Vegetable Seed Calculator: How Much to Plant

Seed quantity is derived by working backwards from the stand you want. The number of planting positions is row length divided by the recommended in-row spacing, multiplied by the number of rows; that figure is multiplied by seeds sown per position and then divided by the germination fraction, because a proportion of seed will never produce a seedling. The germination percentage printed on a seed packet is a laboratory result obtained under prescribed conditions of temperature, moisture and substrate, not a field prediction. Emergence in a real seedbed is normally 10 to 20 percentage points lower, and further below that in cold, crusted or cloddy soil, which is the practical reason growers sow two or three seeds per station for large-seeded crops and thin later. Converting a seed count into a purchase requires seeds per gram, which varies over more than two orders of magnitude: lettuce and carrot run near 800 seeds per gram, tomato near 300, radish near 120, cucumber near 35, while beans are around 4 and sweet corn around 5, which is why a bean crop is bought by the kilogram and a lettuce crop by the gram. Spacing defaults used here follow standard extension recommendations, roughly 5 cm for carrot and radish, 25 cm for lettuce, 45 cm for pepper and 60 cm for tomato and cucumber. The main caveats are that the model assumes discrete stations thinned to one plant, which does not describe broadcast or band-sown salad crops; that it ignores seed lost to damping-off, birds and soil insects after emergence; and that stored seed loses viability at species-specific rates, so an old packet should be retested rather than assumed to meet its label.

Seed quality was effectively unregulated for most of agricultural history, and nineteenth-century seed trade was notorious for adulteration with inert matter, weed seed and dead grain. The response was institutional. Friedrich Nobbe, a botanist at the forestry academy in Tharandt in Saxony, established the first seed control station in 1869 to test purity and germination on behalf of buyers, and he is generally credited as the founder of seed testing; comparable stations spread rapidly across Europe and North America over the following decades. Standardization followed. The Association of Official Seed Analysts was founded in North America in 1908 and the International Seed Testing Association in 1924, and between them they fixed the prescribed substrates, temperatures and counting intervals that make a germination percentage comparable between laboratories. In the United States the Federal Seed Act of 1939 required interstate seed to be labelled with germination percentage, test date and weed seed content, which is why those figures appear on packets today. A faster viability method arrived when Georg Lakon in Germany developed the tetrazolium test around 1940, staining living embryo tissue red so that dormant or slow lots could be assessed in a day rather than a fortnight. Precision seeding equipment and seed pelleting, which coats small irregular seed such as lettuce and carrot into a uniform sphere, later reduced the need for heavy overseeding and thinning.