This grower is fairly sure of this plant identification.
This mint seems to love being in a pot and not in soil with other plants.
Mentha spicata (Spearmint). This is a fast-growing mint with dark crinkly leaves. It will thrive in a sunny or partial-shaded position.
They are very frost hardy, and need sunshine and rich soil with lots of moisture.
Grown for their aromatic foliage, which is both decorative and used as a culinary herb.