The Self-Reliant Garden
field note · garden · winter

Growing herbs indoors through winter: the light is the whole problem

Herbs brought inside in October usually die slowly by January, and people blame the watering. It is almost always the light. A bright kitchen is a dark room by a plant's standards, and the gap between what a windowsill gives and what a herb needs is larger than it looks.

How much light, in numbers

Iowa State University Extension says most herbs need around eight hours of direct light a day when grown indoors, with some tolerating six. Penn State Extension says most need six hours of direct sunlight. Illinois Extension says at least six. So the working range is six to eight hours of direct sun, and a south facing window is the candidate that delivers it. A bright east or west window can work; a north window will not.

Iowa State University Extension, growing herbs indoors, reviewed November 2023. Penn State Extension, growing herbs indoors, 1 November 2024. Illinois Extension, herbs all winter, 8 January 2021.

If the window cannot do it, the lamp has to be close

The single most common mistake with a grow light is hanging it too high. All three extension services give the same distance: 6 to 12 inches from the leaves. Penn State specifies two 40 watt cool white fluorescent tubes at that distance.

And the light runs far longer than daylight does, because it is weaker: 12 to 16 hours a day on a timer (Iowa State), 14 to 16 hours (Penn State), 12 to 14 (Illinois). Put it on a timer rather than trusting yourself, which is also how you avoid leaving it on for 24 hours, which plants do not want either.

Not every herb has the same appetite

Penn State splits the list, and the split is worth using when you choose what to bring in:

  • High light, want the direct sun: bay, rosemary, thyme.
  • Will tolerate low light, two to six hours: lemon balm, chives, cilantro.

If you have exactly one decent window, that decides what earns the spot.

Temperature, and the vent that kills

Penn State gives the range: 65 to 70 °F during the day and 55 to 60 °F at night. Illinois puts optimal growing temperature at 60 to 70 °F. The night drop is not a problem, it is normal and the plants expect it.

What is a problem is the heating vent. Illinois Extension says to keep herbs away from vents, which dry the plant out faster than any watering schedule can compensate for. A pot on a windowsill directly above a radiator gets cold glass on one side and hot dry air on the other, which is the worst of both.

Penn State Extension, growing herbs indoors, 1 November 2024. Illinois Extension, herbs all winter, 8 January 2021.

What this means in practice

  1. Pick the brightest window you have, and be honest about which direction it faces.
  2. Bring in what suits that window rather than everything you grew outside.
  3. If you are adding a lamp, put it 6 to 12 inches above the leaves and on a timer for 12 to 16 hours.
  4. Move the pots away from vents and off the radiator shelf.
  5. Water less than you did outdoors. A cold, dim room dries out slowly, and the plant is barely growing.

Rosemary is the herb most likely to be the reason you are reading this, because in most of the country it cannot stay outside. That is a zone question, and it has a map: herbs by USDA zone.

Sources, with the date of each

  • Iowa State University Extension, growing herbs indoors, reviewed November 2023.
  • Penn State Extension, growing herbs indoors, 1 November 2024.
  • Illinois Extension, herbs all winter, grow them indoors, 8 January 2021.

Where the three sources differ, we printed all three rather than averaging them. Iowa State does not give a temperature range at all, saying only that typical indoor temperatures suit herbs; the numbers above come from Penn State and Illinois.