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Weed Control

The general definition of a weed is a plant growing anywhere it isn’t wanted. Weeds are some of the most persistent and determined plants in nature, often surviving in the most barren of soils. There is no denying that some weeds have beneficial properties, but for the home gardener they can cause no end of problems. Weeds compete for valuable nutrients, light and moisture and can harbour a wide range of pest and diseases that spread throughout the garden.

Weeds germinate from seeds that arrive in your garden in various ways, including sticking to your shoes, attached to your clothes and carried in by birds or the wind. Often just cultivating the soil in your garden can bring seeds to the surface, where they will germinate, even if they have been dormant for years.

Weeds, like plants, are classified in to groups and although many of these are relatively harmless, many fall under the category of noxious weeds. Noxious weeds endanger not only our native forest, but also pasture and precious wetlands.


Annual weeds will grow, flower and produce seed within a season or even a few weeks. Annual weeds are best controlled when they are small before they have a chance to produce seed. Removing by hand or the use of a weed killer is effective on these weeds.

The more common annual weeds are Sorrel, Scarlet Pimpernel, Cress, Nettle and Fat Hen.


These weeds are tougher and harder to get under control. They can multiply by seed and by root division. These weeds have a taproot, this is one main root that supports the plant, complete removal of this root is needed to prevent re growth.

Removing by hand or the use of weed killers is effective on these weeds. If using a weed killer often repeated applications are necessary, especially on weeds like Oxalis and Onion weed, as they have multiple bulbs that will lie dormant. Weed killer needs active green growth to work effectively.

Weeds that fall under this category are Buttercup, Thistle, Dock, Ink weed, Oxalis and Stinging Nettle.

Woody weeds

These plants grow into medium to large shrubs and can strangle the positive plantings in your garden. Removing by hand or the use of weed killer is effective on these weeds. If you are going to use weed killer, it is better and quicker to first remove all the top green growth. Wait for the reappearance of fresh growth and then spray. Once no new growth appears you can remove the roots.

In this category you will find Gorse, Woolly Nightshade, Tree Lupin, Castor Oil Plant and Poroporo.


Getting rid of climbers once they have taken hold comes down to serious hard work and a smattering of elbow grease. Spraying these weeds is often prohibitive because of the sheer spread of the climber and the danger of killing plants that you do want. Removal needs a managed approach and it is important to remember that what makes these climbers tenacious is their ability to grow from a small root left in the ground. An instant solution is hard to find, you may need to keep up the removal of smaller new growth on a regular basis.

  • Find the start of the plant; in some case they will have runners that have formed new vines.
  • Cut all these vines off at the base, this will cause the tops to die away.
  • Wait until the tops have dried off before trying to pull them down, this will make removing them much easier and minimise the damage on the host trees.
  • The stumps will try to re grow, so you can either;

    a. Remove the altogether by digging them up, or

    b. Wait for new growth to appear then apply weed killer.

Plants that fall under this category are Blackberry, Smilax, Morning Glory, Old Man’s Beard, Banana Passion Fruit, Climbing Dock, Honeysuckle and Convolvulus.

Weeds in the lawn

Here are some handy tips for weeds in you lawn:

  • Mowing over dock and dandelion will not remove them as they continue to re grow from a long taproot, you will need to spray of dig out the plant by the root.
  • If you have a large-scale invasion of these broadleaf weeds than the best control is to use a broadleaf weed killer.
  • Onehunga weed, which produces an annoying prickle, is best controlled with a weed killer, typically applied in September through to October when the weed is in flower.

Weeds in the vegetable garden

Keeping the soil in your vegetable garden weed free is an absolute must:

  • By regularly hoeing through of your soil in the garden, you will lift smaller weeds out by the roots.
  • If you are leaving your vegetable garden empty for a season, then plant a cover crop like lupins or mustard. These crops can be dug through the soil and will smother any weeds tempted to make an appearance.

Simple methods of weed control

Mulching - Mulch can be use to smother existing weeds or stop any new ones from emerging. Mulch can come in many organic forms such as straw, pea straw, newspaper and an organic weed mat. Make sure if you are using organic materials that you apply thickly and apply all forms of mulch over bare soil. Organic mulch also has the added benefit of adding nutrients to the soil.

Competition - Simply out plant weeds, with desirable shrubs and flowers. Tilting the competition in your favour will crowd out light and space, giving the weeds less of a chance of taking hold.

Removing flower heads - At the very least, if you remove the weeds flowers by pruning or pulling them off, you will stop the plant from producing seed and starting the cycle all over again.


Annual weeds, like any green growth, are beneficial when added to the compost bin, but there are a few things that you need to take in to consideration. High temperatures within a well-constructed compost heap are needed to kill off many seeds, so if you are not sure that you are achieving this, leave out any seed heads from weeds.

Leave out the more woody stems or vines unless you are going to shred or chop them up before they are added to the compost bin. Also leave out any weeds that have been sprayed with chemicals.

Weed killers

It is important to remember that the war against weeds is best approached on many fronts and that different control methods are needed for different weeds.

Weed killer’s work in various ways, but on the whole weed killer’s that are available for the home need active green growth. They need this growth so that they can be absorbed in to the plants system for effective eradication.

Effective weed spraying

  • Spraying in windy conditions is wasteful and ineffective. The wind can cause spray to drift on to plants you don’t want to kill and give you spotty coverage on the plants you do.
  • Weed killers are not improved by doubling the amount of concentrate, stick to the directions.
  • If it has been raining, wait at least 5 hours before spraying, as the rain will reduce the effectiveness of the product used.

Spraying safety

  • ALWAYS follow the manufactures instructions on mixing and use of the chemicals as they have worked out what you need based on good and responsible practice.
  • Inventing your own brew by mixing chemicals together is not a great idea and should be avoided.
  • Dispose of the empty containers as per the manufacturers instructions.
  • If the name of the chemical on the container is illegible, dispose of the chemical as it has probably become unstable.
  • Old chemicals should not be used, like most things; they have a shelf life and will become ineffective with age.
  • Keep a set of mixing and measuring tools just for mixing chemicals, please store them safely away from children.
  • Wear protective clothing, PVC gloves and a respiratory mask when spraying and mixing chemicals.
Keep out of the reach of children
If poisoning occurs follow the first aid instructions on the bottle and seek medical attention
National poison centre number for 24 hour poisons advice 0800 764 766

How to maintain your sprayer

  • Remove and clean out the nozzle after each use, as this is where debris will get stuck.
  • Leaving unused chemicals in a container creates a health hazard – dispose of them immediately.
  • Wash the sprayer thoroughly with cold water and detergent, ensuring that you run the water right through the spray lance as well.
  • If your sprayer has a filter, regularly check and clean.
  • Keep separate sprayers, one for your weed killers and one for insecticide – clearly mark them.

Spraying weeds and the use of weed killers allow you to maintain your gardens health and promote positive plantings to flourish. Be careful and responsible with weed killers and treat them with the respect they deserve. Always wear protective gear, including respirators and gloves.