This grower is fairly sure of this plant identification.
Definitely phalaris arundinacea. I think it was bought as a named sub-species, but I can't remember which. Sorry!
Reed canary grass, Ribbon grass, Gardener's garters
Phalaris arundinacea (Reed canary grass). This crop has an minimum optimum growing temperature of 17C and a maximum optimum growing temperature of 25C.
They will prefer a damp position, they grow well n most garden soils that do not dry out in summer.
Ground cover, meadow, fodder