DOWNERS’S GROVE, USA: With the continued emphasis on foliage in our gardens, the wide and exciting range of coleus varieties available should nicely augment one’s planting palette. Chosen as the annual for the National Garden Bureau’s 2015 program, coleus is a durable plant with very significant gardening potential for almost all gardeners and their garden situations.
Coleus has gone through various phases of popularity over the past couple of centuries. This member of the mint family comes in a wide range of coloration, leaf texture and plant form. Considered an herbaceous perennial in its native range, coleus are used primarily as annuals. Previously grouped into different species or classified as hybrids, coleus (formerly Coleus blumei and Coleus hybridus) are now all placed under Solenostemon scutellarioides (2006). As of 2012, taxonomic authorities consider the correct name for the coleus to be Plectranthus scutellariodes.
While modern coleus breeding focuses on new color combinations and foliage characteristics, other considerations such as sun tolerance, delayed flowering, more prolific branching and an emphasis on more compact and trailing forms have become more prominent.
The primary ornamental feature of coleus is the foliage which can be green, pink, yellow, orange, red, dark maroon (almost black), brown, cream and white. This plethora of colors and combinations lends itself to the other common names for coleus of painted nettle or flame nettle. While some gardeners will leave the small flowers, it’s recommended that you pinch these back to a leaf node to encourage more energy into stem and foliage growth and not flowering. Coleus left to flower may lose vigor as the plant puts energy into seed production.
The variability in patterns is truly amazing with solid colors, splashes, blotches, streaks, flecks, margins and veins. Color intensity may be affected by sunlight, heat sensitivity and other conditions. The term “sun coleus” refers to selections that tolerate more direct sunlight. Darker cultivars tend to tolerate more sun while lighter varieties benefit from some degree of shade to minimize leaf scorching. Morning sun and dappled afternoon shade tends to maintain consistent foliage coloration. Too little light will encourage a weak-stemmed, less vigorous plant without optimal coloration. For sunny areas consider these varieties: any of the Stained Glassworks varieties, the Wizard, Versa and Marquee series, or any variety with the word sun in its name.
Coleus leaf texture can be quite variable with large, small, twisted, elongated, scalloped, lobed, finger-like, “duck’s foot” (webbed feet), etc. Leaf texture for coleus should be a serious consideration when selecting and using coleus as the visual contribution is significant.
Coleus can be grouped into three basic plant forms including upright, rounded and prostrate/trailing. Frequent snipping, pinching and trimming can help modify form although the trailing forms have great value at the edge of a container, in a hanging basket or as a groundcover becoming a colorful, living mulch. When selecting a variety, there are many to choose from and you can base your choices on foliage color, leaf texture and/or plant form.
Coleus has long been considered a shade plant but their best leaf coloration is achieved with morning sun and some degree of afternoon shade. Many varieties do well in both shade and part sun, such as the ColorBlaze, Fairway, Superfine Rainbow, Main Street and Kong series. Some varieties can take quite a bit of sun as long as they are not allowed to dry out. Coleus are quite tolerant of a wide range of soil conditions. Coleus enjoy the heat and cold, overly damp soils can result in leaf drop and encourage disease. Plant coleus after any danger of frost has passed when soil temperatures have warmed sufficiently and evening temperatures are above 60 degrees F. Light fertilization is recommended, particularly in containers.
To maintain plant form, pinch back most varieties every few weeks to prevent flower formation. This directs the plant’s energy into additional branching and foliage creation instead of flowering, thereby creating a fuller plant. When pinching off flowers, do so throughout the entire summer to create a full, lush plant. Pinch just above a set of leaves or branching junction for the best appearance (don’t leave a stub!).
Raising coleus from seeds is relatively easy. Seed strains offer uniformity and may include mixes or consistent coloration with identical plants. Seed packets can be quite affordable and a wide range of coleus varieties available from seed vendors. Time seed sowing to be 8-12 weeks before the last frost date. Sow seeds in at least three inches of growing medium (maintain at 70 degrees F) and seeds should be sown on the surface as they require light to germinate. Well-timed, even watering, misting (for humidity) and frequent observation are encouraged.
Overwintering coleus plants as houseplants is an option although temperatures near 70 degrees F are required. Rotate plants and pinch back as needed to maintain form. Consider grow lights to provide adequate winter lighting conditions.
Solid color coleus varieties such as Redhead and Lime Delight Premium Sun (both bred for the sun) can be very impactful and make a statement in the mixed border while those with variable coloration may become “color echoes” for neighboring plants with similar (or contrasting) flower and/or foliage colors. The repetition of certain coleus colors and form can lend unity and harmony in the garden. While a solitary specimen can add a “punch” of color, consider the impact of mass planting as well. Foliage with lighter coloration can provide illumination in shadier locations while dark colors (for example, any coleus with Chocolate its name) in the same setting will create depth and contrast. Consider coleus just one of many available tools in your gardening “toolbox.”
All coleus have excellent container potential if they are given adequate well-draining soil mix, reasonable nutrients and the proper sun exposure to thrive. Avoid windy locations as coleus can be prone to breakage in extreme winds. Slow release fertilizers are recommended for your containers although half strength liquid fertilizer applied every 2 weeks over the growing season should be sufficient. Coleus do not show their best coloration if over fertilized so be conservative and consistent. You may want to consider water retention additives to help alleviate some watering needs, particularly in sunny locations. Drainage is vital so consider adding additional drainage holes as needed. The container style, color and ultimate placement should also be considered in advance. Coleus filled containers, if moveable, allow for instant color as they can be positioned as needed and used to add color, provide immediate interest and accent areas of the garden, deck or patio.
Coleus certainly has the potential to be included in hanging basket arrangements. Some of the trailing selections are ideal for the edge of an elevated container while larger varieties can be utilized for a strong foliage contribution in the center of the basket. Consider watering needs as coleus are naturally thirsty and a hanging basket can be one of the most challenging situations in terms of moisture retention and associated watering needs. Wind protection is also warranted.
Coleus may become stressed by lack of heat and moisture. Excessive or inadequate moisture may lead to challenges with insects or diseases. A healthy coleus plant is the best defense against these challenges. Slugs, snails, spider mites, mealybugs, whiteflies and occasionally aphids may be challenges under certain conditions. While there are few fungi, bacteria and viruses that affect coleus, there may be occasional issues of stem rot, root rot or downy mildew which all have a direct relationship to moisture inputs and associated growing conditions. Relocating the plant, pinching healthy cuttings for re-establishment or removing the plant might be options to consider. Healthy, young plants will frequently outgrow some of these challenges if properly “encouraged” or may never exhibit problems because of their vigor.
Founded in 1920, the National Garden Bureau is an American non-profit organisation whose mission is to improve the quality of life through increased use of seeds and plants.