Cutting through the confusion of weedkillers
There are four main types of herbicide available to home gardeners: systemic, residual, contact and hormonal
Systemic herbicides
When applied to plants these products will kill indiscriminately by entering plant tissues from leaf to root. They’re non-selective and quickly break down, making them ideal for tackling deeprooted perennial weeds such as bindweed and dandelions, as well as brushwood and regrowth on tree stumps.
There’s currently huge controversy over glyphosate, the world’s most widely-used systemic weed killer and
the only product of its kind currently available to home gardeners. Used in many popular products, either on its own or in formulations with other herbicides, glyphosate was considered potentially carcinogenic by the World Health Organisation (WHO) cancer agency in 2015. This year a US court found in favour of a legal claim lodged by a professional user with terminal cancer, with other cases still pending.
Manufacturer Monsanto refutes the claim and plans to appeal, producing scientific evidence to the contrary, with other scientific studies supporting their position. It is also alleged by a Reuters investigation that the original finding was misrepresented in the final WHO agency toxicology report.
Glyphosate is currently licensed for sale in the EU until December 2022, when countries such as France, Germany and Holland have said they will withdraw it. The UK has yet to issue such a declaration.
Glyphosate products available include ready-to-use sprays such as WeedFree by Vitax, liquid concentrate Resolva Extra Tough Concentrate by Westland, soluble sachets like Knock Down Super Strength Glyphosate Weed Killer by Doff Portland Ltd or gel such as Roundup Gel Wand.
Glyphosate is also mixed with other herbicides to broaden the nature of its use, or enhance its potency. Mixed with contact weedkiller diquat, which kills above ground parts, glyphosate tackles the roots and is found in formulations such as Westland’s Resolva Path & Patio, or with pyraflufen ethyl in Scotts Weedol Ultra Tough.
Residual herbicides
These products, usually containing glyphosate to kill growing weeds, can remain in the soil for weeks or months, depending on the mode of action of the additional chemical additives which help prevent weed seed germination or regrowth.
Some can be used around plants, whereas others can be used only in uncultivated areas or on hard surfaces, so check the label for suitability before use. Products containing glyphosate and diflufenican include Scotts Weedol Pathclear Weedkiller and Job done Tough Weedkiller by SBM Life Science.
Contact herbicides
Herbicides that kill by contact, particularly those based on ‘natural substances’, such as acetic acid (vinegar) or fatty acids, are judged more environmentally friendly and have become increasingly popular.
Products include Solabiol Super Fast Weedkiller (fatty acids) from SBM Life Science, Origins Weed Control (acetic acid) from Scotts, and Fast Acting Weedkiller (pelargonic acid) and Fast Acting & Long Lasting Weedkiller (pelargonic acid/maleic hydrazide), both from Neudorff.
Killing just the above-ground parts, these herbicides are useful for controlling annual weeds. Their effectiveness against established infestations of perennial weeds, such as bindweed, can be less convincing, often requiring repeated applications to achieve significant control.
They can be used in conjunction with traditional gardening techniques or with other emerging technologies and gadgets, involving electric currents, flamethrowers and ultra-high pressure water jets.
Hormonal herbicides
These types of products mimic natural plant hormones, causing distorted growth and subsequent death of most flowering or broadleaf plants, except grasses at normal dosages, so are mainly used to control lawn weeds or unwanted weeds in rough grassland.
Chemicals include 2,4-D, mecoprop-P, MCPA, clopyralid, fluroxypyr and dicamba, which are absorbed and remain persistent in the plant and soil in the weeks after application. As a consequence you should not compost lawn clippings or use them as a mulch after use, especially if you’re using products containing clopyralid such as Weedol Lawn Weedkiller from Scotts and LawnClear 2 from Vitax as this is the most persistent.
Lawn weedkillers using other active ingredients include Doff Lawn Weedkiller and Resolva Lawn Weedkiller Extra.
Some weedkillers are also mixed with fertiliser blends as ‘weed and feed’ products or with ferrous sulphate to treat moss infestations, such as Aftercut All in One from Westland Horticulture.