Sowing Sesame: Soil Temperature, Depth and Spacing

Wait for warm soil, not for the calendar

Sesame is a hot-country crop and it behaves like one at sowing. Seed sitting in soil below 65F (18C) does not germinate, it rots. The working threshold is 70F (21C) at a depth of one inch (2.5 cm), measured in the morning rather than the afternoon, and germination is fastest and most even between 77F and 86F (25C to 30C). At that temperature seed is up in four to six days. At 68F (20C) the same seed takes two weeks and half of it never appears.

This means sesame is sown well after the last frost, not immediately after it. In most gardens that is two to four weeks later than beans, and it feels uncomfortably late. Sow anyway. A stand sown into warm soil overtakes an earlier sowing within a fortnight, because sesame grows in proportion to heat and simply sits still while nights are cool. Nothing is gained by being early and a great deal is lost when a cold, wet week follows sowing.

Count backwards as well as forwards. From sowing to the first splitting capsules is 90 to 130 days depending on how hot your summer runs, and the plant is killed outright by the first frost. If you cannot find that many frost-free days after the soil reaches 70F (21C), you can still grow sesame for the plant and the flowers, but a seed crop will not finish.

Test the soil rather than guessing. Push a soil thermometer one inch (2.5 cm) down at breakfast time on three consecutive mornings and use the lowest of the three readings. Afternoon readings on a sunny day are flattering and will talk you into sowing a fortnight too early.

Shallow seed, firm contact and no crust

Sesame seed is tiny, around three milligrams, and carries almost no food reserve. It has to reach the light on what it has. Sow half an inch (1 cm) deep at most, and a quarter of an inch (0.5 cm) into a fine seedbed is better. Anything deeper than three quarters of an inch (2 cm) and a large share of the seed exhausts itself underground.

The failure that catches people out is not depth but crusting. A heavy rain or an overhead watering on bare soil, followed by hot sun, sets a hard cap over the row that a sesame seedling cannot break. You will dig up perfectly germinated seed with a bent white shoot pressed flat under the crust. Cover the row with something that does not set: sieved compost, coarse sand, or fine leaf mold rather than the native soil, and water from below or with a very fine rose so the surface is never beaten flat.

Firm the row lightly after sowing. Sesame needs contact with moist soil to take up water, and a loose fluffy seedbed dries out around the seed within a day in the kind of weather you are sowing into. Press the covering down with the back of a rake or the flat of your hand, then keep the top inch (2.5 cm) evenly moist until emergence and no wetter than that.

1. Prepare a fine, free-draining seedbed on a ridge or raised bed Rake out clods until the surface is crumb-sized. On any soil that holds water, raise the row four to six inches (10 to 15 cm). Sesame roots will not tolerate standing water at any stage and least of all as seedlings.
2. Mark out rows 24 to 30 inches (60 to 75 cm) apart The plants finish three to five feet (90 to 150 cm) tall and need air moving between the rows. Crowded rows stay damp low down, which is where every disease of this crop starts.
3. Sow thinly along the row and cover a quarter to half an inch (0.5 to 1 cm) Aim for roughly one seed per inch (2.5 cm). Mixing the seed with a handful of dry sand makes thin, even sowing far easier than trying to place seed this small by hand.
4. Cover with sieved compost or sand rather than garden soil A covering that cannot form a crust is the single most useful thing you can do for germination. Firm it gently and water with a fine rose or from a channel beside the row.
5. Thin in two stages to a final 4 to 6 inches (10 to 15 cm) Thin once when seedlings have two true leaves, then again a fortnight later. Two stages leave you cover against losses and let you keep the strongest plants rather than whichever came up first.

Spacing, and whether to start plants under cover

Sesame is normally direct sown because the taproot goes down early and deep and resents being disturbed. Where the season is genuinely too short, you can start it under cover, but only on strict terms: sow into deep cells or paper pots, three weeks before planting out at the outside, and set the plants out before any root touches the side of the cell. A pot-bound sesame plant stays stunted all season and flowers late, which is the one thing you were trying to avoid.

In-row spacing changes what the plant does. Tight spacing gives tall, single-stemmed plants with capsules concentrated on the main stem, which ripen more evenly and are easier to cut and hang. Wide spacing gives shorter, branched plants with more capsules per plant but a longer, more uneven ripening. For a home crop where you are going to cut the whole plant at once, err towards the tighter end and take the even ripening.

Where you put the row matters as much as how you space it. Full sun all day, the warmest and best-drained ground you have, and shelter from the prevailing wind, because a three-foot (90 cm) plant carrying a stem full of capsules goes over easily in a summer squall.

What to write down at sowing

Record the sowing date together with the soil temperature you actually measured, and then the date the first seedlings broke the surface. The gap between those two dates tells you, better than any general advice, whether you sowed at the right moment where you garden. Four to six days means you judged it well. Twelve days means you should wait another fortnight next year.

Note the position of the row and the bed it went in. Sesame is worth rotating away from itself and from anything else prone to soil-borne wilts, and in three seasons you will not remember which end of which bed carried it. Note the covering material you used and whether the row crusted, because that one detail decides more sesame stands than anything else and you will want to repeat what worked.