。
The plant called friendship needs constant watering.
Literal
Friendship [quotative-という] plant [target-には] constantly water [object-を] give [must-ねばならない].
A beautiful metaphor: friendship is a plant (植物) requiring constant care. 絶えず (たえず, 'incessantly') emphasizes the ongoing obligation. ~ねばならない is a formal/literary variant of ~なければならない — the ね derives from the classical conditional attached to the negative stem. 水をやる is the standard expression for watering plants.