What about organic weed control?
Are there effective alternatives to chemical weedkillers?
LAST week’s article on perennial weeds certainly hit the spot with many readers, most lamenting the presence of these hard-to-control invaders of our garden beds, lawns and pots. I received some quality feedback from perceptive gardeners regarding my apparent promotion of chemical controls over cultural and/or organic methods, but not any real alternative solutions for dealing with these difficult plants.
However, this prompted some deep though about true organic control of garden weeds, and if there are any that are reasonable alternatives to chemical control. “Reasonable alternatives”, in this context, are products or methods that are not too difficult to use or implement, not too costly (overall, and when compared to chemical controls), but most importantly, that do the job for which they were intended i.e. kill weeds effectively.
There are many “home-made” weedkillers that do a reasonable job, and whilst not organic in nature, are worthy of mention here.
One material, iron sulphate, works well on weeds when applied via watering can at a rate of 2 tablespoons of powder in 4.5L water. Wear gloves and don’t get it on pathways or pavers as it stains badly. Iron sulphate works by burning foliage on most broad-leaved annual weeds in lawns.
My horticultural acquaintance, Jerry Coleby-Williams, has released his grandfather’s “weed sand” recipe for all to use. Weed sand is a mix of equal quantities of powdered (not granular) iron sulphate, sulphate of potash, and washed sand. Wear gloves when mixing to avoid stained hands. Apply dry, directly to lawn weeds, including cudweed, white clover, bindii and flatweeds and lightly moisten (sometimes the morning dew will do this if applied in the evening). This is a contact killer so it may need to be reapplied every two weeks for good results.
Vinegar is touted as an effective organic weedkiller, but there are a few things to remember when using this product. Firstly, household vinegar (5% acetic acid) is too weak to be effective. Secondly, horticultural vinegar (20% acetic acid) must be sprayed on the entire plant for a good kill, and reapplication may be necessary. And thirdly, any overspray will affect non-target plants as well. Look for a commercial product that combines horticultural vinegar and clove oil in a pump spray, which is best applied to dry foliage on a warm day. Any type of non-selective spray such as this is better suited for weeds in paths, paved areas, driveways, empty garden beds and under benches in greenhouses and shadehouses, but not in crowded garden beds or lawns.
Now there are a few “organic” recipes out there that are a combination of vinegar and salt but I think it’s wise to avoid any spray with salt as an ingredient. Salty residues will remain in the soil and stifle plant growth and kill soil life.
In the past few years there have been advances in true organic weedkillers on a commercial scale.
One of these is pine oil, having some interesting properties beyond that of some other organic products, which simply burn or desiccate the soft foliage of weed plants. Pine oil will also destroy the outer coating of some seeds, preventing them from germinating and becoming problems down the track. As with many other organic herbicides, complete coverage of the plant is important for the product to have maximum efficacy. I’ve recently sprayed a ready-to-use pine oil product and coverage was achieved on most small weeds with only two sprays, which seems quite efficient. One litre is claimed to cover around 13 square metres which is enough to treat many, many hundreds of small weeds (5cm x 5cm).
Another recent introduction to the organic herbicide repertoire is nonanoic acid, otherwise known as pelargonic acid. This active ingredient occurs naturally in a range of organic materials, including apples, grapes, goat’s milk, beef tallow and, as the name suggests, pelargoniums. A new Australian-made product is derived from oilseed crops, not the environment-destroying palm oil crops from which others are made. Pelargonic acid works by destroying green plant tissue rapidly, with some plants collapsing after only 20-30 minutes after application. As with any contact herbicide that works by destroying plant tissue, if you miss any buds or growing points, the weed is likely to regrow.
Most of these products are relatively easy to apply, either as sprays or from a watering can. What about cost per millilitre (ml) of product?
A readily available vinegar and clove oil spray will come in at 1.8 cents per ml ($18.00 per litre), and pine oil between 1.4 and 2.5 cents per ml ($14.00 to $25.00 per litre - I know, quite a range of prices!). These are both in pre-mixed, ready to use spray bottles.
Pelargonic acid is usually available as a concentrate in a one litre bottle. Cost is about 3 cents per ml, but the mixing rate is 70ml per litre of water in a spray bottle, so each litre of spray will set you back $2.10. Pelargonic acid certainly wins the cost comparison! (incidentally, glyphosate concentrate, made up to 1L of spray, costs about 10 cents)
All these organic herbicides must be used according to directions. Whilst quite safe to use on soil (minimal or no residues, minimal or no effect on microbes, earthworms), and safe for bees and birds, many will be quite irritating to the eyes, noses, throats and skin of humans. Most will biodegrade in 3 to 10 days, and it seems that there are no withholding periods when used near edible plants.
If you have used any of these products, please let me know of your results! wellsleyhorticultural @gmail.com