How to Grow Basil from Seed: Covering Depth and Soil Warmth

Cover the seed only just enough to hide it

Basil seed germinates better in light, so burying it stops the seed responding even when the soil is warm enough. Sift a few millimeters of fine compost over it, just enough that you can still half see the seed here and there.

Do not leave it uncovered, though. Basil seed swells into a jelly-like coat when it takes up water and sticks to the surface, but it still rolls with every watering, and any seed that dries on the surface is dead. A thin covering, gently firmed, puts the seed in contact with damp soil.

1. Level the seedbed first An uneven drill lets seed fall into hollows where the covering ends up thick, and those seedlings come up late. Run a fingertip or the edge of a board along the bottom first and the seedlings emerge level.
2. Sift the covering Lumpy soil drops heavy particles onto the seed that a fine basil shoot cannot lift. Sift compost or vermiculite through a fine sieve and scatter it thinly.
3. Firm after covering Press the surface with your palm. Without firming, watering floats the seed up and washes it into one corner, where it comes up in a dense clump.

Not letting the surface dry before germination matters more than anything else. Cover with newspaper or row cover and take it off the day you see the first shoot.

Wait for the soil to reach 68F

Basil comes from tropical Asia and needs soil at 68 to 77F (20 to 25C) to germinate. At around 59F (15C) the seed does not move, and it often simply rots in wet soil. Seed sown in early spring that never appears has usually failed on soil temperature, not on age.

It grows best at 77 to 86F (25 to 30C). At 68F growth is slow and at 59F it barely moves at all. Sowing outdoors a month early gains you nothing: a plant sown after the last frost catches up within a month.

Direct sowing versus pots

Basil does not object strongly to root disturbance, so either method works. Choose on soil temperature and weeds: when the soil is still cool, or the bed is weedy, raising plants in pots gets them away much more reliably.

1. Sowing in pots Sow three or four seeds in a three and a half inch (9 cm) pot and thin to the strongest seedling once the first true leaves show. On a windowsill or in a cold frame you can start well before the last frost.
2. Direct sowing Once the soil is over 68F (20C), sow three or four seeds per station eight inches (20 cm) apart. With no transplanting check they get away fast. Thin to one.
3. Do not broadcast the seed The seed is fine, and broadcast sowing gives you a dense mat you cannot thin fast enough, so it all stretches. Sow in stations or drills so the plant positions are set from the start.

Watering decides whether it germinates

With such a thin covering, the seed stops the moment the surface dries once. Water from below in a tray, or mist, so the color of the soil never changes. Above 68F expect seedlings in three or four days, at around 68F in five to seven.

1. Water from below Stand the pot in a shallow tray of water and let it draw up. Watering hard from above washes the thin covering and the seed with it, and the seedlings come up piled together and fall over.
2. Get them into the light the same day Move them into full light on the day you see the first shoots. Left in the dark, the stems stretch and flop, and a stretched basil seedling never really thickens up.

Basil seed has no dormancy, so leftover seed from last year germinates perfectly well given the right conditions. If nothing comes up, suspect temperature and water before you blame the seed.

Split a bought pot into several plants

Basil sold in pots usually has several seedlings sown together. Planted as one clump the roots compete and every plant stays thin. Wash the compost off and separate them and one pot becomes three or four plants.

1. Wash the rootball apart Submerge the whole pot in a bucket of water and let the compost fall away. Tearing a dry rootball apart breaks the fine roots and the plants are slow to get going.
2. Separate at the base of the stems Follow each stem down and separate the plants. Do not pull. Untangle the roots with your fingers. Three or four leaves and some white root is enough for one plant.
3. Keep them shaded for a few days Put them in bright shade and keep them watered while they recover. Straight into full sun and they wilt, and that costs a week.

Only divide once temperatures are reliably above 68F (20C). Divided in a cold spell the roots do not move and the plants simply die back.

Space them 10 to 12 inches apart

One basil plant spreads widely, so planted tight the lower leaves sit in still air and drop off black. Because pinching out multiplies the number of shoots, the final spacing needs to be 10 to 12 inches (25 to 30 cm) in the ground, and at least 8 inches (20 cm) in a container.

Crowded plants make smaller leaves with less scent. If you want more basil, grow one plant bigger and pinch it more often rather than squeezing in extra plants.

When nothing comes up

If you see nothing after ten days, check three things in order: soil temperature, covering depth and water. Resist digging the seed up to look, because disturbed seed rarely grows on. Fixing the conditions and waiting gets you there faster.

1. The soil is too cold Below 68F (20C) the seed will not move in a week. If the soil still feels cold to a finger, sow again in pots somewhere warm.
2. The covering is too thick More than a quarter of an inch and neither light nor air reaches the seed. Brush the surface off gently until you can just see the seed through it.
3. It dried out once A surface that dries before germination stops the seed there. Cover with newspaper to hold the moisture and mist morning and evening.
4. When to give up and resow If nothing appears ten days after you have corrected the conditions, use fresh seed. A resow in early summer is still in good time for a summer crop.

Old seed germinates less reliably. If you are using a packet opened last year, sow more thickly than usual and thin to what comes up.