Fan palms come in a number of different varieties, and each variety has a different type of root system. The fan palm that is most common across the United States is the Washingtonia robusta, or "Mexican palm," which grows to around 100 feet tall and develops a spread of 15 feet wide. This is a big tree that is best suited to wide-open spaces, although it grows well indoors while young.
Fan palms have adventitious root systems. This means the roots develop in an unusual position, sprouting from the stem of the tree or from leaf tissue. Large numbers of small, thin roots sprout continually from the base of the trunk, which is called the root initiation zone, and remain the same size in diameter.
Root Ball System
The roots of the fan palm are thin and fibrous and typically form a clump or network called a root ball instead of the primary root system that other plants have. The fan palm primary roots die off and as new roots sprout from the root initiation zone they become part of the root ball. The ball system means the tree's roots do not take up much space, and you can transplant the tree easily as a result.
It is best to retain as much of the root ball as possible when transplanting the tree, although the roots regenerate quite well. A root ball of at least 1 to 2 feet radius from the trunk is necessary to transplant the tree successfully, and root pruning is not necessary. If you have to cut any of the roots away, the percentage of roots that will regenerate will depend on the length of the remaining root stub, so it is best to keep as much of the root as possible.
The Mexican fan palm's roots respond well to a moist, well-drained environment. If the soil is too wet, the roots may begin to rot. Plant fan palm trees 6 to 8 feet apart to prevent the root balls from entangling in the soil. The best soil to use is one that contains clay or loam, and it may be acidic or alkaline.