Concentration of CO2 in the Atmosphere

Weeds, Invasive Species, and Climate Change

Weeping willow, a thirsty tree, is considered a weed in Australia, where water is scarce. But it is useful there for erosion control and helps the land retain water. Photo: Antilived, Wikimedia Commons.

George Harvey

In mid-May, amid the bustle of getting Green Energy Times ready to go to press, I noticed a post at BBC Future, “How Weeds Help Fight Climate Change” ( It is a fascinating article that introduced me to natural sequence farming (NSF), something I had known nothing about.

Those who are somewhat familiar with permaculture might see NSF as a type of that. I think it actually is quite different, because permaculture seeks a permanent environmental solution, while NSF seeks to reclaim land by taking it through a sequence of events, changing it in ways that emulate natural sequences.

NSF restores areas of distressed fertility. Plants are grown that can rebuild the soil and retain it, so it will not wash away. Often this means starting up forests in high areas. Downslope land is rebuilt in much the same way, using species that can grow on the land as it is. Marsh and wetlands are intentionally developed with the help of leaky weirs, which allow water to pass down the streams, but at reduced speed. As NSF slows the flow of water, more of it gets into the water table and there is less erosion.

Experiments with NSF started over forty years ago with Peter Andrews, an Australian farmer ( To deal with repeated droughts, he tried techniques he developed. Unfortunately, his neighbors got upset. He slowed the water in creeks, which they thought would leave them with less. And he encouraged weeds to grow, including thirsty stands of willow, which his neighbors thought was just wasting precious water without any benefit.

Peter Andrews’ story is a saga, with a number of ups and downs. Ultimately, many farmers of New South Wales, including all of his adjoining neighbors, were converted to NSF. Despite the fact that it was developed in the middle of land hit by one of the worst droughts in history, the farms that used the system have remained continually productive. Their sheep and cattle had plenty to eat and drink, while those of farmers not using NSF died of thirst and starvation. And so, the system is spreading to other parts of Australia.

In the course of this story, I was struck by the fact that Andrews stressed the importance of weeds. This was not just the native sort that farmers did not want, such as willows. It even included invasive species. The reason is that when the soil is degraded, the plants we might want to grow there often will not survive. We need to get roots into the ground to build the soil back up (which has a side benefit of drawing carbon dioxide out of the atmosphere). If non-weed plants will not reestablish the soil, weeds have to do it.

Andrews points out that weeds do not take over permanently. They will only remain until they are crowded out by other plants. A field that has been cultivated until it is no longer fertile, for example, could go over to dandelions, knotweed, brambles, and who-know-what else. When trees start to grow, those weeds will disappear. And all the while the soil is built up, restoring it to be whatever it will be, whether that means a meadow, an orchard, or a wildlife sanctuary. Andrews also stressed biodiversity, which is important for a healthy environment.

The form of the land may be changed, as well, to slow the progress of water. Even slopes are encouraged to have “steps.” The steps, areas of nearly level land on a slope, slow the progress of water down, reducing erosion, and retaining water.

Though I did not see references to it in Andrews’ videos or material written about him, it has become clear to me that we really need to revisit the question of how we see weeds, alien species, and invasive species. To be brief, a weed is a plant that is where we do not want it. An alien is a plant from some other place. And an invasive species is an alien that spreads as a nuisance.

A weed might be a sugar maple seedling that comes up in the tomato bed. Alien species include peach trees, and if a volunteer peach happens to be too close to the foundation of a house, it is also a weed. (Yes, with climate change we have volunteer peaches in Vermont.) Lamb’s quarter and tilapia are invasive in some parts of the world. So are garlic mustard, kudzu, and Japanese knotweed, and though each can be a nuisance, each also has possible benefits.

In a time of climate change, we may have to change our views about what we encourage or discourage to grow. If we get to the point that the only way to rebuild the soil is to allow weeds to grow, then they are no longer weeds.

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