Grow Light for Herbs: How Long, How Close and Which Light?
From October to February, the light on your windowsill isn't enough for most herbs. A grow light closes that gap. But plenty of questions remain: How long should the light stay on? How close should it hang? And do you really need that purple light? Here you'll get clear answers, no jargon.
Do herbs really need a grow light in winter?
In short: if you want to harvest in winter, yes. Herbs like basil, parsley or coriander grow long and thin in low light. Gardeners call this "legginess". The leaves turn small and pale, and at some point the plant flops over. A light keeps your herbs compact and growing. If you only want to overwinter thyme, sage or rosemary, a bright, cool spot is often enough.
How long should the grow light be on?
- For most kitchen herbs: 12 to 14 hours a day.
- For basil and other sun lovers: 14 to 16 hours.
- For seedlings and sprouts: 14 to 16 hours, so the seedlings don't get leggy.
- Switch it off at night. Plants need a dark phase of at least eight hours. Light around the clock does more harm than good.
- Stick to a schedule. A fixed rhythm matters more than the exact number of hours. Use a plug-in timer or a light with a built-in timer.
How close should the light be to the plants?
The further away the light is, the less of it reaches the plants. Even double the distance means a lot less light. Typical LED grow lights hang 20 to 40 centimetres above the leaves. Follow the manufacturer's advice. These signs help you fine-tune:
- Too far away: long, thin stems, big gaps between pairs of leaves, plants leaning towards the light.
- Too close or too bright: pale or brownish spots on the top leaves, curled leaf edges.
- Move it up as they grow: When your herbs get taller, hang the light higher or trim the plants.
Which light colour is right?
- Purple light (red and blue): It works for plants, but it's unpleasant in a kitchen or living room. Plants look grey, and it's harder to spot pests.
- White full-spectrum light: The best choice for your home. It contains all the colours plants need and looks natural.
- Cool or warm white: Cooler light around 5000 to 6500 Kelvin suits leafy growth. Warmer light around 3000 Kelvin feels cosier in the evening. An adjustable light colour gives you both.
What to look for when buying
- Made for plants. A regular desk lamp or LED bulb usually gives too little usable light. Look for products sold as grow lights and for details on the spectrum.
- Built-in timer. Saves you an extra plug-in timer and forgotten evenings.
- Dimmable. Seedlings need less light than mature plants.
- The right size. The lit area should be as big as the area of your pots.
- Power use. LEDs are efficient. A 20-watt light that runs 14 hours a day uses about 8.5 kWh a month. At 35 cents per kWh, that's around 3 euros.
The most common mistakes
- Light too far away, because it looks nice hanging from the ceiling.
- Light around the clock, with no break for the plants.
- Irregular hours, because the light is switched on and off by hand.
- Wrong expectations: A light makes up for missing light, not for water and nutrients. Keep watering sparingly and feed little in winter.
Light, schedule and water in one device
If you don't want to hunt down a light, timer and pots separately, take a look at the GreenBox. The smart indoor garden made of birch wood has a built-in LED. It runs on your schedule, and you set brightness and light colour from 3000 to 6000 Kelvin in the app. A sensor measures the water level and reminds you to top up. That way herbs, lettuce, microgreens or chillies grow even in winter, no windowsill needed.
In a nutshell
- In winter, herbs need 12 to 16 hours of light, but also a dark phase.
- The light usually hangs 20 to 40 centimetres above the leaves.
- White full-spectrum light is the most pleasant choice for your home.
- A timer matters more than the perfect light.
