John Campbell, of Christchurch, asks :-

When belatedly weeding the garden on a very hot day, I received a surprise when one weed type I touched threw seeds several tens of centimetres into the air. Is this a standard seed dispersal mechanism?

George Hill, an agronomist at Lincoln University's Department of Agricultural Sciences, responded.

Plants have their greatest chance for germinating and surviving when their seed drops at some distance from the parent plant.

There are a number of mechanisms involved in seed dispersal, including wind, water, animals, and dehiscence.

Plants of the Compositae - (the daisy family) such as in thistles and dandelions have a parachute effect via the pappus which has a little tuft of fluffy feather like structures attached to the seed by a stalk. Sycamore and silver birch have developed seeds with helicopter-like wings, which aid long-distance dispersal. Pine seed is also dispersed downwind thus the presence of wilding pines. Some grasses develop fluffy seeds and a brittle rachis. As the wind blows the rachis breaks into small segments and the seed is carried on the wind.. In Canterbury, in particular, large numbers of weed seeds are carried by North West winds in late summer. Plants with seed modified for wind carriage are difficult to keep off other properties and can be carried considerable distances.

Some plants produces seed which floats on water. They usually develop a thick buoyant seed coat - a coconut is an extreme example. However, dock (which is common weed in wet areas in New Zealand) develops seeds which are enclosed in cork and thus will float in streams, irrigation canals and water races.

Seed can be adapted to be carried outside animals in species such as medics (burrs), stork’s bill, cleavers and barley grass where the seeds become attached to the pelt or are carried in the hooves of domestic livestock. Other plants have fleshy fruits that are eaten by animal and are then carried some distance before being dropped in faeces. In Canterbury it is very common to find elderberry seedlings growing under trees in domestic gardens after their seed has been dropped by birds. Seed of many plant species can pass undamaged through an animal and can still germinate. In some species such as fleshy fruits they are specifically able to do this. The animal deposits the seed ready to germinate complete with manure at some distance from the parent plant. Wild animals such as rodents store seed for the winter. Uneaten seed germinates in the spring. Insects also collect and store seed in their nests which, if uneaten, can germinate in the following season.

In legumes the reproductive structure is the pod. As the pods dry out in the summer the fibres present in the pod valves dry out and shorten. At maturity when the pods are dry the pods shatter and twist and can throw seed a considerable distance from the parent plant (dehiscence). We have found lupins growing at nine metres from where the nearest parent plants were in the previous season. Gorse, a very common weedy legume in New Zealand uses this method to spread from the edges of clumps. There is a similar situation in Brassica plants such as wild turnip, and wild radish have a two-valve capsule which also splits explosively when dry.