Is an ordinary LED bulb enough for houseplants?

For houseplants kept away from windows, an ordinary LED bulb sometimes helps and sometimes falls short. What really decides it: amount of light, distance and hours.

In winter, or in flats with north-facing windows, houseplants often stretch, drop leaves and lose colour. The first idea is to put a lamp next to them. That raises a question: is an ordinary LED bulb from around the house enough, or do you need a special purple-glowing grow light? The answer depends less on the type of bulb than on how it is used.

What plants take from light

Plants use light for photosynthesis, which works with visible light in roughly the 400–700 nanometre range. Chlorophyll absorbs blue and red light most strongly and reflects part of the green, which is why leaves look green. That does not make green light useless: it penetrates deeper into the leaf and works in the lower layers too.

So while red-blue lamps that look purple-pink are efficient for plants, white light is also fully usable for photosynthesis. Most modern professional grow lights in fact produce broad-spectrum white light.

The main issue: the amount of light

When an ordinary LED bulb is not enough for a plant, the reason is usually not the spectrum but the lack of light. Human eyes adapt well to dim light, so a room looks bright to us. A plant does not “perceive” light; it simply uses it. The light a plant receives in front of a window, even on a cloudy day, is many times more than what reaches the middle of a room from a ceiling bulb.

Light falls off quickly with distance. For a point source, doubling the distance cuts the light reaching a surface to roughly a quarter. So a bulb hanging from the ceiling delivers very little light to a plant below, while the same bulb a few tens of centimetres above the plant can make a noticeable difference.

When an ordinary LED bulb helps

An ordinary white LED bulb is appropriate when:

• the plants need little light, such as ZZ plant, snake plant, peace lily and some ferns;
• it supplements window light during winter's short days;
• the bulb is close to the plant, aimed from above or slightly to the side;
• the light is on for long enough each day.

The low heat of LED bulbs is an advantage here: an incandescent bulb close to a leaf can scorch it, while an LED bulb is safe even at short distances. Still, make sure leaves do not touch the bulb.

When a dedicated lamp is needed

In these situations an ordinary bulb is unlikely to be enough:

• light-loving plants such as cacti, succulents, fruiting citrus and flowering species;
• rooms without windows, or growing entirely under artificial light;
• raising seedlings, which become thin and leggy without enough light;
• growing microgreens and herbs.

In such cases a fitting designed for plants, with its light output and beam angle calculated for the job, gives better results.

Does colour temperature matter?

A common claim is cool light for foliage and warm light for flowering. There is some basis for it, since cool light contains more blue and warm light more red. But for decorative houseplants at home, this difference is not as decisive as the amount of light. For a lamp used in a living space, neutral or cool white is a practical choice: it suits the plant and does not look odd in the room.

Hours and rhythm

Most plants do not like continuous light; they need a dark period too. For houseplants, 12–16 hours of light a day is usually considered enough, with the exact figure varying by species. A simple plug-in timer is very handy for switching the lamp on and off at the same time every day. Watch how the plant responds: pale leaves and stems stretching towards the light mean too little light; scorch marks on leaves mean the lamp is too close.

Come back to these questions after a few weeks to judge the result:

• Is the species light-loving or shade-tolerant?
• How far is the bulb from the plant?
• Is there window light, and how much?
• How many hours a day will the lamp be on, and is there a timer?
• After a few weeks, do new leaves look healthier than before?

Zəfər İşığı general-purpose LED bulbs come in a range of wattages and bases, but they are not specialised grow lights. You can filter the catalogue to find the right cap for a desk lamp or pendant in your plant corner.