Peel the ginger with the edge of a spoon and slice it thin. Zest the lemon until you have 1 tsp, then juice it to get about 45 ml.
Pour the coconut water into a 1.5 L / 50 oz blender jar. Add the wheatgrass powder, maple syrup, ginger, lemon juice and zest, then top with the frozen banana and pineapple.
Blend on low for 15 seconds, then on high for 45 to 60 seconds, until no ginger flecks are visible and the smoothie is as thick as soft-serve but still pours. Scrape down the sides once if the fruit sticks.
Taste and add a small squeeze more lemon if it's flat. Pour into two 350 ml / 12 oz glasses and drink within 15 minutes.