Coleus

Identification and origin

Coleus belongs to the mint family, Lamiaceae, along with oregano and deadnettle. It is native to a vast tropical region stretching from Indochina, southeastern China, Taiwan and the Ryukyu Islands through island Southeast Asia to New Guinea, the Solomon Islands, Vanuatu and northern Australia. It is thought to have reached England in the mid-1800s, where it became a prized Victorian parlor plant and a favorite for carpet bedding.

It is a frost-tender evergreen perennial, most often grown as an annual. Its square, somewhat succulent stems carry opposite, oval to oblong leaves 3–6 in long, with toothed, scalloped or deeply cut edges. Their velvety texture and colors, mixed in blotches, margins or contrasting veins, are the whole point of the plant. Depending on the cultivar, it grows 6 in to 3 ft tall and about as wide; trailing types, ideal for hanging baskets, can spread more than 3 ft.

The small, two-lipped, blue-violet or whitish flowers form slender spikes at the stem tips from summer into fall. They are not showy, yet they attract bees and other pollinators. The calyx persists after flowering as a tiny saucer, which explains the name scutellarioides. The plant is still often sold under its former names, Coleus blumei or Solenostemon scutellarioides.

Light

Indoors, it needs bright light: an unobstructed east- or north-facing window works, or a grow light if no window is available. It enjoys a few hours of gentle sun, but avoid long hours of strong direct sun through glass.

Outdoors, most coleus prefer part shade or dappled light. In full sun, cultivars that are not suited to it bleach and can wilt; deep shade, on the other hand, gives long, sparse stems. Cultivars sold as “sun coleus” do handle full sun and heat.

Watering

Coleus likes a mix that stays moist but never soggy. After planting, keep the root ball evenly moist, not wet, for about ten days. After that, water when the top 1–1 in of the mix is dry: check regularly by pushing a finger into it. Never let it dry out completely.

In containers, the soil dries faster and plants need water more often. Water at the base without wetting the foliage. Above 64 °F, a tray of damp gravel under the pot keeps the air humid; in a warm, dry room, misting also helps keep spider mites away.

Potting mix

It needs rich, loose, moist soil that above all drains well, with a slightly acidic pH. In containers, use a very free-draining, peat-free mix; in the garden, work organic matter into the soil.

During active growth, feed with a complete, nitrogen-rich fertilizer, for example every two weeks for potted plants. Avoid bloom-booster fertilizers: their phosphorus makes the plant stretch and flower at the expense of the foliage.

Temperature

Coleus is a warm-weather plant. It gets off to a good start around 64 °F, then grows best at 70 °F to 84 °F during the day, with nights of 55–64 °F. Growth is vigorous above 61 °F; below 50 °F it suffers cold injury, and when nights fall to between 39 °F and 50 °F, its leaves blacken and die. The first frost kills it.

So move it outdoors only once mild nights have settled in, and bring it in, or take cuttings, before the first cold spells of fall. It can stay in the ground year-round only in USDA zones 10a to 11b.

Repotting

Coleus grows very fast and reaches full size in a single season. For pots and window boxes, choose a large enough container with drainage holes and a very free-draining mix from the start. Growing in containers also has the advantage of keeping slugs at bay.

For winter, don’t bother repotting an old, tired plant: young plants grown from cuttings get through the cold season much better. Before frost, take cuttings, pot them up and set them by a bright window to enjoy their foliage year-round.

Pruning

Pinch the tips of young shoots often, just above a pair of leaves: the plant branches and forms a dense mound. Also remove flower spikes as soon as they appear, by pinching or cutting them: a plant allowed to set seed wears itself out and declines. So-called vegetative coleus, sterile cultivars propagated by cuttings, flower little and need less upkeep.

Propagation

Cuttings are remarkably easy and work in any season. Cut a 2–3 in stem tip, remove the lower leaves and set it in water or in clean potting mix. Kept warm and in bright light, it roots in about two weeks. Keep the mix just moist. Patented cultivars, marked as such on the plant label, should not be propagated.

Some series, such as ‘Wizard’ and ‘Kong’, are grown from seed. Sow indoors, leaving the seeds uncovered, on a sterile medium kept at 70–75 °F. Germination takes ten to fifteen days, and seedlings are large enough to plant outdoors six to eight weeks later.

Among the countless cultivars: ‘Alabama Sunset’ (brick red with a thin yellow edge, tolerates full sun), ‘Wasabi’ (deeply cut chartreuse leaves), ‘Redhead’ (bright red), ‘Pineapple’ (lime gold with burgundy accents, seldom blooms), ‘Dipt in Wine’ (large burgundy leaves) and ‘Black Magic’ (mahogany purple), plus the trailing ‘Trailing Red’.

Common problems

• Muddy brown leaves, scorched edges, stunted plant: wet feet, poorly drained soil or overwatering.
• Leaves that blacken and die: cold, nights that are too cool, or frost.
• Washed-out colors: too much sun for the cultivar.
• Long, sparse stems: shade too deep, not enough pinching, or a fertilizer too high in phosphorus.
• Plant declining in late summer: it has gone to seed.
• Diseases: downy mildew, stem rot and root rot, encouraged by wet foliage and overly wet soil.
• Pests: aphids, whiteflies, mealybugs, spider mites (especially indoors), thrips that leave silvery, yellow or black spots, and slugs on plants grown in the ground.

The same symptom can have several causes: look at light, watering, temperature and the condition of the roots together.

Toxicity

Coleus has low toxicity for people. Its leaves contain diterpenes, including coleonol, and repeated, frequent contact can cause allergic dermatitis with a red rash. Wear gloves if you handle the plant often. A closely related species in the same genus, Coleus amboinicus, is listed as toxic to dogs, cats and horses because of its essential oils, so keep the plant out of reach of pets as a precaution and, if any is eaten, contact a veterinarian.