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Onion
Density
Onions are a species that are highly influenced by their
plant density, particularly in terms of their shape and
size. Onions that are grown at above optimal density,
may be misshapen, may not grow to their full size
potential and will mature later than expected. Onions
that are grown at low densities, can exhibit an increased
percentage of visible doubles and give a reduced yield.
In general, 80 bulbs per m2 is a reasonable target,
although the optimal density for any particular
circumstance depends on the variety, season and
location. Here's a way that you can calculate your final
onion plant stand, given variable germination, bed width
and seed spacing.
Onion plant density(per m2) =
(Seed Label germination % x 9) x number rows per
bed
------------------------------------------------------------------------------(actual
bed width (m) x in row seed distance (mm))
Notes on the equation:
 Seed label germination - straight from the pack.
9 - a field factor, assuming 90% of seeds that are
sown, survive, remembering that label germination
results are from tests carried out in optimal
germination conditions.
Number of rows per bed - four double rows, would
be entered as eight rows and so on.
Bed width - measure the width of the top of the bed,
in metres. This is not centre to centre, but the actual
width of the bed.
In row seed distance - distance between seed in each
individual row, in millimeters.
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For example, if you had an onion variety with a
germination of 85%, sown in four double rows, on a
1.5 m wide bed with an in-row spacing of 50 mm, your
final standing plant density would be 81.6 bulbs per m2,
calculated as follows:
Onion plant density(per m2) =
(85% germination x 9) x 8 (four double rows)
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(1.5 m wide bed x 50 mm between seeds)
To get number of bulbs per hectare, simply change the
bed width to centre to centre width and multiply the
result by 10 000. As a check, if the centres bed width
was 1.8m, the result from the figures above would be
680 000 bulbs per hectare.
by: Bruno Tigani
Grower
Solutions Magazine
Lefroy
Valley Magazine
June
2003
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