Kratky Grow Lights: Window Light, LEDs and Hours
Light is the plant's energy supply. A Kratky kit can have perfect nutrients and pH and still grow slowly if the light is weak. This guide covers when a window is enough and when Kratky grow lights make the difference.
Is a window enough?
Sometimes. A bright, unobstructed window that gets several hours of direct sun can grow herbs and lettuce, especially in spring and summer. In the Northern Hemisphere a south-facing window usually gets the most light. In winter, or in a window shaded by trees or buildings, light levels drop sharply. Signs that a plant needs more light include long, thin stems, pale leaves and plants leaning hard toward the glass.
Windows also bring heat. Direct sun through glass can warm a container, so keep an eye on solution temperature, as described in the Kratky containers guide.
| Light source | Suits | Notes |
|---|---|---|
| South-facing window (northern hemisphere) | Herbs and greens most of the year | The brightest window; can heat a reservoir in summer |
| East- or west-facing window | Lettuce and leafy greens | Half a day of direct light; growth slows in winter |
| North-facing window | Little; mostly too dim | Expect pale, stretched plants without extra light |
| LED grow light on a timer | Any leafy crop, any season | About 12 to 16 hours a day for greens and herbs |
Choosing a grow light
LED grow lights are the usual choice for home Kratky systems. They use little power, run cool compared with older lamps, and come in sizes from a single clip-on light to panels that cover a shelf.
- Full spectrum. White "full spectrum" LEDs suit leafy greens and herbs and make plants look natural.
- Coverage. The light should cover all the plants in the kit, not just the middle.
- Adjustable height. As plants grow, the light needs to move up.
- Brightness. Manufacturers increasingly list PPFD (light intensity at the plant) or coverage area for leafy greens. These are more useful than watts alone.
Some countertop kits include a light on an arm. Check that it can be raised and that it covers every planting site, as noted in how to choose a Kratky kit.
How many hours of light?
Leafy greens and herbs grown indoors are commonly given around 12 to 16 hours of light per day. A plug-in timer keeps the schedule steady. Plants also need a dark period each day, so leaving lights on around the clock is not recommended. Some plants, such as lettuce, can be encouraged to bolt (flower and turn bitter) by very long days combined with warmth, so a moderate schedule is a good default for greens.
How high should the light be?
Follow the light manufacturer's recommendation, because LED output varies a lot. A light that is too far away produces stretched, leggy seedlings. A light that is too close can bleach or scorch leaves. Many small LED grow lights are used somewhere around 12 to 18 inches above leafy plants, but the right distance depends on the specific light. Raise the light as plants grow so the distance stays similar.
Light and water use
More light means faster growth, and faster growth means the plant drinks more. A Kratky kit under strong light will use solution more quickly than the same kit on a dim windowsill. That is normal, but it is worth remembering when judging container size and when deciding how often to refill a Kratky kit.
Keeping light off the solution
Grow lights are bright, so any gap in the lid or a clear container will let light reach the nutrient solution and feed algae. Cover clear containers and unused holes. The Kratky problems and fixes guide covers algae if it does appear.
Signs of too little or too much light
- Too little: tall, thin, floppy stems; small pale leaves; slow growth; plants leaning.
- Too much or too close: bleached or white patches, crispy leaf edges, leaves curling away from the light.
Go deeper: choosing and comparing LED grow lights is covered at Awesome LED Grow Lights, our sister site on LED grow lights for indoor plants.
Frequently asked questions
Can I use a regular LED bulb as a grow light?
A regular household bulb gives far less usable light than a grow light at the same distance. It can keep herbs alive near a window, but leafy crops usually grow much better under a light made for plants.
Should grow lights stay on all night?
No. Plants need a daily dark period. Leafy greens and herbs are commonly grown with about 12 to 16 hours of light, controlled by a timer.
Does a grow light make the water run out faster?
It can. More light means faster growth, and faster-growing plants drink more, so a kit under a strong light may use its solution sooner than one on a dim windowsill.
What color grow light is best for herbs and lettuce?
White full-spectrum LEDs work well for leafy greens and herbs and make it easy to see the true color of the leaves, which helps you spot problems early. Purple-looking red and blue lights also grow plants but make leaves harder to judge.
Key points
- A very bright window can work; most indoor kits do better with an LED grow light.
- Give leafy greens and herbs roughly 12 to 16 hours of light, with a dark period.
- Follow the light maker's height guidance and raise the light as plants grow.
- More light means faster growth and faster water use.
Matching light to the crop is part of choosing what to grow: see the best plants for Kratky.