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Isolation
To ensure that pollination occurs only among plants of
the desired variety, fields must be isolated either by
distance or flowering time from potentially
contaminating pollen sources.
The isolation required
depends on flower characteristics, sexual compatibility
with neighboring crops, pollen quantity and viability,
mode of pollen dissemination and purity standard
required for specific class of seed.
Self-pollinating
crops such as rice or wheat require relatively small
isolation distances of 0 to 30 feet because their flower
characteristics limit movement of pollen among plants.
Isolation is primarily to prevent mechanical mixtures
during harvesting.
Cross-pollinating crops, on the other
hand, can require distances greater than 1 mile (1.6 km)
of isolation from sexually compatible plants to prevent
out-crossing, depending upon the flower structure, the
mode of pollen transfer, the duration of pollen
viability, and the type of seed being produced (e.g.,
foundation versus certified or hybrid versus
open-pollinated).
Recommended isolation distances may need to be increased
depending upon the economic impact of contamination. For
example, hybrid seed production or production of seeds
in which contamination would be readily apparent (e.g.,
contamination of seeds for white onions with seeds of
red onions) often requires greater isolation to achieve
purity standards expected in the marketplace. Similarly,
greater isolation is required reduce the likelihood that
pollen from outside the field will introduce undesired
traits intro the crop, such as biotech traits into an
organic field.
Isolation can also be achieved by planting crops at
different times such that their flowering periods do not
overlap. Border rows around the field that are not
harvested with the seed crop can also be employed to
intercept stray pollen and reduce contamination of the
remainder of the field. Related weeds and volunteer crop
plants or home gardens can also be sources of pollen
contamination. Certifying agencies inspect fields and
the surrounding areas to ensure that isolation standards
are met.
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