Veggies

Scope

The focus of this article is on weedicides that can be manufactured at home, there are new generation organic herbicides that are becoming available like nonanoic acid also known as pelargonic acid, as present in the commercial herbicide ‘slasher’. These are beyond the scope of this article.

Introduction

When the term ‘weed management’ or ‘weed control’ is mention, I would bet the majority of gardeners will think first of a spray of some description. This has been due to the impact of advertising by the agricultural chemical industry over many years. Sprays do form a part of weed management, but should not necessarily be the first choice because there are many non-chemical controls that may be more appropriate, depending on the circumstances. (see the previous article here)

Non-organic weedicides are plentiful, easily available and to be avoided!

Having said that, if you are one who wants to formulate a spray so you know and understand what goes into it, and what the consequences of its use will be, keep reading.

What we look for in a weed spray

From what I can gather, based on the thoughts of myself and others, what we are looking for in an organic weedicide spray is something that is –

• already on hand in our homes, (Preferably)
• Cheap and easily available,
• Effective,
• Safe for us, non-target organisms (like bees, butterflies and worms) and the soil,
• That doesn’t build up and cause problems in the long run, but that will break down quickly and easily in the soil.

Weedicide formulations that are written down in books and available on the internet can be touted as being ‘organic’ but not necessarily comply with all of the above criteria. It is important to understand the materials we are dealing with, how they work and what downsides they have (if any).

For the most part, the herbicides covered in this article are

• Non-selective; that is to say, they are equally effective on all weeds.
• Contact rather than systemic herbicides so they act on the part of the plant that they are applied to, rather than being absorbed by the plant and travelling to all parts of the plant (salt solution can be an exception).
• Generally short term in their action (Salt solutions can be an exception),
• Several applications may be required for them to be effective, particularly for perennial weeds.
• They are post-emergent, that is to say they will kill weeds that are growing but will have little or no effect on weed seeds that have not germinated.

Weedicide Application

To get the best from organic weedicide sprays the following points should be borne in mind –

• Ensure good spray coverage of your target weeds. Due to fact that they are contact herbicides, they can’t kill what they don’t touch so the spray needs to cover most of the plant.
• spray in warm weather (24°C+);
• Treat weeds when they are small; and use repeat applications on larger weeds.
• Spraying should be done under favourable weather conditions (low/no wind, no imminent rain) to ensure that the sprayed materials remain on the treated plants long enough to be effective.

Application methods

The following link is to an article in the ‘Protecting Your Investment’ section of the website about various types of sprayers and their use with pesticides. The link is included here because a lot of the information also works well for weedicides. Link: Sprays and Sprayers

While spraying is a common way of applying weedicides, for small areas or single plants, particularly where they are nestled in amongst your flowers or veggies, the weedicide can be applied to the leaves of the weed with a paintbrush.

Types of Weedicides

Acids

Vinegar – is a cheap, easily available and very effective weedicide, it is my go-to when I need a chemical weed killer. It kills weeds by rupturing the plant cell walls where it applied and allows the plants’ fluids to leak out, drying them out and killing them. It is not a residual herbicide so it will break down quickly and so won’t build up in the soil, but it may require multiple applications to be effective, particularly for perennial weeds. It is sprayed undiluted, with the highest concentration of acetic acid that is available.

Lemon juice – may also be used as a weed killer because of its acidic nature, but will be less effective than vinegar, because citric acid is a weaker acid than the acetic acid in vinegar. Its advantage is that it can be home produced.

Day one: prior to application of vinegar spray

Day Four: after application of vinegar spray

Inorganic acids – like hydrochloric acid, sulphuric acid and nitric acid will most certainly kill weeds and, given the chance, do very unpleasant things to the person applying them. They will also do very unpleasant things to the soil and are NOT recommended for use.

Salts

While the term ‘salt’ is usually used to describe table salt (sodium chloride), in this case the term is used to refer to a number of sodium containing chemicals which are used as weedicides and work in a broadly similar fashion. They include table salt, sodium bicarbonate (bicarb, baking soda) and borax (sodium tetraborate).

