Monday, March 22, 2021

A Slow Motion Carnage

The dried brownish seedheads of Andropogon sp face off with the white fuzzy spikes of Cogon grass (I. cylindrica)
The scenery that lazy afternoon seemed almost pastoral to human eyes, a landscape of tall trees and grasses waving in the breeze. But in reality, such a depiction was quite deceptive.

Grasses and other plants mostly live in slow time. We see them as a green backdrop to the fast movements of the animals that depend and live with them, but they are of course living things. They are dynamic entities, not passive observers. They  fight and cooperate with others and die messy deaths just like animals, albeit at a pace that is glacial relative to our own fast paced lives. They can in fact be every bit as vicious and aggressive as animals.

The forces arrayed. Red arrow shows main cluster of Cogon grass
In this case, the location is Lakes Park in Fort Myers, FL. The combatants are a native grass (Andropogon glomeratus) on one end, and a world-conquering invasive (Imperata cylindrica) at the other end. The situation is dire for the native stand, with the cogon grass massed in an extremely dense cluster next to it. In the photograph above, the erect light-green blades of I. cylindrica are to the right, with its main boundaries marked by the red arrow. To the left are individuals of A. glomeratus, their seedheads rising above the fray like flags set out for battle. Unlike the cogon grass, the native grasses exist in a looser grouping with more forbs in it.

Beautiful seedhead of I. cylindrica
It is easy to tell the two apart. The seedhead on Cogon grass is much slimmer and almost pure white. It is fluffy in texture and quite distinctive. I've seen videos of entire fields of the cogon grass flowers, and they are simply beautiful, notwithstanding the fact the species is a weed in almost every single place where it has taken hold.

Anthers and stigma of I. cylindrica
The battle is joined when the I. cylindrica rhizomes encounter the roots of the native grass. The rhizomes of cogon grass comprise up to 60% of the total biomass of the plant, and not only are their sharp tips capable of piercing through the flesh of their adversaries, but they also inhibit germination and growth of nearby plants through the excretion of allelopathic chemicals. In addition, cogon grass roots are strong competitors for soil nutrients such as phosphorus. 

Cogon grass rhizome
But their most effective attribute is that they quickly allow cogon grass to push into nearby territories as they produce shoots along their length. Established stands of cogon grass can have from 3 to 11 tonnes per hectare of rhizomes, supporting 3 to 6 million shoots.

The battle is joined
It is a silent battle that is waged for nutrients underground, and for light and space above - one whose ferocity and destructiveness is undetectable to passing human beings.

But in the end, this is probably going to be less of a battle than a carnage. The dense cluster of cogon grass will slowly expand into the A. glomeratus territory, following in the wake of the underground rhizomes. When I peered into the area that still had native forbs and grasses in it, I could see numerous cogon grass blades pushing up from the soil (see photo below), the underground rhizomes moving relentlessly outward. 

I knew that if nothing were done about it, that these small blades of grass would ultimately engulf the remaining natives. The doomed natives would put up a fight, but I thought that the next time I visited the park that I would most likely see a much larger mass of white seedheads, and perhaps only a few scattered individuals of Andropogon left. 

The young yellow-green blades of I. cylindrica pushing into enemy territory


  1. I enjoy your informative posts. Had to close my eyes in horror at places in this one.

    1. Thanks! I enjoy writing about the topic. I admit I never thought about the competitive interactions of these species as a "battle" until I saw that the inflorescence of both resembled flying banners, as if from two opposing armies.