Water at Root Depth

At 4 pm on a July afternoon your hanging basket droops. Before you reach for the can, picture what water does inside a plant.

A plant is a wick. Water enters through the roots, travels up the stems and leaves through tiny pores in the leaves called stomata, evaporating as it goes. That steady pull from the leaves is transpiration, and it's what lifts water to the top of a plant with no pump at all. The leaves also need the water to stay firm: a leaf is held out by water pressure inside its cells, like a balloon, and it goes limp when the pressure drops.

The pull depends on the weather. Sun, warmth and wind make leaves lose water fast; a still, cool night slows it almost to nothing. The roots, meanwhile, can only take water up so fast, however wet the compost.