Giant kelp can grow half a metre in a single day

Macrocystis pyrifera, giant kelp, is the fastest-growing marine plant known, adding up to 60cm of new frond in a single day under good light and nutrients. A single plant can climb 30–45 metres from the seafloor to the surface within one growing season, its fronds knitting together into underwater forests that shelter otters, fish and countless invertebrates.

— Wikipedia contributors, Macrocystis pyrifera — giant kelp, family Laminariaceae