Growing Herbs on a Windowsill: How Many Hours of Direct Sun Is Enough?
A pot of basil from the supermarket often looks great for a week on the kitchen windowsill, then stretches, pales and flops. The usual cause is not water or feeding but light. A window that looks bright to us delivers far less light to a plant than the open garden, and many herbs are sun plants. The way to choose herbs that will last is to count the hours of direct sun the pot actually gets, then match herbs to that number with the Sunlight Requirements Guide.
The guide does not measure light. You count the hours and type them in; it sorts them into full sun, partial sun, partial shade or full shade and lists plants for each. So the first job is the count.
Counting sun on a windowsill
Direct sun on a windowsill means sunlight falling on the pot itself, strong enough that your hand casts a sharp shadow on the soil. Bright daylight from a window that never sees the sun is indirect light, and it counts for much less. Check the sill every hour on a clear day and note when direct sun is on the pot. Do not look at the sun to check; look for the shadow.
Which way the window faces makes the biggest difference. In the Northern Hemisphere a south-facing window gets the most direct sun, west-facing windows get afternoon sun, east-facing windows get morning sun, and north-facing windows get little or none. In the Southern Hemisphere, swap north and south. Buildings, trees and roof overhangs opposite the window can cut hours from any of them, and the count changes with the season as the sun moves higher or lower.
Two things reduce the value of the hours you count. Window glass, especially coated or double glazing, filters some of the light. And light falls off quickly with distance: a pot on the sill gets far more than one on a table a few feet back. Keep herbs right at the glass.
Matching herbs to the hours
Here is how the guide sorts several windowsill counts:
| Direct sun on the pot | Category | Herbs from the guide's list |
|---|---|---|
| 2 or 3 hours | Partial Shade | Mint, cilantro |
| 4 or 5 hours | Partial Sun | Parsley |
| 6 or 8 hours | Full Sun | Rosemary, lavender |
The guide's lists cover garden plants generally, so they name only a few herbs. Others fall into the same pattern:
- Six hours or more: rosemary, lavender, thyme, oregano, sage and basil. These are the hardest herbs to keep happy indoors, because few windows give six hours of direct sun once glass and distance are taken into account. A south window in summer can manage it.
- Four to six hours: parsley, chives and dill do reasonably well. Growth is slower than outdoors but steady.
- Two to four hours: mint, cilantro and leafy parsley cope with less light. Cilantro tends to bolt to seed quickly in warm rooms whatever the light, so sow a few seeds every few weeks.
- Under two hours: no culinary herb will do well on direct sun alone. This is where a grow light earns its place.
The signs of too little light are the same for every herb: long gaps between leaves, stems leaning hard toward the glass, small pale leaves and no new growth. Turn pots a quarter turn every few days so they grow evenly, and move the leggiest ones to the sunniest window.
Adding a grow light
A small LED grow light can make up for a dim window or a short winter day. Keep it close to the plants as the maker directs, since output falls off quickly with distance, and run it on a plug-in timer for a consistent day length. Herbs need a dark period too, so do not leave lights on around the clock.
Lights and water share a windowsill, so take care. Use a light rated for the purpose, keep plugs, cords and power strips away from drip trays and spills, and never water over a lamp or its cable. Unplug before moving a light or wiping it down.
Watering: a rough ceiling, then the soil decides
The Watering Schedule Planner is built for outdoor beds, and it has no indoor climate option, so treat its numbers for a windowsill as a rough ceiling rather than a schedule. For an herb planter about 3 feet by 1 foot (3 square feet) moved outdoors for the summer, with loam-type mix, temperate climate and no rain, it gives 2.6 gallons a week, or about 0.4 gallons a day. In spring with the same inputs it is 1.9 gallons a week. Indoors, out of wind and summer heat, herbs usually use less.
Containers behave differently from beds. A small pot holds little reserve and can dry out in a day on a sunny sill, then sit wet for a week in a cool, dull spell. So water by the soil:
- Push a finger an inch or so into the mix. Water when it feels dry at that depth.
- Water until a little runs from the drainage hole, then empty the saucer. Roots sitting in water rot; rosemary, thyme and lavender are especially sensitive.
- Use pots with drainage holes. A decorative pot without one should hold a draining pot inside it, not soil.
- Mint and parsley like steadily moist soil; Mediterranean herbs like rosemary prefer to dry out a little between waterings.
Outdoors in summer the picture flips. A small bed of herbs in sandy soil, 12 square feet, comes out at 13.6 gallons a week in the planner, with the suggested frequency of daily or every other day because sand drains fast. For more on how soil changes the numbers, see clay, loam or sand.
Pets, children and the windowsill
A sunny windowsill is also a favorite spot for cats, so check what shares it with your herbs. Lilies are extremely dangerous to cats; even a little pollen or water from the vase can cause kidney failure, so keep them out of homes with cats entirely. Chives and other onion-family plants are toxic to cats and dogs, and aloe, often grown on the same sill, can make pets ill if chewed. Potting mix may contain added fertilizer, which is not something pets or small children should eat, and some herb seeds come treated with fungicide. Keep pots, mix and seed packets out of reach, and wash hands after handling them.
Only eat herbs you have grown from a labeled seed packet or a plant you can positively identify. A mystery seedling in a pot is a weed until proven otherwise.
Feeding
Herbs in fresh potting mix need little feeding for the first month or two. After that, a diluted liquid feed during active growth keeps leaves coming. Follow the product label for strength and frequency, since container plants are easy to overfeed and salts build up in a pot with nowhere to go. Too much fertilizer gives soft, weak growth and can burn roots. Our guide to reading N-P-K numbers explains what the three numbers on the bottle mean.
What the tools do not cover
- The Sunlight Requirements Guide does not measure light, and it does not account for glass, distance from the window or the strength of a grow light. It sorts the hours you enter.
- Its plant lists are short. Basil, thyme, oregano, sage and chives are not on them; the ranges above are general horticultural guidance, not output from the tool.
- The Watering Schedule Planner is for outdoor beds. There is no indoor setting and no container setting. Its Watering Time is minutes at the flow rate you enter for a hose or sprinkler; with a watering can, go by the gallons and the feel of the soil instead.
The short version
- Count the hours of direct sun on the pot on a clear day.
- Six or more: rosemary, basil, thyme. Four to six: parsley, chives. Two to four: mint, cilantro. Less: add a grow light.
- Keep pots right at the glass and turn them every few days.
- Water when the top inch is dry, and never leave pots standing in water.
- Keep lilies away from cats, and keep herbs, mix and seed packets out of reach of pets and small children.