How weeds are able to survive

Farmers often struggle to grow crops for food, yet weeds flourish on their own without any effort on our part. Bill Kerr explains why.

How weeds are able to survive
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Weeds will always be with us. We simply have to adapt our practices to reduce the impact of weeds as far as possible.

But why is it that weeds spring up so readily whereas our crops are so difficult to grow?

The main reason is that weeds have developed highly effective survival strategies over thousands of years, evolving into many species adapted to numerous environmental conditions.

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The most common survival mechanism is ‘hardseededness’: a water-impervious outer seed coat has to rupture after being worn away, being exposed in a fire, or experiencing large temperature fluctuations before germination can occur. All of this ensures that the seeds germinate at different times.

If they germinated at the same time (like our crops), a drought or some other calamity could kill them all before they produce seed.

Some pasture seeds use this mechanism too. So, in order to ensure immediate germination, the seed coat must be scarified (mechanically damaged). This is usually mentioned on the label.

Other survival mechanisms include:

  • Thriving through fire: Some plant species, such as fynbos, have evolved to survive fire, and this has benefited them. When a fire rages through an area where fynbos species grow, it kills the non-fynbos plants, removing shade and giving the new fynbos seeds time to get established. At the same time, the heat and smoke help to trigger germination of the fynbos seeds. Wattle seeds are also stimulated by fire.
  • Chemical protection: Plants such as some of those in the Chenopodiaceae family use a chemical barrier (tannin, in in this case) to ensure survival. The barrier inhibits germination until it is leached from the seed coat. But because leaching by rain is often not uniform and the seeds have different amounts of this chemical, the seeds germinate at different times. The leaching of the chemical in this way also ensures that there is sufficient moisture in the soil to establish the plant.
  • Stacking the odds: Many weeds also produce a vast number of seeds to give the next generation a better chance of survival. A single Portulaca plant, for example, can produce 50 000 seeds.
  • Life light: After maturing, some seeds may be consumed by foraging animals. They pass through the dung and are buried, where they remain until brought to the surface by digging animals. Even a brief flash of daylight can then break the dormancy.
  • Cold spells: Some seeds need to go through a cold spell before germination is triggered. This mechanism stems from adaptation to climatic conditions where survival is only possible in spring.
  • Thermal dormancy: A change of weather, which would perhaps affect the survival of a germinated seed in nature, could cause prolonged seed dormancy. The conditions may have to remain favourable for a long period before the seed has the ‘confidence’ to germinate.

Using various mechanisms, some species may remain dormant in the soil for up to 40 years. Cultivation often provides the right conditions for them, and they finally germinate. An example is the bindweed Convolvulus arvensis.

I encountered it when ploughing what I thought was virgin veld and a huge crop of the weed rapidly appeared. Not even the oldest employee in the company could remember when the land had last been used for cropping – that’s how long the weed had lain dormant.

Get used to it! Weeds are smart enough to be with us always.