But the risk of it doing very much damage when it does spread far and wide need not be great.
Pollen is spread in one of two ways in a country like Britain - on the wind, or by insects.
How far it will travel by either method depends on what crop it comes from. If you want to have no danger at all of pollination of other crops, then no barrier zone could prevent it. In theory, pollen can travel for hundreds of miles. At 200 metres, it will not be very concentrated. Time also takes its toll on pollen, which gradually loses its potency.