By personifying both the chainsaw and the pampas grass, the poem's speaker hints that the struggle he describes—violent force versus calm persistence—might appear within human society, not just in the battle between humanity and nature.
Right from the start, the chainsaw is a violent, shady character. Even before it's plugged in, it lies in wait "grinding its teeth," as if anger is its natural state. It "knock[s] back a quarter-pint of engine oil" like a cantankerous drunk at a bar. When the speaker turns it on, its nasty character becomes even clearer: with its "perfect disregard" for human flesh and its "bloody desire" to chew up whatever comes in its path, the chainsaw sounds practically psychopathic. It's a killing machine, and it seems to love its job. All these images of uncontrolled rage, violence, and binge drinking have led some readers to interpret the chainsaw as a symbol of stereotypical violent male dominance in particular.
By contrast, the pampas grass is a placid, gentle creature: it "sun[s] itself" like a cat (though it also selfishly takes the "warmth and light" that would otherwise go to "cuttings and bulbs" at its feet). It's showy and dramatic, setting out "footstools, cushions and tufts" to decorate its surroundings. However, it also has "twelve-foot spears," a reminder that it can defend itself in its own way. While those spears "swoon[]" when the speaker applies the chainsaw's blade to them, they also spring right back up in mere weeks, donning a "new crown" that shows the grass is the true ruler of this garden.
In personifying these nemeses, the speaker also sets up a contrast between two ways of being in the world: aggressive and active versus passive-but-persistent. The latter way, the poem suggests, tends to win out in the end.