Light
The University of Maryland Extension asks for six or more hours of direct sun a day, and Penn State Extension uses the same figure for most herbs grown inside. Indoors that means the sill of a south window, or a west window in summer. The University of Illinois Extension measures about 1,500 foot-candles directly in front of a south window, and anything set back from the glass gets a small fraction of that.
Short winter days rarely provide enough. Penn State suggests two 40-watt cool white fluorescent tubes hung 6 to 12 inches above the plants for 14 to 16 hours a day, and LED grow lights work the same way. Signs of too little light are easy to read: long, thin stems, smaller leaves and a weaker aroma. Rotate the pot often so the plant grows evenly rather than leaning toward the glass.
| Range | Bright light to direct sun, best in direct sun |
|---|---|
| Foot-candles | 300 to 1,500 fc (3,229 to 16,146 lux) |
| Windows | South, West · needs some direct sun |
| Sources | North Carolina Extension Gardener Plant Toolbox; Penn State Extension; University of Illinois Extension; University of Maryland Extension |
Water
Basil is thirstier than the Mediterranean herbs it is usually grouped with. Penn State Extension says never to let the potting mix dry out completely, and the RHS asks for compost kept evenly moist. In practice, feel the surface each day in warm weather and water as soon as it is dry, slowly, until a little runs from the drainage hole. A small pot in a sunny summer window dries remarkably fast, so check it more often than your other plants.
Moist is not soggy, though. Clemson Extension advises watering regularly but not overwatering, and never letting the pot stand with water in its saucer, because Pythium and Rhizoctonia root rots yellow the plant and kill branches from the tips down. The RHS adds two habits worth adopting: water in the morning, since basil hates wet roots overnight, and avoid splashing the leaves.
Dry spells have a second cost. The RHS notes that basil starts to flower sooner when it runs dry, and once it flowers the leaf harvest winds down. Steady moisture keeps the plant in leafy mode for longer.
| Rule | When the top inch is dry: keep the mix steadily moist: water as soon as the surface feels dry, and never let the pot dry out completely |
|---|---|
| Winter | Growth slows in short days and cooler rooms, so the pot dries more slowly; keep watering whenever the surface is dry, in the morning, and never leave water in the saucer. |
| Watch for | Basil may flower sooner when it runs dry, the RHS notes, which ends the leafy harvest. |
| Water the mix, not the leaves: wet foliage invites gray mold, leaf spots and downy mildew. | |
| Water in the morning if possible; the RHS says basil hates having wet roots overnight. | |
| Sources | Clemson Cooperative Extension; Royal Horticultural Society; Penn State Extension |
Soil and pH
Clemson Extension says basil grows best in rich, well-drained soil with plenty of organic matter, at a pH of 6.0 to 6.5. A good multipurpose potting mix with some compost or composted bark mixed in fits that description. The RHS recommends a container at least 8 inches wide and deep for one plant outdoors; indoors a pot of similar size holds enough moisture that you are not watering several times a day.
| Mix | A rich, well-drained potting mix with plenty of organic matter, in a pot at least 8 inches wide and deep. |
|---|---|
| Ingredients | multipurpose potting mix, compost or composted bark for organic matter, container with drainage holes |
| pH | 6 to 6.5 |
| Sources | Clemson Cooperative Extension; Royal Horticultural Society |
Humidity
Average indoor humidity is fine for basil. The real concern is the opposite of dryness: UF/IFAS and the University of Maryland both note that warm, humid, wet conditions spread downy mildew, and that good air movement around the leaves reduces the risk. Space pots so air can circulate, keep water off the foliage, and avoid enclosing the plant in a humid corner with tropical houseplants.
Temperature
Warmth is non-negotiable. Penn State recommends herb rooms of 65 to 70°F (18 to 21°C) by day and says basil specifically cannot survive below 50°F (10°C), so it should not sit on a windowsill in winter, where glass gets cold at night. Illinois Extension reports damage and blackened leaves at 50 degrees, and UF/IFAS says the leaves of many varieties turn black and drop below 40°F.
If your sunniest window is drafty, pull the pot back from the glass at night or move it under a grow light in a warm room. Outdoors, Clemson Extension advises waiting until days are consistently in the 70s and nights stay above 50°F before setting basil out.
| Minimum | 50°F (10°C) |
|---|---|
| Ideal | 65 to 70°F (18 to 21°C) |
| Humidity | Average room humidity. Normal room air is fine; what matters is that leaves dry quickly, because humid, stagnant air spreads downy mildew. |
| Sources | Clemson Cooperative Extension; UF/IFAS Gardening Solutions; University of Illinois Extension; Penn State Extension; University of Maryland Extension |
Fertilizer
Feed modestly and often. Penn State Extension recommends a low dose of water-soluble fertilizer every 2 weeks for indoor herbs, and warns that more frequent feeding can hurt aroma and taste. Clemson and the University of Maryland agree that overly lush growth dilutes flavor. The RHS suggests a balanced liquid fertilizer for container basil and advises against potassium-rich tomato feeds, which encourage flowering. Cut back in winter, when growth slows.
| When | While the plant is producing leaves, spring to fall indoors or out, every 2 weeks, reduced in winter |
|---|---|
| What | Low dose of a balanced water-soluble fertilizer; avoid high-potassium tomato feeds, which push flowering, half strength |
| Sources | Clemson Cooperative Extension; Royal Horticultural Society; Penn State Extension; University of Maryland Extension |
Work out the dose for your watering can with the fertilizer dilution calculator →
Repotting
Store-bought basil usually needs attention on day one. The RHS explains that supermarket pots are packed with thin, overcrowded seedlings meant for short-term harvesting, which soon flower or rot, although they may survive if divided and repotted promptly. Tease the root ball into three or four clumps and pot each into its own container of fresh mix. Seedlings you raise yourself go into individual 3-inch pots once they have several leaves, then into larger containers as roots fill them.
- Roots grow through the drainage hole
- The pot wilts within hours of watering on a warm day
- A store-bought pot is crammed with dozens of thin stems