They can work in two ways. When applied as a solid or, more usually as a solution, to the leaves and stems of a plant will act as a contact herbicide. However, if a salt solution is applied to the ground around the plant deeply enough that it can be absorbed by the roots, it disrupts the water balance in plants, eventually killing them.

While salts are cheap and effective weedicides they have one major disadvantage in that they do not break down in the soil. They will remain there until diluted by rain or irrigation and flushed deeper into the soil, down to the water table or by run off to nearby streams, rivers or lakes. Salts will also have a negative effect on soil structure and soil biota as well as surrounding plants. There are many areas around Australia that are already struggling with soil salinity.

Due to all these negative impacts, salts as weedicides or weedicides that contain salts as a component are NOT recommended for use.

Epsom Salts (Magnesium sulphate) is a non-sodium containing salt that I have seen on some websites recommended as a component of weed killer, but I can find no evidence that it actually works. In dilute solution it is a fertiliser (supplying magnesium and sulphur) and may have some weedicidal effects in concentrated solution, but you would have to apply a lot and it would be detrimental to the soil in such high concentrations. It is not recommended as an effective weedicide.

Bleach

Bleach is a reactive and aggressive chemical that is comparatively cheap and readily available. The active constituent of liquid bleach is sodium hypochlorite. There are also likely to be other corrosive compounds like sodium hydroxide present, as well as less nasty compounds like sodium chloride, sodium carbonate and surfactants, all depending on the formulation of the particular bleach you are using. Over time bleach will break down to what is effectively salty water.

The downsides of using bleach is that it is corrosive and can damage your skin and eyes, it is quite alkaline and will impact the soil biota negatively and affect soil pH. As it breaks down it will contribute salt to the soil (check out ‘salt’ above). While it could be useful in small amounts in isolated areas on particularly difficult weeds, the use of bleach as a weed killer is generally not recommended.

Surfactants

Surfactants (or surface active agents) are general found as one of two types: soaps, which are produced by the reaction of an alkali and a fatty acid of one type or another, and detergents, which are a mixture of synthetic compounds based on petrochemicals. Either are likely to contain sodium.

When applied to plants they can strip off the protective layer from plant leaves, allowing them to dry out and the plant to die. Surfactants applied to the soil can result in raising soil pH and a reduced availability of nitrogen and potassium in the soil. Given time, both soaps and surfactants will eventually break down in the soil albeit possibly leaving some sodium residues behind.

Surfactants also have another property, and that is to reduce the surface tension of water. This allows solutions to cover surfaces, like the waxy surface of a leaf, more completely and thus allowing any weedicidal components of a mixture to work more effectively. This is why many weedicide formulae on the internet contain ‘dish soap’ as a component. Note: ‘dish soap’ is a detergent not a soap.

While a small amount of these chemicals when combined with other weed killers may help to improve their effectiveness, using surfactants as weedicides is not recommended.

Oils

Oils used as weedicides or in weedicide formulations may be vegetable oils like cooking oil, petroleum hydrocarbon oils like white oil or kerosene, diesel or turpentine, or they may be essential (‘botanical’) oils like clove oil.

Petroleum hydrocarbon oils – In terms of effectiveness, I don’t see any advantage of white oil over the more easily broken down vegetable oil. Regarding other hydrocarbons, while I understand that they are effective weed killers, I think they would take considerable time to break down and would therefore pollute the soil, causing problems for the soil biota and so I would NOT recommend their use.

Vegetable oils – I have read that vegetable oil can be used as a weedicide, so I got hold of some of our used olive oil, filtered it and sprayed it I sprayed it on some annual (chickweed) and perennial (dandelion) weeds. In terms of the annual weeds all it did was make the leaves look glossy, but after a week on the dandelion leaves they were looking a bit sad, but not decimated by any meansl (this does not show up well in the photos, but differences are there, trust me!). A caveat here: we are in autumn and although the weather has been fine and clear, it has also been cooler than normal. I think if the vegetable oil were to be applied at a hotter time of the year, it would be more effective.

