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Wally Richards
Wally Richardshttp://www.gardenews.co.nz
Wally Richards has been a gardening columnist for over 30 years. Check his websites - for gardening news and tips visit www.gardenews.co.nz. For mail order products visit www.0800466464.co.nz. Wally also has a gardening problem help line on 0800 466 464.

Gardening with Wally Richards: Alternative weed controls

Weed control news

Chemical herbicides aren’t the only option available.

There are a number of other products which are handy weedkillers but which don’t do as much damage as do the chemical herbicide products.

Oils added to water and sprayed over the foliage of plants in certain conditions will dehydrate or bleach the foliage, destroying everything growing above the ground.

Thus any cheap cooking oil mixed with water adding the same amount of dish washing liquid as oil then sprayed over weeds on a hot sunny day when the soil is on the dry side, the foliage of the sprayed plants will begin wilting very quickly – within minutes even.

If it is applied in cooler weather, or when the soil is moist, the killing action will take longer, and might not even work at all.

Ratio to use will depend on several factors you could start off say 100 mils of used cooking oil with 100mils of dish washing liquid into a litre of water.

That may dehydrate some weeds and other weeds may need a stronger solution. Of course you can spray it undiluted for maximum effect.

Plants are at their most vulnerable in sunlight on hot sunny days when moisture levels in the soil are low.

It is then that the roots of the plants will be gathering moisture as fast as possible to send upwards to replace the moisture lost through leaf transpiration.

When water is being lost from the leaves faster than it can be replaced, plants will be seen to wilt or droop.

This is most noticeable on hot sunny days, when leaves wilt during the day but come right as the sun goes down and the moisture level of the plant’s cells is replaced.

Foliage that remains starved of moisture for too long will dry out and is unlikely to recover.

You might notice that only parts of a leaf will be affected, perhaps only the tip or the edges. When oil is applied to the foliage in these conditions, all the leaves and stems are likely to wither and die, effectively killing the plant’s foliage.

If the plant is an annual, this will deprive the root system of energy, and it too will wither and die.

If the plant is a perennial, it will have the ability to send up new foliage from it roots, tuber or bulb, and it may well survive.

But if we keep spraying new foliage as it appears, doing the work in ideal hot sunny conditions, the roots or the bulb will eventually run out of energy and fail completely, causing the plant to finally die.

By adopting this method, we’re simply applying the basic principle that no plant can survive indefinitely without foliage as it loses its ability to gather energy from the sun.

If we simply keep cutting the foliage at ground level (as soon after it appears as possible), the plant will eventually die. The well-known Dutch Hoe, with its sharp edges, was designed for just this purpose.

Used against weed seedlings, the hoe would be placed just under the surface of the soil and then pushed forwards so that its sharp edges sliced off the weeds just under the surface, killing annual weeds and knocking back perennial weeds.

The fallen foliage is left on the soil to be re-absorbed back into the soil.

The latter may need a further treatment or two using the same method to finally finish them off.

If the hoe is used when the weeds are very young, it will successfully kill both annual and perennial weeds which have grown from seed. The older and more established the perennials, the more treatments will be required to get rid of them.

There are several common household products which can be used for non-selective weed killing, including any salad or cooking oils, vinegar and salt plus bleaches and acids.

(I spoke to a old farmer recently who told me they used to use sulfuric acid to remove the potato crop foliage prior to harvesting the tubers. Said it worked very quickly but could not remember the strength of the solution)

You need only to experiment a little with various dilution rates to see what works best for you.

Good old common table salt, purchased in bulk or in kilo bags, is probably the cheapest natural weedkiller available to everyone.

Use it at the rate of 240 grams (about 12 heaped tablespoons) to a litre of warm or hot water to dissolve it, and then spray it on the foliage of the weeds, again in sunny dry conditions.

Then stand back and watch the plants shrivel over time. Annuals will be seen going off quite fast, with perennials probably needing further salt treatments.

If you are finding that you are not getting the kill coverage you want, then increase the amount of salt to about 500 grams to a litre of water.

You will find that a little trial and error will indicate the level of salt needed to do the job well, without using too much. Applying extra amounts of salt over an extended period of time will eventually harm the soil.

If you are spraying only the foliage, and at the above rates, there will be, little residual damage done to the soil.

To make absolutely sure, give the area a good watering after the weeds have died. Adding Raingard to the salty water at a rate of 1ml per litre of water should also help the salt adhere to the surface of the foliage.

Salt can also be applied dry on cobbles to kill weeds in joints or cracks.

Another home product which is useful is white vinegar. It is made from ascetic acid which, in its undiluted state, can be quite dangerous to play with if you don’t know what you are doing.

Only small amounts of ascetic acid, probably about 100ml to 1 litre of water, are needed to make a strong white vinegar.

If I remember correctly, it becomes a good weedkiller if diluted to the rate of about 15-20% acid. Salt is certainly much safer to use, and can be purchased cheaply in bulk.

Many years ago, and probably still today in some places, many people did their own oil changes on their vehicles, primarily to save on the cost of having a garage do it.

The old oil from the sump would then be poured over areas where gardeners didn’t want plants to grow – it was often applied to grassy areas of parks to mark out the lines for football fields and the like.

The strip over which the oil was poured would be bare of grasses for a long period of time. Diesel, at about $2.50 plus a litre, (price varies) is a very good weed killer,

used either as a spray to knock out weeds with a short-term residue growing in waste areas, or as a drench for longer term control.

Likewise, salt applied directly to the soil in reasonable quantities will also give a long-term control, depending on the amount applied to an area.

Simply apply the salt and leave it to get all those unwanted plants out of our cobbles or pathways – it’s a cheap and easy solution.

Sulphate of ammonia (which is another type of salt with nitrogen), was once another very popular weedkiller for spot control. It was also relatively inexpensive if purchased in 25kg bags from stock and station agents.

All that’s required is about a tablespoon of sulphate of ammonia powder placed onto the crown or centre of a weed. Left alone, the dry salts will burn out the crown of the weed, killing it outright.

As it is nitrogen, the control residue level lasts only for a short time. Some gardeners use this product on flat weeds in lawns – and indeed it can be a reasonably successful lawn weedkiller if mixed in with sand to get an even spread over the lawn.

This is called Lawn Sand, and here’s how to make your own.

Take 5kg of sharp sand (plasters sand preferably – don’t use river or beach sand as it can contain weeds of its own), together with 700 grams of sulphate of ammonia, and 300 grams of sulphate of iron.

The iron aids in making the area more acidic, and helps to burn mosses. Mix this all well together, and apply at a rate of 30 grams per square metre.

Do it ideally in dry weather conditions when rain is unlikely – if it does rain soon after application, the Lawn Sand will be less effective. Grasses which come into contact with the Lawn Sand will turn brown, but should bounce back after a week or two.

Another compound called Ammonium sulphamate mixed at 200 grams per litre of water and sprayed over weeds is also a very effective way of getting rid of them as they quickly compost down.

Watered over the foliage of oxalis into the soil it will compost the bulbs and help you get rid of this pest weed.

Sold as a Compost Accelerator or Super Stump Rotter and as Ammonium sulphamate in 2kg jars.

Image credit: Ylanite Koppens

Products mentioned are from Wallys Range of products and can be found in some garden shops or by Mail Order on www.0800466464.co.nz

Problems ring me at: Phone 0800 466464
Garden Pages and News at www.gardenews.co.nz
Shar Pei pages at www.sharpei.co.nz
Mail Order products at www.0800466464.co.nz

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