Grow lights for coleus – sowing and winter under lamps

Coleus get by on ordinary window light for longer than many people think. But if you sow early or want to keep the colors going through a dark winter, a lamp earns its keep. The good news is that you do not need an expensive grow light. An ordinary white LED bulb gives coleus exactly the light they need. Both researchers and hobby growers with light meters have shown it time and again. Here we cover when grow lights are needed, what to buy and how to use them correctly.

Do coleus need a grow light?

To survive the winter, coleus need no extra grow light. A bright window is enough for the plant to rest its way through the dark season. But a lamp really does make a difference in two situations.

Sowing while the days are short – seedlings that sprout in winter darkness grow leggy and pale without extra light. The earlier in the year you sow, the more important the lamp becomes. In northern Europe it is in practice a requirement throughout the winter months.

Color and growth in winter – a plant you want to keep growing and hold its color when the days are at their shortest needs more light than the window provides. Remember to feed only plants that stand under grow lights.

There is more about sowing itself in How to grow coleus plants from seeds and about winter rest in Coleus in winter.

Does it have to be a grow light?

No. Plants use the energy in light, not the color we see. An ordinary white LED bulb contains every wavelength the plant wants. Research also shows that white light works better than the pinkish-purple specialty lamps that glow only red and blue.

Swedish hobby growers have also measured both ordinary LED bulbs and grow lights with proper light meters, and the results are clear. An ordinary white LED bulb delivered more plant-usable light than branded grow lights costing six times as much. The cheapest white bulbs in the stores also beat several products sold as "grow lights." So the words "grow light" on the box say nothing about the light. What matters is the lamp's strength, the distance and the hours.

When you buy grow lighting, there are two traps to avoid.

The cheapest purple lamps. They look professional but often give so little light that the plants never get what they need. An ordinary white bulb gives more light and is nicer to live with too.

Grow lights that give too little light. In measurements, several have delivered roughly half the light of an ordinary LED of the same wattage and still cost more.

And the Kelvin number? It only describes whether the light looks warm or cool to the eye. Choose the tone you like in the room – the plants will not notice any difference.

How much grow light your coleus need

What matters is the total amount of light per day. The lamp's strength, the distance to the plant and the number of hours work together. Three things to keep in mind if you are going to use grow lights.

Count on roughly 10 watts of LED per 25×25 cm of growing area.

Distance matters more than you think. Double the distance and the plant receives only a quarter of the light. Hang the lamp about 10 cm above the seedlings and raise it as the plants grow. Above larger plants, around 20 cm is about right.

Let the lamp run 10–16 hours a day – 16 for sowing in winter darkness, while plants that also get daylight in a window make do with around 10. Many hours of light favor leaf growth rather than flowering.

Many people think coleus demand a lot of light. In fact they are low-light plants that make do with a fraction of the light vegetable seedlings require. Nor is more light necessarily better. Plants that get too much respond with darker, smaller leaves. A simple lamp at the right distance therefore often gives nicer-looking plants than a needlessly strong one.

Which lamp should you buy?

An ordinary white LED bulb (E27) of 15–20 watts is the simplest buy for a small sowing area or a single plant. Choose a well-known manufacturer with a high stated light output in lumens. A clamp lamp or desk lamp works perfectly as a holder.

An LED strip or LED fixture suits long narrow surfaces such as windowsills and shelves, instead of several ordinary bulbs in a row.

Do you have old fluorescent tubes? Existing tubes work fine for sowing, so throw nothing away that still lights up. Fluorescents are being phased out of the stores, though, and if you buy new it is usually simplest to choose LED.

More tips on the equipment around sowing and growing can be found in 5 accessory tips for coleus.

IKEA VÄXER is gone. Buy this instead

IKEA SOLHETTA LED E27 1521 lumen is the closest successor on the IKEA shelf. It works just as well for sowing as for overwintering plants with their color intact.

Airam Superlux 20 W is a completely ordinary household bulb and the test winner in measurements from Swedish growers' forums. It beats most lamps sold as grow lights.

How we do it ourselves

We sow under grow lights from early winter, with the lamp just above the seedlings and lit from morning to evening. The same lamps then bring on cuttings and keep the color in the plants that go on display in the winter window. The rest of the collection stands in bright windows and rests until spring.

Frequently asked questions

Do coleus need a grow light in winter?

No, not to survive. A bright window is enough for winter rest. A grow light is only needed if you sow during the winter months or want continued growth and strong color in the middle of winter.

Do I have to buy a special grow light for coleus?

No. An ordinary white LED bulb gives coleus exactly the light they need. Count on roughly 10 watts per 25×25 cm of growing area. What matters is the lamp's strength, the distance and the hours. Not whether the box says "grow light."

How far from the plant should the lamp be, and how long should it run?

About 10 cm above the seedlings and roughly 20 cm above larger plants. Raise the lamp as the plants grow, because the light falls off quickly with distance. Let it run 10–16 hours a day. The less daylight the plants get, the more hours are needed.

Articles