Herbicides, also called weedkillers, are chemicals used to kill the growth of unwanted weeds. They work by changing soil pH so that the balance of which nutrients were potentially available to the plants changes and by disturbing or destroying the soil’s microbial life whatever vegetation has been weed-killed then takes much longer to decompose and release its nutrients back to the soil. Herbicides vary in their mode of action, persistence in soil, and the timing and method of application. The two main herbicide types are called pre-emergent and post-emergent.
Pre-emergent (‘residual’) Herbicides: These act against germinating seedlings of the weeds. Pre-emergent (soil applied) herbicides are applied directly on the soil surface before seed germination and growth of the weeds. Weeds are killed as they germinate. This type of treatment does not typically control established weeds or dormant weed seed. Examples of pre-emergent (soil applied) herbicides include Atrazine, Simazine, Diuron (Karmex), Oxyfluorfen (“Goal”), Oryzalin (‘Surflan’), Pendimethalin (‘Stomp’), Napropamide (‘Devrinol’), Norflurazon (‘Zoliar’), Trifluralin and (‘Treflan’). See also Dichlobenil (“Casoron”).
Post-emergent (‘knockdown’) Herbicides: These damage growing weeds and come in two main types–contact and systemic herbicides.
Contact: Post-emergent (foliar applied) contact herbicides counter established weeds during summer. Usually, clay is treated with moderate doses at short intervals, limestone sites are given a single hefty dose per season, while sandy soils are treated little but often. Examples of post-emergence (foliar applied) contact herbicides include Glufosinate ammonium (‘Basta’), Paraquat and Diquat.
Systemic: Post-emergent (foliar applied) systemic herbicides are applied to established, growing weeds. Post emergent herbicides may kill tissue directly contacted (contact herbicides) or they may translocate within the plant (systemic herbicides). Examples of post-emergent (foliar applied) systemic herbicides include Glyphosate (‘Round-Up’), Fluazifop (‘Fusilade’), and Amitrol + Ammonium thiocyanate.
How used: ‘Typically, herbicides are applied only to the strip of ground directly under the vine, and weeds growing between the rows are controlled by cultivation or mowing. The practice of using herbicides across the entire vineyard–both under vine and across row middles–is declining, partly due to pressure from environmentalists, but also because of increased risk of soil erosion, and reduced potential water infiltration due to diminished organic matter provided by either a sown cover crop or the native sward,’ says Dr Richard Smart (2015).
Potential problems: Dr Richard Smart (2015) says ‘continued use of some herbicides leads to the increased presence of so-called ‘escape’ weeds, which were previously suppressed by competition from other weeds.‘ Smart identifies other potential problems with weedkillers, namely higher soil surface temperatures, a loss of organic matter (fewer earthworms), damage to either the vine leaves via inadvertent spray drift, or to vine roots with young vines, sandy soil and irrigation, and shallower rooting as weedkillers encourage vine roots to go to the more fertile soils nearer the surface.
Philip Moraghan of Curly Flat winery in the Macedon Ranges, Victoria, Australia says of moving towards organic practices that ‘I reckon it’s not using herbicide that makes the difference. You have water running down the grass to the roots. Also there’s a much greater temperature variation in the soil,’ (quoted by Jancis Robinson MW, 15 Apr 2013).
Cmmonly used herbicides: ‘The four commonly used herbicides: mesotrione, topramezone, nicosulfuron and atrazine. These are often used in combination with safeners, which are chemicals that selectively help protect crops from herbicide damage,’ (‘For better or worse’, The Economist March 3rd 2018, p.68).
Natural alternatives: Pelargonic acid.
Dr Richard Smart (2015), entry for Herbicides in Oxford Companion to Wine 4th edition ed. Jancis Robinson MW and Julia Harding MW (Oxford University Press, 2015), p.357.
Jancis Robinson MW (15 Apr 2013), ‘Curly Flat’s recent gems’, www.jancisrobinson.com