Ask the experts: Effect of herbicides on weed seeds

13 March 2024

Question: Are there weedkillers that will also combat or suppress the weed seeds, or are weedkillers only suitable for combating already growing or emerging weeds?

Answer: Weedkillers that are applied directly to the soil and are intended to combat weeds during emergence and before they appear above the ground are called pre-emergence weed killers. 

The one characteristic that all soil-applied (pre-emergence) weedkillers have in common is that they do not hinder or inhibit the germination of weed seeds, but they prevent seedlings from appearing above the ground (any seedlings that do emerge quickly die). Weed seed management is crucial for effective weed control, primarily for two reasons:

  1. There are millions upon millions of weed seeds in the soil, and only a small percentage of them germinate at any given time and develop into seedlings. The seeds that do not germinate immediately remain dormant in the soil until conditions become favourable. That is why there are waves of weed emergence in a field – the pre-emergence weedkillers are depleted by the time seeds germinate later in the season. This leads to seedlings that become increasingly difficult to combat.
  2. The longevity of weed seeds in the soil varies greatly among weed species. Some remain viable for less than a year (e.g., grass seeds), while especially broadleaf species’ seeds can remain viable for years and even decades. Small seeds prefer shallow placement in or on the soil for optimal germination, while large seeds have enough energy reserves to emerge from deeper soil layers. Soil tillage is a good practice for weed seed management, but remember, burying seeds to prevent germination can simultaneously bring dormant seeds from deep soil zones to the surface where they can germinate.

Inquiries: Dr Charlie Reinhardt, professor at North-West University. Email: dr.charlie.reinhardt@gmail.com

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