Fog develops within the marine layer on the Pacific Ocean and then makes its way into the foothills of the Santa Cruz Mountains where the redwoods live. The fog is a crucial part of the redwood ecosystem. It provides 30% to 40% of the water the thirsty redwoods require.
There are several ways a redwood tree gets its water. The first is that the roots reach down into the soil and pull water from there. The second is that the redwood leaves catch and condense fog, dripping it down onto the ground where the roots can pick it up. The third and I think most interesting is that the upper parts of tall redwoods absorb moisture directly from the fog.
How they do this is still something of a mystery, but the leading theory is that upper branches will catch needles, seeds and dust, creating a soil mat where small roots form to capture water from fog and rain. This phenomenon happens more in the northern range of the coast redwoods and not as much down south where we live but it’s thought to happen some with the larger trees in the foggiest of canyons. Another theory gaining steam is that the trees can absorb the fog directly through their leaves.
A mature redwood uses anywhere from 100 to 500 gallons of water every day depending on its size and location. During the winter, redwoods get most of that from rainwater but during the long rainless summers, much of their water comes from the marine layer fog.