The wide variety of Tolly’s crops is likely to be a further defence against pests, as no insect that specializes in attacking a particular vegetable can proliferate for long or spread very far. His method also hampers invasive weeds: one scientific review suggests that weeds are reduced by 49 per cent in complex rotations like his.10 Because different crops are sown and harvested at different times, and compete with wild plants in different ways, weed species cannot easily hop from one stage of the rotation to the next.
