Wily weeds win the war

Farmers have been battling with weeds since the advent of agriculture, about 6,000 years ago.  Most of the weapons used against weeds were mechanical until the last century or so when herbicides became the primary weapon.  At the same time that the weapon became more lethal, farming techniques changed to give weeds the advantage.  Farms became huge monocultures and crop rotations were abandoned in favor of the most profitable crop. 


When weeds evolved defenses against the herbicides, farmers responded by increasing doses and manufacturers created new products to which weeds hadn’t yet evolved resistance.  Finally, the use of herbicides skyrocketed when crop seeds were invented that weren’t killed by the herbicides, so that huge amounts of herbicides can be used without killing the crop. 

When Roundup with the active ingredient glyphosate went on the market in the 1970s, its manufacturer, Monsanto, claimed that weeds would not be able to evolve resistance to it.  And apparently that was initially true until Roundup was used on a huge scale when herbicide-resistant seeds were put on the market in the 1980s.   Norman Ellstrand of UC Riverside explains why:  “He argues that the reason was that farmers applied glyphosate to relatively little farmland.  As they applied it to more and more acreage, they raised the evolutionary reward for mutations that allowed weeds to resist glyphosate.  ‘That ups the selection pressure tremendously,’ he said.” *

There are now 24 species of weeds that are resistant to glyphosate and they are rapidly expanding their range in agricultural areas all over the world.  In 2012, an agricultural consulting firm reported that 34% of farms in the US had glyphosate-resistant weeds.  In the first half of 2013, half of all farms in the US are reporting glyphosate-resistant weeds.

Let’s take another approach

Obviously, we are losing the war against weeds.  So, let’s examine the strategy we have been using and try another approach.  Weed ecologists are now studying the strategies that weeds have used to cope with the weapons we have been using against them.  Here are a few of those strategies:

  • Some weeds have changed color so that they are indistinguishable from the crop they are hiding in.
  • Weeds that grew in dry ground, evolved to thrive in wet ground in rice fields that are flooded much of the crop season.
  • Some weeds became shorter to escape the mowing and harvesting of the agricultural crop.
  • Some weeds drop their seeds and go dormant before the crop is harvested and create seed banks that can sprout when conditions are more favorable for them.
  • Parasitic weeds wrap around their host and steal nutrients from them.

Some weed ecologists believe that a better understanding of the mechanisms used by weeds to foil our attempts to control them will enable us to devise better weapons against them.  They believe that developing new herbicides and/or using more of them will always be a short-term solution. 


For example, David Mortensen of Penn State is “investigating controlling weeds by planting crops like winter rye that can kill weeds by blocking sunlight and releasing toxins.  ‘You want to spread the selection pressure across a number of things that you’re doing so that the selection pressure is not riding on one tactic,’ he said.”*

Regardless of what method is used to control weeds in the future, let’s consider the toxicity of the method the most important criterion for judging their effectiveness.  Even if it kills fewer weeds, the least toxic alternative is the best alternative in our opinion.


*Carl Zimmer, “Looking for Ways to Beat the Weeds,” New York Times, July 15, 2013.

4 thoughts on “Wily weeds win the war”

  1. The real culprit among farmers or growers is their lack of study, or understanding, of basic biology. It’s been schooled right out of this society, engineered out. People have been taught not to think critically; thus, we seem to need all these experts to tell us what to do, which is why as a society we have become so vulnerable as to think what many modern growers do, that it is possible (or even right) to expect that we should be able to harvest all that grows in a field or orchard. That’s just not realistic, but it is the attitude that has developed. The new idea is that if you have a thousand fruits on a tree, you should be able to harvest a thousand perfect pieces of fruit to sell. And that you should be able to intensively grow on every acre of land, every season of every year. This defies biologic logic.How do humans do when they try to avoid sleeping? If it’s upon occasion they will survive but if they keep avoiding sleep first they develop chronic health problems, then they die much earlier than they were genetically pre-disposed to do. It’s really quite simple. So it goes in fields and orchards where first what is produced starts to suffer, then production drops off. Agribusiness can make it through such times, as can Starbucks or McDonalds, by shifting economic focus as a corporation (though the individual growers with contracts can lose their business, as can a franchise be shut down for lack of business as a result of production issues).

    If one takes a look from the “outside” she can only shake her head and say, “What are they thinking?” For humans in this society have actually been convinced that this is a reasonable expectation, and therefore have been convinced to assume there is a problem if an insect is present, or some unplanted vegetation appears. It’s always a great joy to hear biodynamic farmer Bob Cannard talk about the ebb and flow in nature. Rather than the head-banging of truly modern agriculture, he has the relaxed attitude that comes from working alongside nature; he points out that he could spend months poisoning the (beautiful) mustard that grows between grape rows but he enjoys the yellow flowers (as do bees) and before harvest time the mustard dies back to reveal the plump grapes. Bob is one of the longtime food suppliers to Alice Waters at Chez Panisse.

    In the late 90’s when we lived in the Carneros (Southern Sonoma Wine Country), I had occasion to be in Napa two times one week, once during the famed Mustard Festival, once two days after. As I drove into Napa it was a feast for eyes of yellow flowers in a field by the side of the road just South of town. When I returned a couple days later it all was dead, that Roundup orange/red that shows up 7-10 days after application. Things had been perfectly timed to allow the mustard to seem to be alive during the festival but poison fiends had actually started the killing process before the festival, carefully-timed to pull the wool over the eyes of tourists who’d be less likely to be in a spending mood if they had just driven by a field of death.

    1. This is very true in that we never really ask what are those “weeds doing?” Why is that weed growing and not another? What is missing in the soil and the environment that the weed is changing? It certainly isn’t random. What benefits are from those weeds? We have this plant called multiflora rose. Although viewed as the most evil of weeds, studies have shown it turns out it have the highest biodiversity than any native shrub by many fold. Yet huge legislation and chemical warefare has been created just for this plant. Why is it then we want to get rid of it? Oh that’s right. It’s a weed!

  2. “Weeds” are what humans decide to be considered problem plants, but which are revered in other cultures. Many of what are considered weeds are far more nutritious than the fields they are growing in.

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