Weeds

Weeds – an overview:

This page provides an overview of weeds. The related tools listed at the end of the page provided detailed information about identification and management of weeds.

Impact of weeds:

Weeds compete with agricultural crops for plant nutrients and water and are one of the most significant sources of pests and diseases. Weed control, and the control of pests such as aphids and thrips in weeds near crops, is important for the management of viral diseases.

Effective control of weeds is critical for maximising moisture storage and crop yields, reducing the weed seed bank, and meeting quality standards at harvest. It is very important to prevent weeds from setting seed, as a single parent can produce hundreds to thousands of weeds. Many species also have an inherent dormancy period, so seeds persist in the soil seed-bank. These offspring can be a problem not only in the next season, but for many future seasons.

Identification of weeds:

State government agricultural departments have weed identification guides and information on a selection of weeds, particularly those that are declared noxious or are listed in the ‘Weeds of National Significance’ (WONS) or ‘National Environmental Alert List’. It is important to have weeds identified as some serious weeds are required by law to be controlled by all landholders in an area.

Management:

Weed management and control is an essential component of Integrated Pest Management (IPM) for a range of insect pest and disease problems. It is essential to consider the range of potential host plants for key diseases and their vectors, as well as insect pests.

Weeds can also act as a reservoir for beneficials so consideration must be given to the weeds present and the risks they may be to crop production. Plants or vegetation that are not a host to diseases or insect pests may help support a beneficial population. Weed management can take up a significant proportion of pre-harvest variable costs in vegetable production. In-crop weed management requires a long-term integrated approach. Weed management commences prior to planting of the crop and does not stop until the crop has been harvested and residual produce/weeds are destroyed or cultivated. There are three key steps to effective weed management:

Planning your rotations

Selective crop rotation to minimise weed build up is an important weed management strategy.

Identifying your weeds

Weeds may be broadleaf or grasses, and the potential for management depends on the type of weed present and crop being grown. The key is to understand the weeds which will be a problem in your crop. For more information on the different groups of weeds, click integrated weed management component for vegetables; VIDP: A Guide to Effective Weed Control in Australian Brassicas and http://www.weeds.gov.au/cgi-bin/weedspeciesindex.pl?id=701

Developing your weed management strategy

There are five possible stages in which weed control will be most effective. These are pre-planting, transplanting, early in-crop, late in-crop, and post-harvest, when the paddock is fallow.

Pre-planting

Management is easier and more effective where longer rotations and suitable break crops to reduce weed seed bank are used. Successful weed control commences with transplanting into a weed free seed bed. Best results are obtained using a combination of cultural and chemical controls. But this can be difficult in intensive production systems where crop rotation times are tight.

Transplanting

This is the most critical time for obtaining good in-crop weed control, and is when most available herbicides for use are applied.

Early in-crop

Options for good control are limited if weeds develop in-crop. Broadleaf weeds in most vegetable crops can only be killed by cultivation or by inter-row spraying with a shielded spray unit. An alternative option is to control weeds through the use of directed inter-row spraying with non-selective herbicides using shielded spraying equipment.

Late in-crop

Once a crop is established and canopy closure achieved, it will suppress the growth of most weed species. However, if some weeds grow rapidly and rise above the crop during the last few weeks and interfere with the harvest, hand weeding is the only way to reduce their numbers prior to harvest. While this may be expensive in the short-run, the longer-term benefits of reducing the weed seed bank will be seen in future crops.

Post-harvest weed control

Following harvest, effective weed clean-up and maintenance of a clean fallow between crops is essential. The use of cultivation or non-selective herbicide sprays, or a combination of both will ensure clean fields and easier and more cost-effective weed management in future crops.

Integrated weed management components include:

Cultural methods

include good management practices generally used in the course of production, such as crop rotations, farm hygiene, crop health maintenance, and correct weed identification.

Mechanical and physical methods

are relatively easy, may be inexpensive, and do not create chemical residues. However, they may damage soil structure and promote erosion. Some mechanical and physical methods include cultivation, mowing, and chipping; heat treatment (flaming, steam, or hot water treatments are knockdown options for organic farmers); and mulches (plastic, paper, organic, and living).

Herbicides

are designed to kill either selected weeds or a broad range of plant species. A range of herbicides are currently available for use on broadleaf and grass weeds. Choosing which herbicide to use will depend on which weeds are likely to be a problem, what you plan as the following crop in the rotation, and the types of herbicides used in previous crops. To avoid build-up of resistant weeds, it is important not to continuously use herbicides with the same modes of action. Take careful notice of the registrations and restrictions on different herbicides, and plan so that future rotations are not likely to be adversely affected. A variety of application methods are available, with the most common being sprays or wick-wipers.

Preventing weeds in the greenhouse:

Weeds are one of the most significant sources of insect pests and diseases. There is a very high chance that the insect pests and diseases that affect greenhouse crops have come from weeds in and around the greenhouse. It is very important to keep a weed-free area around the greenhouse, and not even leave weed seedlings. Flowering plants and especially any plant that is in the same family as the crop that is being grown are serious weeds. Grasses are generally a lesser problem although they must be kept short and free from broadleaf and flowering plants.