Answer: Glyphosate is absorbed through the foliage and moves through the plant to the roots where it actually kills the plant. The plant must have enough foliage to absorb the chemical or it will not be effective, it must also be in active growth or the chemical will not be absorbed. Applying it in early fall is usually most effective because at that time of year the plant is naturally storing energy in its roots and will move the chemical to the roots most effectively.
But, you probably want to get rid of your clump of pampas grass (Cortaderia selloana) sooner rather than wait until next fall. It does grow to a large size when mature! Certainly remove any plumes as this is where the seeds form and it spreads easily by seed if conditions are right.
Repeated cutting of the foliage can help weaken the plant, then allow it to grow and spray the regrowth foliage in mid to late summer. Or, you can try to dig out the crown and roots and then spray any regrowth. Digging it out is a really big job. You will probably need to use both mechanical and chemical controls to remove it completely.
Good luck with your project!
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