Sowing Sorghum: Soil Temperature, Depth and Spacing for Grain, Syrup or Broom

Wait for the soil, not the calendar

Sorghum seed sitting in cold, wet ground does not wait patiently the way a pea does. It swells, sits, and rots. The threshold that matters is soil temperature at two inches (5 cm) at mid-morning, measured on three consecutive days: below 60F (16C) you should not be sowing, at 65F (18C) you will get a usable stand, and at 70F (21C) and above the seed is up in four to six days and comes up level. That last part matters more than speed. A sorghum block that emerges over four days grows as one crop and heads as one crop; a block that straggles out over two weeks gives you a harvest you have to walk three times.

In practice this puts sorghum a good two to three weeks after the last frost, later than you would sow corn in the same garden. Resist the urge to match the corn. Sorghum is being grown here precisely because it handles the hot, dry middle of the season, and it has no need to start early to reach that. A crop sown into properly warm soil catches and passes an earlier sowing within a month.

The other half of the timing question is the far end. Grain sorghum needs roughly 100 to 120 days from sowing to a hard, dry seed head, and syrup types need a similar stretch to fill the cane with sugar. Count backwards from your first expected frost and leave two weeks of margin. If the arithmetic does not close, grow it as a forage or broom crop rather than pushing a grain crop into weather that will not finish it.

Depth: deeper than most small seed, and for a reason

Sow one inch (2.5 cm) deep in a bed you have watered, and up to one and a half inches (4 cm) in sandy or fast-draining ground. This is deeper than the seed size suggests and deeper than many people go with it. The reason is that sorghum sets its crown roots just below the surface, and a seed placed too shallow puts that crown into the top half inch (1 cm) of soil, which is exactly the layer that bakes dry in the first hot week. Crops sown at half an inch (1 cm) look identical for a fortnight and then fall over in the first wind, because there is nothing anchoring them.

Firm the soil over the seed with the back of a rake or the flat of your hand. Sorghum seed is small enough that a loose, fluffy seedbed leaves half of it suspended in air pockets, and those seeds simply never germinate. Good contact is worth more than perfect depth.

1. Water the bed the day before, not after Soak the bed thoroughly 24 hours ahead so the moisture has settled evenly through the top four inches (10 cm). Sowing into pre-moistened ground and then leaving it alone gives far better emergence than sowing dry and watering on top, which caps the surface and can seal seedlings under a crust.
2. Open a shallow furrow and drop the seed Draw a furrow one inch (2.5 cm) deep with a hoe corner. Drop seed every two inches (5 cm) along it. Sowing thicker than your final spacing is deliberate here: sorghum germination in a home packet is rarely above 80 percent, and you thin down to the stand you want rather than trying to fill gaps later.
3. Cover and firm Pull the soil back over and press it down along the length of the row. The surface should be visibly compressed, not packed hard. If you can still see loose crumbs sitting on air, press again.
4. Leave it dry for a week if you can Do not water again until the seedlings are up unless the top two inches (5 cm) go bone dry. Sorghum roots chase moisture downward from the start, and a crop that has to reach for water in its first fortnight is the one that stands through a drought in midsummer.

Spacing depends entirely on which sorghum you have

Grain, syrup cane and broom types are all the same species and all look like sorghum for the first six weeks, but they want quite different room. Grain types are shorter and are grown to produce a heavy, compact head on every stalk, so they are spaced close and grown thick, and the crop supports itself as a block. Syrup and broom types are tall, sometimes ten to twelve feet (3 to 3.7 m), and a crowded stand of those gives you thin canes with no sugar and short, sparse broom heads. Give them room.

Whatever the type, grow sorghum as a block of at least four short rows rather than one long row. It is wind-pollinated, and while it is largely self-fertile, a block sets a fuller head than a single line does. A block also shelters itself: sorghum standing five deep bends together in a storm and comes back up, while a single row goes flat.

Thin when seedlings are four to six inches (10-15 cm) tall, and cut the extras off at ground level with scissors rather than pulling them. Sorghum roots knit together early, and pulling one seedling lifts the neighbor you meant to keep.

What to write down at sowing

Record the soil temperature you actually measured on the day you sowed, not the air temperature, and record the date emergence was complete rather than the date the first seedling appeared. The gap between those two dates is the single most useful number you will carry into next year: a three-day gap means you got the timing right, and a ten-day gap means you sowed into ground that was still too cold and should push the sowing later next season.

Also note which type each block is, and where it sits in the garden. By late summer a syrup cane and a grain type are unmistakable, but in early summer they are identical, and people routinely lose track of which end of the bed holds which. Write down the row positions and the final plant spacing you settled on, so that if the block lodges or the heads come in thin you know whether crowding was the cause.