Milkweed growth rates vary greatly. Started at about the same time. The equatorial Calotropis grows quickly while the temperate Asclepias is much slower.

From the growth you may assume the root of Calotropis will be bigger. However the proportional mass of the Asclepias is expanding at a quicker rate. The additional carbohydrates in the root will allow it to become dormant in the winter. Calotropis will freeze and die next winter.

Both benefit the monarch butterfly and other pollinators.