There are a great number of varieties, obtained by hybridization of different species, which are grouped by appearance, size and shape of the leaves, colour, etc.
1. Taxonomy and Origin
It is native to India, Java and tropical regions of Asia.
- It is a fast-growing, shrubby plant. In general, it is 0.5-1m tall, but it can reach the height of 2m.
- Leaves: The leaves are opposite, simple, peciolate, cordiform and usually dented and they stand out because of their colours ranging from yellow to purple, from brown to green and, sometimes, to scarlet. These colours cover the surface of the leaves in spots, strips and concentric shapes.
Coleus leaves. Photo: Jim the Photographer
[click to enlarge image]
- Flowers: The small bluish flowers form a spike which has no ornamental value. In fact, they are usually eliminated.
There is a great number of varieties, obtained by hybridization of different species, which are grouped by appearance, size and shape of the leaves, colour, etc. Almost all varieties are the result of crossing Coleus blumei, originally red, with Coleus verschaffeltii. There is another crop species called Coleus pumilus, but it is not so common. It looks like a pendulum and it has small, dark brown leaves with light green edges. Among the most prominent varieties of Coleus, we can mention: “Arc-en-ciel” (yellowish or pink leaves, edged with a greenish strip), “Iroquois” (long, bright red leaves, with a green spot where the stalk starts) and “Otoño” (coppery leaves, edged with a yellow strip).
3. EDAPHOCLIMATIC REQUIREMENTS
- Temperature and Light: General temperature should not be below 13ºC. It requires 25,000 to 40,000 lux and a temperature of 20-25ºC. The colour of the leaves depends mainly on light and temperature; with short days of low temperature, the leaves are small and the colour centers along the central vein; with long days, all the leaf is colored; with low temperature during the day or high temperature during the night, the colour of the leaves fades. Temperatures of 23ºC during the day and 17ºC during the night combined with long days guarantee strong colours all over the foliar surface and the absence of flowers, which are not very attractive.
- Soil/Substratum: The plant is not very demanding, so soft soil rich in organic matter is enough. In general, it is advisable to use acid substratum, such as peat with sand and its corresponding manure.
- Humidity: The plant should be in a breezy place but protected from draughts. If the area is very dry, it is convenient to spray it. Once transplanted, the cuttings require a high level of humidity.
- Watering: The plant should be watered very frequently, especially in summer and when it is growing. However, it is not advisable to water it abundantly. So as to choose the best moment for watering the plant, you should pay attention to the dryness of the soil and the condition of the leaves; if they are withered, the plant needs water.
The reproduction of commercial varieties is usually done by seeds, which are sown at the end of winter in substratum with a mixture of peat and perlite and grown at a constant temperature of 23ºC. Germination occurs around two weeks later.
Propagation can also be done by 5-7cm long cuttings, planted, from February onwards, in sterile sand substratum or in a mixture of peat and sand. With a constant temperature of 23ºC, rooting easily occurs 10-15 days later. Throughout autumn and winter, the flowers of the mother plants should be removed and the tips of the leaves cut so as to favour their ramification.
5. GROWING TECHNIQUES
- Pruning and flowers removal: Trim the vertices when the plant is 20cm high. If you also remove the flowers when they bloom, the leaves of the coleus will grow further and the foliage will look really appealing.
- Fertilization: As this is a fast-growing plant, fertilize it often, with a balance of 2:1:2 at a ratio of 200-250ppm.
6. PESTS AND ILLNESSES
In general, coleuses are attacked by few pests and suffer few illnesses. The most common are:
- Woodlouse (Ortheza insignis and Pseudococcus citri)
- Whitefly (Trialeurodes vaporariorum)
- Red spider
- Pythium and Rhizoctonia are the main fungi that attack this plant.
It is important to use substratum and vegetal material free of nematodes, as they are very difficult to eradicate; Heterodera radicola causes knots in the roots and Aphelenchoides olesistus attacks the leaves, producing transparent spots that then become yellowish brown and finally black.
- Abnormal development (the plant languishes and even dies) due to an inadequate pH.
- Yellowing of the leaves due to excessive sunlight.
- Defoliation as a consequence of low temperatures, excessive humidity or the combination of both factors.
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