Initially

After one week

After two weeks

I have also read on the internet about injecting vegetable oil into the soil, wording goes something like this – “For bulbous weeds, like onion weed and oxalis, you can inject vegetable oil into the ground surrounding the bulbs. The oil will coat the bulbs so they suffocate and will then rot into the soil.” I have not tried this so I don’t know if it works or not, however, the wording is almost the same in references on a number of websites so I think that someone tried this once, and everyone else just copied. There is no way of know who the original poster was or if it really worked. There is room for more research here.

Botanical oils or ‘essential’ oils – There are e number of essential oil used as weed killers, such as oil of wintergreen, clove oil, orange oil or cinnamon oil in combination with other chemicals. They seem mostly to be used in combination with vinegar, presumably to improve its performance. I have only tried one combination that was vinegar and clove oil I extracted from cloves, but there did not seem to be much improvement over vinegar by itself. That story can be accessed here. I suspect that the vinegar would work just as well by itself and be considerably cheaper.

Miscellaneous

Aged urine – This is not one that I have tried but is advocated by no less of an authority than Jackie French in her book ‘Organic Control of Common Weeds’. Her recommendation is to leave the urine for 24 hours to become ‘ammoniacal’ and apply it undiluted when the soil is dry, twice over a period of 24 hours. She finds that the main disadvantage of this approach is getting hold of enough of the stuff, so next time you have a party…… provide a bucket!

Alcohol – There are two alcohol compounds that are generally used when weedicides are mentioned, the so-called rubbing alcohol, which is 60% - 70% solution of isopropyl alcohol and the ubiquitous methylated spirits (metho, meths) which is greater than 95% ethanol plus chemicals to make it taste bad so people won’t drink it.

Formulations generally suggest one or two tablespoons of alcohol per litre of water, so I added one tablespoon of methylated spirits to 500mls of water, swirled it around to disperse and had a spray on some annual (chickweed) and some perennial (dandelion)  weeds.  After a week there was no observable differences between ‘before’ and ‘after’ spraying. Just to check, on another patch of dandelions I sprayed 'neat' methylated spirits to see what would happen. Nothing did, I suspect that the metho evaporated before it could do much damage.

To be fair, the references did talk more about using isopropanol as a weedicide so I got hold of some Isocol (64% isopropanol) and mixed in the ratio of 2 tablespoons to 500ml of water and sprayed it on another patch of weeds. The results were the same, after checking for several weeks there were no changes to the weeds

Mixtures

There seems to be one main weedicidal mixture out there on the internet, and it is a combination of various ratios of –

• Vinegar,
• Epsom salts or table salt or bicarbonate of soda, and
• Dish ‘soap’ which is, of course, a detergent. If a true soap is mixed with vinegar it will break down to an oily residue and will not be able to function as a surfactant.

Following is one example, but there are many –

• 1 gallon white vinegar
• 2 cups Epsom salt or table salt
• ¼ cup dawn dishwashing soap

From the detailed information about the ingredients above, it can be seen that there are some problems with this formulation, particularly in terms of the previously referred to ‘salt’ component.  My suggestion would be to go with the vinegar, and perhaps a small amount of dishwashing detergent to help the vinegar wet out the leaves of the target plants.

Conclusion

Having looked around at the recommendations I have been able to track down it seems to me that vinegar is still the best bet option with or without a small amount of surfactant added to help its coverage and with or without a small amount of essential oil(s), but for me they are yet to be proven. There are other chemical products that are easily available and out there, they may even be effective, but they tend to bring other drawbacks with them. Based on the information I have provided (and you own researches) it is up to you to make the choice of which weedicide(s) will work best for you in your situation. Although it is worth progressing non-chemical weed control strategies first.