Although this treatment is not optimal, the coleus recovers quite well from these oversights, but will undoubtedly lose a few basal leaves.

It’s the same thing outdoors: on hot days, it’s not unusual to have to water your coleus several times a week.

Atmospheric Humidity

Like many tropical plants, coleus prefers good atmospheric humidity, but tolerates the dry air of our homes. There is little damage to coleus leaves caused by atmospheric drought, apart from the drying out of some new shoots.

Beware, however, of humid conditions, which can cause leaf diseases or mildew.

Soil and Potting

Coleus tolerates all types of soil. I recommend coir-based soils, which are easy to keep slightly moist at all times. A few draining elements, such as perlite, reduce the risk (rare, but it does exist) of root rot.

With its overflowing growth, it’s not unusual to have to repot coleus frequently. It recovers very quickly from these repottings, not really suffering any significant shock.

Fertilizers

It can be fertilized with an all-purpose fertilizer – a quarter of the recommended dose is enough. Since coleus has no resting period, it can be fertilized at any time. That said, it can grow in pride (wither) during low-light months, so if you don’t have good light in winter, don’t fertilize it until the weather’s better.

Temperature

Coleus are strictly tropical plants that are completely intolerant of frost. They can be damaged, or even die, when temperatures drop below 10°C (50°F). It’s best to keep them above 15°C (59°F) at all times.

Maintenance

As mentioned above, the main stages of coleus care boil down to watering, more and more frequently, until repotting is unavoidable. In this sense, coleus are reminiscent of Tradescantia and Chlorophytum. What’s more, coleus are very prone to phototropism, i.e. they grow towards the light. They therefore require frequent turning.

Depending on the cultivar, it may be necessary to pinch the coleus, i.e. remove the terminal shoot to encourage the plant to branch. Coleus branches easily and, with regular pinching, a bushy, attractive habit can easily be achieved. It may be necessary to support the plant with stakes. You can also give coleus the shape of a small tree.

Again, depending on the cultivar, it may be necessary to remove the flower stems as they grow. It’s not that coleus dies after flowering, but rather that it tends to become uglier in the process. Flowers require a great deal of energy, which is not put into foliage.

Flowering

Of course, it’s perfectly acceptable to leave the plant to bloom and see the flowers, at least once. Personally, although they are rather insignificant, I find that their bluish color can provide an interesting contrast with pink or red foliage. However, what a mess to clean up when they start to dry out and fall everywhere…!

For more information on coleus flowers, read this article on how to pinch coleus flowers.

Coleus flowering is a “real” problem indoors, where even varieties that are less quick to bloom will eventually do so. Long days (i.e. days with more than 12 hours of light) can be used to delay flowering, for example by placing coleus under supplementary lights (before the animal gets too big… which, like flowering, seems inevitable with coleus!)

In my experience, stems that have decided to flower fool themselves into continuing to try, even though I remove the flower spikes day after day. With coleus, it’s sometimes time to restart the plant from scratch with cuttings and discard the mother plant, especially when it’s balding since we can’t keep it sufficiently watered. Fortunately, nothing is easier than taking coleus cuttings.

Multiplication

You can start with a small three-leaf coleus in early summer and give a small bush to each of your friends at the end! It’s simple: coleus cuttings are quick and easy to take. Whether cuttings are made in soil or water, there’s no difficulty. Roots often appear in less than a week. For cuttings planted directly in soil, a supply of moisture can be beneficial, but avoid greenhouse conditions, which can cause more foliar problems than they solve.

It is sometimes recommended to cut the broad leaves in half, to prevent the cutting from losing too much water. Personally, I never do this, but it’s true that smaller cuttings are more successful than a full branch. Don’t worry: little baby coleus will soon grow big!

Coleus can also be easily grown from seed. In this case, plant the seeds on the surface of potting soil (they need to be exposed to light to germinate) at a temperature of at least 21°C. The first leaves to appear will be entirely green, but the next ones will be the color of the chosen cultivar. Mixtures of coleus seeds are often sold, resulting in a wide variety of subjects. Coleus seeds are also rarely true to type, meaning that the plants that grow from them will not necessarily be identical to the plant from which the seeds originated.

Problems

In general, coleus plants are disease- and insect-free. Here, however, are the main problems:

• Soft petioles, wilted leaves: the plant needs water. If potting soil is soggy, take cuttings: roots may be affected by rot.
• Powdery leaves, as if covered with a white powder: mildew. Remove affected leaves. Increase air circulation or slightly reduce ambient humidity.
• Pests: whiteflies, mealybugs of all kinds, cyclamen moths, aphids and spider mites.

Toxicity

Coleus are not toxic. They are said to have mild hallucinogenic properties, which have so far been little studied. (Having said that, I don’t recommend you try them!)

Buying Advice

Although it may be tempting to buy a large coleus, it’s so easy to buy a small one and so much fun to watch it grow. As well as saving money, you can have fun pruning or taking cuttings. Don’t hesitate to buy a slightly dried-out coleus in the “discount” section of garden centers: unless it’s as crisp as a packet of potato chips, it’s surely very easy to water it properly and watch it magically come back to life (big-box garden centers often struggle to keep plants sufficiently moist, especially as coleus are often sold in temporary pots that are far too small!)

Even better: watch out for friends who buy ready-made planters: they’ll probably be willing to take small cuttings for you, free of charge. Coleus are very common in summer arrangements. Also keep an eye on classified ads: it’s not uncommon for altruistic gardeners to give cuttings away for free, since coleus are so prolific.

Conclusion

Don’t be fooled by salesmen who swear that coleus are annuals: to buy one is to have enough coleus to fill the whole house within a few weeks. The care is simple, the growth rapid and the water requirements high – but they’re worth it with their colorful foliage! Whether on our terraces in summer or on our brightest windows in winter, coleus absolutely belong in our hearts.