Weed Seed Dispersal Mechanisms

Weeds reproduce and spread by many methods and may have special adaptions to assist their dispersal. Vegetable integrated pest management weeds view page

Weed Seed Dispersal Mechanisms

Weeds reproduce and spread by many methods and may have special adaptions to assist their dispersal. These include seeds, spores, runners and separated root and shoot fragments. Nature plays a big part in spreading weeds over small distances in wind and water. Humans, unfortunately, are by far the worst offenders at spreading weeds, particularly on dirty tools, machinery, vehicles, clothing and transported animals. Some of the more common methods of weed spread and methods used to prevent this are discussed as follows:

Stock Feed – Contamination of hay and grains with wet seeds is one of the most common means by which weeds are spread. Feeding animals in a confined area or in one paddock reduces the risk of weeds invading the rest of the property.

Stock – Weed seeds ingested by stock can remain viable after passing through the digestive tract. New stock should therefore be confined to one paddock for a week after arrival. This allows time for any viable seeds that have been ingested by the stock to be expelled. Seeds which are sticky or spiny can spread on the animals, for example in sheep fleece.

Machinery – After using machinery in weed-infested areas ensure they are thoroughly cleaned. Weed seed can be transported in tyres and in other road materials.

Soil Disturbance – Minimise the amount of soil and vegetation disturbance when carrying out work. Disturbed ground creates an ideal seed bed for both existing and introduced weed seeds to germinate.

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Humans And Animals – Check your own clothing, socks, cuffs, jumpers, boots etc. after walking through weed-infested areas. Remove and destroy and seeds you find. Dogs and cats can also disturb seeds in their coats, as can wild animals, particularly vermin such as foxes and rabbits. Birds also transport seeds when they feed on wet fruits and seeds such as blackberry and cotoneaster.

Garden Escapees – Many weeds were introduced to Australia as ornamental plants or for herbal medicine, which have since “escaped” from our gardens and become wild. For example – pampas grass, broom, Spanish heath and cotoneaster. It is best to avoid “weedy” species when choosing plants for your garden.

Water And Wind – Wet seeds entering waterways or drains can be spread to new areas downstream. On windy days when plants are seeding, the wind can easily disperse the seed quite some distance. Many weed species have seeds especially adapted to be carried by the wind.

Explosive Projection – Many weeds such as gorse disperse their seeds through explosive projection. The seeds are encased in pods, which can be thrown up to several metres from the parent bush.

Weed Seed Dispersal Mechanisms

The tolerance for weeds in vegetable crops grown in this area is very low and some fields are kept almost weed free. It is hard to understand why so any weeds appear the next season in fields that are kept weed free. There is no simple answer to this question. Part of the reason lies in the fact that weed seed, unlike crop seed, does not germinate all at the same time. Some may germinate immediately and some may not germinate for years. Another reason is that weeds produce millions of seeds and these move from field to field. In an area like this where irrigation is so intensive, water is moving into and out of fields all of the time. Most weed seeds are very small and float. There is just no practical way to filter small seeds out of large volumes of water. Filters that are capable of removing small seeds restrict water flow too much. Water would have to be diverted from the ditch or canal to successfully filter the seed. After water, wind is probably the next major means by which weed seed is dispersed from field to field. Some weeds have developed structural characteristics that allow them to blow long distances.

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The composites like sowthistle, groundsel, prickly lettuce and others, have fuzzy seed heads that blow in the wind. Some seeds have spinners, gliders and other structures that help them travel. All of the tillage, cultivation and spraying we do provide a mechanism for weed seeds to be moved mechanically. Sandbur, puncturevine, clovers and other seeds are hard to keep off equipment, boots and clothes and are difficult to control. Birds can disperse weed seed, tubers and bulbs. Some eat and defecate seeds, fruit or pond sediment that can be deposited in other fields. Seeds can attach feathers, claws and beaks. The physical damage that birds do to lettuce and Cole crops is far worse than spreading weed seeds. They pull young seedlings out of the ground and cause direct crop loss.