Sowing Buckwheat: Soil Temperature, Seeding Rate and Depth

Warm soil, no frost, and no hurry to be early

Buckwheat is a warm-season crop with no frost tolerance at any stage. Seedlings are killed outright at around 28F (minus 2C), and even a light late frost on a young stand will set it back so far that it never recovers. There is nothing to gain from sowing early. Wait until the last frost date has genuinely passed and the top two inches (5 cm) of soil sit above 55F (13C), and the crop will overtake anything sown two weeks sooner into cold ground.

The reward for waiting is speed. Seed sown into warm, moist soil emerges in three to five days, covers the ground in two to three weeks, and opens its first flowers around 30 to 40 days from sowing. Nothing else you can sow will do that. It is this speed that makes buckwheat the standard gap-filler between two other crops, and it is why the sowing window is so wide: any time from late spring through midsummer works, as long as there are still 35 frost-free days ahead for a green manure crop, or 70 to 90 for a seed crop.

Buckwheat is not a grass and not a cereal, despite the name. It belongs with rhubarb and sorrel, and its seed is a small three-sided achene rather than a grain. That matters practically, because it does not fit into a cereal rotation the way oats or rye would, and it will not regrow after cutting the way a grass does. Once you mow it past a certain point, it is finished.

If a late frost is still possible where you garden, delay by a week rather than covering the bed. Buckwheat sown a week late catches up within a fortnight. Buckwheat sown a week early and frosted does not catch up at all.

Rate, depth and why you broadcast rather than drill

For a cover crop, broadcast the seed thickly and evenly at roughly two to three ounces per square yard (60 to 100 g per square meter), then rake it in. A thick, even stand is the entire weed-suppression mechanism: the canopy closes over bare soil before any weed seedling gets going, and gaps in the stand are exactly where the weeds come back. Sowing thin to save seed defeats the purpose of the crop.

For a seed crop, sow at roughly half that rate. Plants given a little more room branch more, carry more flowers on side shoots and set more seed each, and a thinner stand also dries down faster at harvest and lodges less. You will lose some weed suppression in exchange, so a seed crop needs cleaner ground to start with.

Sow half an inch to one inch (1.5 to 2.5 cm) deep. The seed is large enough to push through that easily and shallow sowing on bare soil simply feeds the birds. Rake the bed after broadcasting so most seed is buried, then firm the surface with the back of the rake or by treading. Water once if the soil is dry. After that, leave it alone. There is no thinning, no transplanting and no second operation.

1. Wait for the soil to reach 55F (13C) Measure in the top two inches (5 cm) in the morning rather than the afternoon. Below this the seed sits and sulks and slugs find it. Above it, emergence takes three to five days.
2. Clear and rake the bed, but do not enrich it Pull large weeds and rake to a rough tilth. Skip the compost and the fertilizer entirely. Buckwheat performs well on poor and even acidic ground, and rich soil produces tall soft growth that falls over.
3. Broadcast thickly and evenly Two to three ounces per square yard (60 to 100 g per square meter) for a cover crop, half that for seed. Scatter in two passes at right angles to each other, which evens out the gaps better than one careful pass ever does.
4. Rake in to half an inch to one inch (1.5 to 2.5 cm) Rake lightly in both directions so most seed is covered, then firm the surface. Uncovered seed on the surface is bird food and germinates unevenly.
5. Water once, then leave it One good soak if the soil is dry at sowing is enough to trigger germination. Buckwheat needs steady moisture in its first fortnight but repeated light watering packs the surface and does more harm than good.

Choosing a sowing date by what you want from the crop

The single decision that shapes everything else is whether you are growing buckwheat for the soil, for the bees, or for the seed. All three are legitimate and all three want a different sowing date, because they end at different points in the plant's life.

A green manure crop is cut at first flower, roughly five weeks after sowing, so it can be sown as late as five or six weeks before your first expected frost. A pollinator crop is left to flower for its full run, which is four to six weeks, so it needs about ten frost-free weeks. A seed crop needs the whole 70 to 90 days plus dry weather at the end, so it has to be sown early enough that ripening lands before the first frost and before your wettest autumn weather.

Because the crop is so fast, successive sowings three weeks apart work well and are worth doing deliberately if you want flowers for bees over a long stretch. Each sowing carries the flowering forward while the previous one is going over.

What to write down at sowing

Record the sowing date and the bed. With a crop this fast, the sowing date is what every later judgement is measured against: you will want to know that emergence took four days and not eight, and that first flower came at 33 days and not 45, because those two numbers tell you whether the soil was warm enough when you sowed.

Note the rate you used and whether the stand closed over the ground completely. A patchy stand almost always traces back to sowing too thin, sowing too shallow or sowing into cold soil, and you cannot tell which unless you wrote down what you did. Note the bed position as well, because buckwheat sheds seed easily and next year's volunteers will tell you exactly where this year's crop stood.