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Organic control of weeds, plant pests and diseases

What is a weed?

It’s simply a plant out of place in the opinion of the farmer. But remember that weeds can be valuable natural indicators.

Why are weeds growing here?

This has got to be your first question, and finding an answer could save you a lot of time, frustration and money. Weeds can be great indicators of soil conditions and poor drainage.

What damage are they doing?

Farmers always assume weeds are doing great damage. They may be - by using up nutrients that could be used for animal production, and smothering important crop plants.

Weeds add variety to diets

Weeds provide variety to diets for stock. Goats seek out weeds in their diet and when well fenced can be used very effectively to clean up a weed problem.

Control Methods
Mechanical control

There is a range of machines on the market for mechanical weeding in arable crops. Timing operation and weather are critical to get an effective kill. Good cultivation prior to planting is important.


In small situations such as around trees, you can use heavy mulches to prevent weed seedlings getting light. Mulch that comes in close contact with the tree trunk can cause fungal infections.


Rotations can be used to prevent weeds building up and are also the basis of all good husbandry. Weeds are often an indication that management procedures need attention.

Cover crops

This is where the seeds of one crop are sown into an existing crop. This saves time and prevents a lot of weed build up on the bare ground in the existing crop. An example is where grass and clover seeds are sown into a cereal crop when it is about 150mm high. When the grain is harvested the new pasture is well established and ready to grow.


This is used in Biodynamic systems where the weed seed heads are collected and burned, and the ash returned to the soil. For full information, see the Bio-Dynamic Association.

Specific Weeds

Gorse is a legume so fixes atmospheric nitrogen. Cutting mature plants is effective but the ground below the plant will yield masses of new seedlings once the old plant has gone. Don’t burn the dead plant no matter how tempting it is! It will only encourage germination of new seeds. The ground must not be left bare - use some old hay as mulch.

Graze any new seedlings with stock such as goats or grub them out. Liming will help to slow up growth and there is gorse grub beetle, which is having a reasonable impact. Biological control methods are being investigated.


Chopping, grubbing, slashing and liming will help. Goats will eat new shoots and reduce the plant over time.


Continual cutting or hard grazing with mature stock will exhaust root reserves. Drainage and liming will help as they prefer wet acid areas.


These come in usually after dry spells when grasses die off and bare ground is exposed. Avoid overgrazing to expose bare ground.

Californian thistles grow in great patches joined together by masses of rhizomes or underground roots. Often a whole bed will be only one plant. Cut in December when at the peak of their growth. The new laterals will come up a month later. Keep grazing the new regrowth (preferably with sheep or goats) until the plant’s food store is exhausted. Keep repeating this treatment.

Scotch thistles, nodding thistles, winged thistles that grow as individual plants. They can be grubbed or cut when at their maximum size before they flower to exhaust the root reserves.

Biological control methods are being investigated.


The young plant is most toxic, although dead plants in hay can still be poisonous. Plants can be grubbed just before they flower to exhaust root reserves. Sheep will eat ragwort with no ill effects. The ragwort Cinola moth is proving to be effective in control.


Stock will eat buttercup so grazing is one way of trying to exhaust the root reserves of the plant.

Barley grass

Can be controlled by early grazing and encouraging competition from other species as well as lifting fertility.

Couch grass

Regular cultivation to bring roots to the surface for the sun to kill them is ideal.

Chickweed/ Redroot/ Storksbill/ Black Nightshade

These are common weeds on dairy farms and respond well to high fertility. They come in when pastures open up and are grazed laxly by dairy cows. Avoid pugging in wet conditions and retain pasture covers in dry periods.

Pasture pests

These are grass grub, porina beetle, sod web worm and black beetle. Their larvae (caterpillars) all eat the roots of grasses effectively cutting the plant’s food supply off.

Stock treading and mechanical damage by consolidation can help when the larvae are at their most vulnerable in autumn and near the surface. Encouraging birds such as starlings and magpies many help although this will cause other challenges.


These are mainly fungal diseases such as ryegrass rust and mineral deficiencies. Developing healthy soils will help solve these challenges.

Information provided by:
Mr Denis Cadwallader, Organic Farming Specialist. 22 May Avenue, Napier, New Zealand
Phone (06) 834-3405, Fax (06) 834-3406, Mobile 025-481-782, Email This email address is being protected from spambots. You need JavaScript enabled to view it.

Mr Cadwallader is guest tutor in Organic Farming at the Waikato Polytechnic.
Phone (07) 834-8806 for further information on courses.