Short answer: treat a north window as a bonus, not your main light. For a 1 × 3 foot shelf of herbs, aim for about 210 to 240 PPFD for 14 to 16 hours a day. A basic LED bar setup drawing roughly 30 to 50 actual watts should get you there.
Why north windows fall short
In the Northern Hemisphere, a north-facing window gets little or no direct sun for much of the year. Plants there get soft, indirect light that drops off quickly as you move away from the glass. Leggy stems and pale leaves are the usual result.
Michigan State lists a DLI of 12 or more for leafy greens and herbs. We don't give a typical DLI for a north window, because it depends on your latitude, season, glass and what's outside. If you want to know yours, a light meter or PPFD meter app will give a rough reading, but it's safest to size the light as though the window adds little.
Which herbs cope best?
| Herb | Light demand | North window notes |
|---|---|---|
| Mint | Lower | Often the easiest choice. Keep it in its own pot. |
| Parsley | Lower to medium | Slow but steady with a small light. |
| Chives | Medium | Works with added light. |
| Cilantro | Medium | Short-lived indoors, so sow small batches often. |
| Basil | High | Needs a proper light and warmth. |
| Rosemary, thyme | High | Hard in low light; give them your brightest spot. |
These rankings reflect common growing advice rather than measured crop data.
How much light to add
| Hours on | PPFD needed | Approx. actual watts |
|---|---|---|
| 12 | About 280 | About 55 W |
| 14 | About 240 | About 50 W |
| 16 | About 210 | About 40 W |
Longer hours let you use a smaller light. A cheap outlet timer is well worth having, because plants like a steady routine and you won't have to remember it.
What to buy
LED bars under a shelf
A good fit for a window shelf. Bars link together and stay out of sight. Check the length against your shelf. See LED grow light bars on Amazon.
Clip-on or stand lights
Fine for one or two pots. Many are low powered, so expect slower growth if the listing has no PPFD data. See clip-on grow lights on Amazon.
A simple timer
Some lights have one built in. If yours doesn't, use a plug-in timer. See outlet timers on Amazon.
Getting the setup right
- Pots need drainage holes. Herbs in low light dry out slowly, so water less often.
- Turn pots regularly if light comes from one side.
- Keep leaves off cold glass in winter.
- Harvest little and often to keep plants compact.
