Weeds and Pests

Organic agriculture is a holistic approach built upon natural ecosystems which include insects, wildlife, diverse plant life and the biological activities of organisms living within the soil and other parts of the growing environment.

Using good organic practices will encourage healthy plants that are resistant to pests and diseases and a diversity of plants and varieties suited to the climate and soil will encourage a growing environment that promotes beneficial insects, wildlife and microorganisms.

There will always be pests in the garden.  Of all the huge numbers of living organisms in the system, only a very small number of species cause problems.  In fact, most are beneficial and some essential to the health of many plants.  Organic gardening techniques are key to prevention – by maintaining the ecological balance of the garden so that pests and diseases are kept in check by their natural enemies and the healthy soil food web that facilitates nutrition of plants and their natural defence mechanisms to pests and diseases.

More information about Beneficial Insects.

Organic controls

If a problem with a pest, disease or weed arises then an organic gardener will use targeted and sustainable strategies to deal with it based on knowledge of the pest, disease or weed and its life-cycle and minimise any adverse impact on the growing environment.  Here is a list of some useful techniques that are aimed at prevention or targeted treatment of problem pests, diseases or weeds: 

Weeds

  • Take the time to clear perennial weeds before planting. some of these weeds are noxious (eg

    Capeweed (Arctotheca calendula) Photo: Agriculture Victoria; a favourite host of the Red-legged earth mite (Halotydeus destructor) – size 0.5mm

    nutgrass and oxalis) and will need persistent effort.
  • Weeds are opportunistic and prolific seed producers. Don’t encourage invasion or regrowth by leaving soil bare or by excessive tillage.  Green manure planting in a crop rotation cycle helps prevent this exposure to weed infestation or regrowth.
  • Mulches can help prevent weeds and provide other benefits in organic soil management.  Close spacing and intercropping can also prevent germination and growth of weeds. 
  • Some weeds can be good in the garden; for example, by attracting beneficial predatory insectsand pollinators.  However be aware of the weeds that are likely hosts to serious pests (eg cape weed and nightshade) and diseases.  Keep them under control and well away from food plants.
  • While some herbicides may be permitted under the Australian Certified Organic standards these are subject to restrictions and should only be used as a last resort when other management practices are not effective.  These chemicals can still have an adverse impact on soil biology.

COGS has a recommended approach to managing weeds in its community gardens outlined in this paper.

Pests and diseases

  • Grow a diversity of plants to provide food, shelter and habitats for predator insects and mites and other wildlife (eg birds).  Choose heirloom varieties suited to the climate and with known resistance to pests and disease.
  • Use a crop rotation method over a 3 to 4 year period to avoid reinfestation of pests and pathogenic or parasitic fungi and other pest organisms that live in the soil.  A healthy soil food web with beneficial bacteria and fungi will help resistance to these organisms.
  • Check your garden regularly and at different times of the day.  Just because pests are present it doesn’t necessarily mean they are a problem.
  • Clean up your garden at the end of the season and compost organic waste.  This material can harbour pathogens and other pests (eg nematodes and wilt) that can contaminate new season plantings.  Most of this material can be composted safely. Keep your garden tools clean.
  • Growing a variety of plants together (companion planting) can attract beneficial insects and deter pests because the different odours that they give off confuse these pests. Herbs and some flowers have a strong deterrent effect; this includes basil, sage, hyssop, garlic, nasturtium and French marigolds (tagetes patula).  Try different combinations of herbs and flowers in your vegetable garden and see what works.
  • Physical controls can be highly effective when pest numbers are low.  This includes shaking plants, hand picking or using a jet of water from a hose (which works well on aphids). A zone of fine, sharp or gritty material such as sand or diatomaceous earth (food grade) can deter slugs, snails and slaters from seedlings – but less so in wet weather.
  • Slugs and snails can be a serious problem for seedlings and plants with tender leaves (eg lettuce).  A combination of collection (eg after rain and in cool shaded places), baiting with Comfrey leaves and traps should keep them under control.  Trap them in small containers filled with a 50/50 mixture of stale bee and water.  Wink these into the soil in strategic spots near seedlings.
  • Some gardeners use exclusion nets over their vegetable plots or fruit to prevent access by flying pests.  These nets are organically acceptable providing they are fine grade with less than 5mm spacing and stretched taut and pinned to the ground to prevent wildlife from being tangled or trapped.   (For more information on wildlife friendly netting) Unfortunately some fine grade nets or shade cloth over the plants also exclude pollinators and other beneficial insects, so these devices should only be used as a last resort.
  • Commercial sticky bands placed around the trunks of fruit trees or other traps designed to attract and trap insects can be used if organically certified and do not contain pesticide.
  • When the weather is hot and humid, especially late in summer, tomatoes and cucurbits may get infected with a range of fungal diseases; eg in tomatoes, Fusarium Wilt or (rarely) late blight; and in cucurbits, powdery mildew.  As with other pests and diseases, reading the signs early and a correct diagnosis should guide the response. Powdery mildew and Fusarium wilt only weaken a plant and affect production but blight is fatal and in this latter case the plant should be removed entirely and destroyed.
  • An organic bacterial pesticide, Bacillus Thuringiensis (Bt), can be very effective against cabbage moth butterfly caterpillars (and some others).  Consider its use on young brassica seedlings and other susceptible young plants but larger plants in an organic garden with plenty of wildlife predators (including birds) should not need this help.  Some brassicas have good natural resistance to this pest.
  • There are some organically acceptable sprays that can deal effectively with pests through physical action (eg plant based oils or starch that suffocate).  While less harmful than most insecticides they are still toxic and can still disrupt the natural ecosystem in your garden and harm beneficial insects.

Broad spectrum sprays (those that kill a large number of organisms), such as pyrethrum, are particularly destructive of the ecological balance if used indiscriminately.  Such sprays also destroy the natural predators and parasites of our pests.  Some pests always survive any spray program and those survivors can rapidly multiply, unchecked by their natural enemies.

The problem of creating ecological imbalance by use of aggressive chemical methods of pest control in crops is well known and such methods have no place in organic food production.  Integrated Pest Management (IPM) provides an effective alternative.  It applies techniques in common use by any good organic gardener, but its advantage is that it applies those techniques in a systematic and integrated manner.  It aims to maintain or enhance rather than destroy the ecological balance of living organisms within our soils and plants. The attached paper describes the main features of Integrated Pest Management   While it was developed for use on pests and disease of plants, the approach can be broadly applied to managing weeds in our gardens.

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