Joshua Tree National Monument. Joshua trees can grow from seed or from an underground rhizome of another Joshua tree. They are slow growers; new seedlings may reach a height of 10-20 cm in their first few years, then only grow about 10 cm per year thereafter. The trunk of a Joshua tree is made of thousands of small fibers and lacks annual growth rings, making it difficult to determine the tree's age. This tree is not very sturdy because of its shallow root area and top-heavy branch system, but if it survives the rigors of the desert it can live to two hundred years of age. The tallest trees reach about 15 m tall.