Giant sequoia grow so large because they live a very long time and grow quickly. To thrive, giant sequoia require a great amount of water, which they primarily receive from the Sierra snowpack that accumulates over the winter months and soaks into the ground when it melts. Because they need well-drained soil, walking around the base of giant sequoia can cause them harm, as it compacts the soil around their shallow roots and prevents the trees from getting enough water.