Spray or Not to Spray
Part VI: Perfect Herbicides
by M.J. Harvey September 2005
In a previous newsletter I left readers with my Christmas letter to Santa begging for a can of the perfect herbicide – one that would kill plants but be perfectly harmless to animals; that would not persist in the environment but decompose rapidly to harmless residues; something that I could chug-a-lug with impunity.
So what happened? Well, Santa did leave a 10 litre container of concentrate under the tree (we have nearly 11 acres) and a note that read: “Dear Little Joe, I get this request all the time and the herbicide you want is readily available, so I did not have to bother the elves to invent anything new. Happy gardening. Santa: Needless to say, I was delighted.
Looking into the history of what I received I found that in the 1960’s or earlier a group of scientists did a survey of all the reactions that plants have to carry out (and this is a huge list since plants have to synthesize everything from carbon dioxide, water and a few elements), but which animals did not. The logic behind this search was that if a reaction going on in plants could be stopped, this might be fatal to the plant. But if animals did not carry out this reaction then there would be no effect on them. Ergo – the perfect herbicide.
The SAP in Plants
In the book ‘1006 and All That’ the question is posed to readers ‘Why are you so numb and vague about Arabella Stuart?’ The reason being that this is a really obscure bit of British history. I anticipate that many of my readers are numb and vague about the Shikimic Acid Pathway (SAP) for much the same reason – this is a really, really obscure bit of biochemistry.
To give an explanation, the SAP is long chain of reactions leading to the production of the three essential amino acids that possess an aromatic (benzene) ring. Block a single enzyme in this long chain and none of these three amino acids are produced, with fatal results to the plant.
So why is this particularly obscure bit of knowledge useful? Well, plants use SAP but animals don’t. Why don’t animals possess SAP? They get their essential amino acids by eating either plants or other animals. So, the scientists speculated, if we can block SAP in plants they will develop deficiencies of these amino acids, but animals will not be affected.
The search was on for an enzyme blocker, one that would stop a single reaction in the SAP. The reason for targeting an enzyme is because enzymes exist in extremely minute amounts in cells so the quantity of specific enzyme blocker required will be correspondingly minute. This could lead to an effective herbicide if such a substance could be produced economically.
So, the race was on. There were potential profits to a company whose scientists could come up with a SAP-enzyme blocker. Who won? Monsanto, with the patenting of a chemical they named glyphosate. This chemical is sold as a formulation under several names, most commonly Roundup which is available in several strengths of which I recommend the concentration of 356 g/l of glyphosate. The other concentrations may be more convenient for amateurs but you are buying very expensive water.
Does Roundup live up to the claims?
It depends on who is making what claims. To some people it is the Devil incarnate. But let us look at the claims. First, is it non-toxic? Yes, glyphosate is completely inactive in animals. As is claimed, it is less toxic than common salt which some people think of as the staff of life, but you eat a kilo of salt and you are a goner (of course you have to keep it down). So would I drink a cupful of roundup? Certainly not! Roundup is a formulation that in addition to glyphosate contains a detergent. The detergent lowers the surface tension and helps to get the glyphosate into the plant. So drinking a cup of Roundup would be the equivalent of drinking a cup of liquid Tide – not fatal but not nice. Word of advice: when diluting Roundup add water to the container first – that way you avoid foam.
Second, does Roundup persist in the environment? No, it really does decompose in a few days to carbon dioxide, water and a trace of nitrate. In this respect it is diametrically the opposite of the early chlorinated pesticides which persist for years, evaporate into the atmosphere and eventually condense at the poles. This is the model of the Earth as a giant still. So seals in the arctic can get their pesticide burden from pesticide spraying in the tropics. (Incidentally there is a developing worry about another class of compounds: brominated ethers, which are used to make fabrics and plastics non-flammable. These are parallel to the chlorinated compounds but contain bromine, the next heavier halogen).
I grow peonies and have very carefully sprayed the grass around them in spring avoiding the young shoots. One can be very specific in what one sprays. Several plants have developed distorted shoots with streaks lacking chlorophyll. This can mimic natural variegation. I think this must be a case of transmission through the soil. In other words, although Roundup does decompose in the soil, for some plants that are really sensitive it can be absorbed by surface roots in the few days it takes to decompose. There is also a hint that Rhododendrons can be damaged (not killed) because they have surface roots and careless overuse around them may result in absorption. Roundup damage is basically chloroplast damage and evidences itself in pale areas or streaks on leaves.
So although Roundup is one of the most benign chemicals yet to come on the market, it is highly potent and overenthusiastic use by yourself or hired help can result in disaster. This is the fault of the operator not the chemical. It is particularly important to not use too fine a mist when spraying to avoid drift on to desired plants, and to avoid spraying in a wind.
I had better wrap up this series in the next article (they could go on indefinitely), where I will get back to the need for knowledge and the frequent lack of it exhibited in the organic movement. I have to thank my old colleague Jeff Child who supplies some of the chemical data – he has a computer, I don’t. I say ‘old’ since we were students together in Newcastle 50 years ago and it is a strange coincidence that has caused our paths to coincide in retirement.