Growth rate is a function of water. If you water saguaro well (but not enough to rot the roots) they actually grow reasonably quickly for a large cactus. But, o

Growth rate is a function of water. If you water saguaro well (but not enough to rot the roots) they actually grow reasonably quickly for a large cactus. But, over-watering can cause them to split so it's a very fine line. This is one of the reason saguaro (and other cactus) have ribs. So they can expand and contract as they store/use water. In nature they grow when it rains, which is not that often in their native climate. Their roots go out quiet a ways, fine roots go out around as tall as the plant is, but they are surprisingly shallow. A large specimen can weigh several tons. This large shallow root system allows them to capture much shallow monsoon rain. When we moved them, we took off all roots leaving only the large base roots maybe 2 or 3 feet from the trunk. We would have to brace them for a few years before they established enough of a root system to hold themselves up.