Rinse 4 cups of fresh rhubarb stalks under cold water and chop into ½-inch pieces, discarding leaves.

Wash and zest 4 fresh lemons, then juice them, straining to remove seeds.

Combine 4 cups of water, 1 to 1.5 cups of sugar, chopped rhubarb, lemon juice, and zest in a medium saucepan and bring to a boil.

Reduce heat and cook for about 20 minutes, stirring occasionally until rhubarb is tender.

Remove from heat and mash the rhubarb mixture gently to release more flavor, then cool for 10 minutes.

Strain the mixture through a fine-mesh strainer into a bowl, pressing down on solids to extract liquid.

To serve, mix concentrate with water in a 3:1 ratio or store in an airtight container for up to 5 days.