Weed Management

Weed Descriptions

There are approximately 200 plant species that make up the world’s most important crop weeds.

These species are spread over approximately 12 plant families. These 200 odd species have some common characteristics, which make them important weed species. These characteristics include high seed viability, rapid seed emergence, rapid initial plant growth, strong competitors and the production of large numbers of seeds.

Weeds do not fall outside the classification of plants. The major groupings important in weed management, are broad-leaved weeds and grass species. Approximately two-thirds of the important weeds are broad-leaved weeds, the rest are grasses, sedges or ferns.

Weeds can be further classified according to the time it takes for the plant to complete its life cycle. There are three groups of weeds classified according to the time for completion of the lifecycle as Annual weeds, Biennial weeds and Perennial weeds. Approximately two-thirds of this important weed species are annuals, with the rest predominantly perennials with only a small proportion being biennial plants. The lifecycle of a plant is defined as the time from seed emerging to the time of seed production i.e. from seed to seed.

Annual Weeds

Annual weeds complete their life cycle in one growing season, thus in less than one year. Due to their short lifecycle annuals produce large numbers of seeds and grow very rapidly. These plants are sensitive to weed control practices.

Annual weeds can further be divided into two groups. Summer annuals, which germinate in the spring or early summer, grow in the summer, mature and form seeds in the autumn. Summer Annuals complete the cycle seed to seed across two calendar years but in one growing season. Winter annuals germinate in autumn or early winter and mature and form seeds in spring or early summer. So winter annuals complete the cycle in one calendar year and in one growing season. Annuals do not live for time periods exceeding 12 months and reproduce primarily through seed.

Biennial Weeds

Biennial weeds live longer than one year but not longer than two years. Biennial weeds usually have a fleshy taproot system for the storage of food sources. Biennials are not abundant in South Africa.

Perennial Weeds

Perennial weeds reproduce every year through vegetative organs such as rhizomes and stolons, bulbs and tubers, root and stem cuttings and can generally also form seeds. Perennials can be sub-divided into two groups; namely simple perennials and creeping perennials. Simple perennials reproduce only by means of seed. Should the shoot of a simple perennial be damaged, new buds can form. Creeping perennials reproduce through seed and vegetative parts. Vegetative reproduction usually occurs through creeping surface roots or stolons, underground stems or rhizomes, tubers and bulbs.

Weeds are generally known by common names that may be unique to regions of the country. There are approximately 5 species of weeds that are all known as the “Khaki-bos”. Within the scientific community, this problem of confusing species is overcome by specific scientific, Latin names. It is useful to learn the Latin names of weeds rather than common names, in order to avoid the potential confusion of species.

Weed Control

Weed management can be done by applying various methods:

Preventative Weed Management

Most weeds produce large amounts of seeds per growing season that build up as a ‘seed bank’ in the soil. The seed of many weed species also has complicated dormancy behaviour with the result that all the seeds do not germinate when conditions are favourable. Weed seed produced in the same season can therefore germinate over an extended period of several years. It is therefore important to prevent weeds from producing seeds. Suitable cultivation techniques should be applied to ensure weeds are controlled early before they become competitive. The use of mulch is effective in preventing seed germination and weed growth. Pruned leaves and branches, straw, bark mulch, or black plastic will control weed growth considerably. However, you should ensure that especially pruned leaves and branches are well composted before applying as mulch, as these could carry diseases that could infect the crop.

Biological Control

Some insects and animals can be used in certain crops to keep weeds under control.

Mechanical Weed Control: In fruit orchards, weeds between rows are slashed by means of hand-lashers or machine lashers pulled by tractors. In agronomic crops, weeds between rows can be removed using specialised machinery. Weed slashing involves walking along the rows of the crop, slashing the weeds with a hoe, leaving the weeds on the ground as mulch.

Chemical Control: A very effective way of controlling weeds is by using chemicals known as herbicides. A herbicide is a chemical specifically designed to control killing plants. Herbicides are developed with specific modes of action. In broad terms, there are two main groups of weeds, broad-leaved weeds and grasses. A herbicide like ‘Roundup’ will kill both narrow as well as broadleaf weeds, including annual crops.

When using herbicides to kill weeds, care must be taken not to accidentally use a broadleaf herbicide on a broadleaf crop or vice versa. Herbicides can be highly poisonous and must be used very carefully, and only with competent handlers and equipment.

Remember: It is impossible to remove all weeds from a crop as new infestations occur regularly. It is best to keep a weed problem under control, trying to remove weeds before they get the chance to form seeds.