Lesson 5 - Weeds
We could spend a bit of time discussing what it is that makes a plant a weed but at the end of the day, a weed is a plant that you define as undesirable. The reasons for undesirability are various, but the main one is competition with the crop. Weed plants growing around your crop can compete for water, nutrients, and space. Uncontrolled weed growth can severely reduce productivity. Other potential issues with unwanted plants can be hosting of pest insects, or pest animals, or acting as alternative hosts of plant disease. While any plant can be a weed depending on circumstance, in crop production the same plants tend to turn up in weed lists. Weed species can be classified in various ways. They can be annuals or biennials, always regenerating from seed, or perennials, often with underground reproductive organs like bulbs or corms, rhizomes, or stolons. Approaches to control may vary depending on the type of plant causing your weed problem.
Site preparation
Virtually any soil that you might consider using for growing crops will come with a seed bank of potential weed species. These seeds are from previous generations of weed species growing there, and many of them have lifespans in the soil of years. Clearing a bed for a planting will stimulate a round of germination, as will any subsequent clearing, even in the unlikely situation that you have not allowed any new weed seeds to set. Over time, without new seed set, you could anticipate that the seed bank will be reduced, but it will take years.
A possibly more important issue to consider is where there are perennial weeds growing in your proposed bed(s). The most serious of these are plants that can regenerate from cut roots (e.g. comfrey, blackberry, horseradish, dock), from rhizome or stolon pieces (e.g. johnson grass, kikuyu grass, couch grass, twitch grass), or from bulbs and corms (e.g. nutgrass, oxalis, Italian arum, watsonia). Such plants can be very difficult to remove mechanically because of the problems of regeneration from small pieces. Where weed species like this are present you could consider the use of impermeable mulch materials like degradable jute matting, plastic sheet or weed mat (will have to be removed at some point), carpet (with no synthetic component, or layers of cardboard. The herbicide glyphosate could be used in this situation as part of a pre-treatment process if that fits your philosophy of growing.
For perennial plantings (perennial vegetables, fruit trees) removal of serious perennial weeds before planting is vital because it is difficult to remove them from established plantings. Take time to do this work carefully, and aim to maintain a weed free area around each planting. Grasses in particular are serious competitors and during establishment aim to keep an area of about one square metre around each tree free of competing vegetation. Mowing isn’t enough.
Controls and management strategies
Reducing weed competition usually involves removal of the plant(s) in question so that your crop plant is the dominant component of the vegetation in the bed. The table below lists various approaches to weed management and identifies circumstances where they are most useful.
| Approaches to weed management | ||
| Control method | Target plant types | Notes |
| hygiene | All plant types. | Try to prevent seed set by timely removal of weeds, don’t import weed contaminated soil, compost, or mulch |
| physical removal by: | ||
| -hand weeding | Most useful against young plants, especially annual/biennial. | Time consuming but often essential with young plants to avoid mechanical damage from tools. |
| -hoeing | Most useful against young plants, especially annual/biennial. | Effective and quick, especially if the surface soil is dry. |
| -digging (spade or fork) | Most useful against young plants, especially annual/biennial but can remove larger plants. | Best used in bed preparation. Will cause root damage if used near established crops. |
| -mechanical cultivation (e.g. rotary hoe) | Most useful against young plants, especially annual/biennial. | Repeated use is very harmful to soil structure and soil health. Very effective at spreading perennial weeds that regenerate from cut pieces (roots, rhizomes/stolons, bulbs) |
| mulching | All weed types but most useful to prevent germination of seedlings. | Established weeds can grow through mulch, especially granular types. Difficult perennial weeds may need impermeable mulches like fabric for suppression. (Figure 13) |
| crop rotation | All weed types, but especially annual/biennial types. | Prevents weeds that are associated with particular crops or cultivation regimes from becoming established. |
| cover crops | All weed types, but especially annual/biennial types. | Can help interrupt growth and seed set of weeds by competition, but cover crops are also subject to weed competition in their own right. |
| plant coverage | All weed types, but especially annual/biennial types. | Using planting densities that result in total soil coverage by the crop will assist in reducing weed control in maturing crops but the crop will still need early weed control at establishment. (Figure 14) |
| herbicides | All weed types, but especially annual/biennial types. Generally only useful for cleaning up planting areas before cropping. Herbicide use in growing crops may not be feasible or desirable. | In the non-commercial sphere, access to herbicides is quite limited and generally only non-selective knock down types are available (e.g. glyphosate, nonanoic acid). Do not use broad leaf herbicides (2,4 _D, MCPA, dicamba) in or near crop plants. |
| mowing/slashing | All types, if access for machinery is possible. | Only removes plant tops and does not kills weeds, does not reduce competition from grass swards. Useful for reducing plant cover to facilitate the use of other methods such as cultivation or the use of herbicides. |
Further learning
- Weeds Australia
- Weeds of the South-East: an indentification guide for Australia by FJ Richardson, RG Richardson and RCH Shepherd (RG ad FJ Richardson, Third edition, 2016)
- Western Weeds: A guide to weeks of Western Australia by BMJ Hussey, GJ Keighery, J Dodd, SG Lloyd and RD Cousens (The Weeds Society of Western Australia, second edition, 2007)
- Weeds of Central and North Queensland (Weed Society of Queensland, 2019)