How do pesticides work?

From an anonymous grower. How do pesticides work?
I recently had a can containing some two-stroke petrol, which had gone off. Having seen where spilt petrol/oil has killed grass, I decided to use it as a weed killer using a rough hand sprayer. Later on that same hot day I sprayed another weed patch with ‘Roundup’ herbicide at the recommended strength, using the same hand sprayer. It is now a week later and I can see the results.

 Those weeds sprayed with the herbicide are wilting, and the entire plant has turned yellow. Different types of weeds are coloured to different degrees, but all are obviously dying. 

The weeds sprayed with the petrol are different. There are strong yellow spots on the leaves, obviously where the actual petrol droplets have hit. The remainder of the leaf is still green.

Can you explain this difference?

It might be helpful to start by defining what a ‘weed’ actually is.

A weed is a plant which is growing somewhere that it is not wanted, rather than being inherently a weed. They are usually a significant problem when they are fast growing and invasive, so that they swamp any wanted plants.
What is considered to be a weed is quite subjective; that is, one person’s beloved plant can be someone else’s weed. There are many examples in Australia of ‘pretty’ ornamental plants (usually imported), such as lantana, which have gone feral. Also, cultivated trees such as olive, willow and camphor laurel have become widespread weeds. On the other hand, many other countries have problems with Australian eucalypts (gum trees) also going feral.

Systemic chemicals
Roundup is the original brand name for glyphosate herbicide and, as its patent has run out, it is now sold under many brand names and formulations. It is a ‘systemic’ chemical. That is, once sprayed onto a leaf, the chemical is absorbed into the leaf and from there mixes into the water streams circulating around the plant. It is therefore distributed around the plant and hence impacts upon the entire plant.

How quickly and effectively this works depends upon a number of factors, some of which are:

Species of weed
Glyphosate works on a very wide range of plants. The product label lists different species for which it can be used and the associated concentrations.  It varies in its effectiveness on different species and works best on soft annual species and grasses.

It only works upon actively growing plants, not upon seeds; hence, it is useless as a pre-emergent herbicide.

The plant needs exposure of several hours of sunshine after spraying, or at least six hours if dull and cool, in order to absorb and distribute the herbicide. A similar time is needed before the plant is washed by rain.

While petrol is severely toxic to plant tissue, it is obviously not systemic. Hence, it kills where it hits, but does not get into the plant solutions. Time will tell whether it kills the plants that you sprayed. Some factors involved would be the percentage of damage to the plant and how vulnerable it is to damage. For a moderate percentage of damage to a tolerant plant, the plant will probably survive and continue to grow. A susceptible plant will continue to struggle and probably eventually die.

Reader Inquiries – PH&G March 2013 /Issue 129