Okay. Did some reading.
Have you heard of Isoprene?
It is released by trees and shrubs possibly as a way to deal with heat stress. Also A. Donax.
Plants do not produce it at night.
Fosmidomycin(which is available for purchase) inhibits isoprene synthesis from dimethylallyl pyrophosphate.
Isoprene accounts for 1/3 of hydrocarbons released into atmosphere.
"After release, isoprene is converted by free radicals (like the hydroxyl (OH) radical) and to a lesser extent by ozone into various species, such as aldehydes, hydroperoxides, organic nitrates, and epoxides, which mix into water droplets and help create aerosols and haze."-wiki
Many plants emit isoprene but am not certain of which ones.
So if in fact isoprene is affecting extractions then destructive distillation or pyrolysis might need to be performed.
Read up on "cracking" in chemistry.
Possibly this is affecting the yields or at least the effectiveness.
Perhaps a nice cool setting in Autumn or Winter on a night with not a cloud in the sky would be the precise method to harvest these reeds.