Ferula means "rod" in Latin. Ferula is also the genus of about 170 species of flowering plants in the family Apiaceae, native to the Mediterranean region east to central Asia, mostly growing in arid climates.
The Romans used the hollow light rod made from this plant for various purposes: for walking sticks, splints, for stirring boiling liquids, and for corporal punishment. So the term ferula could refer to various things, including a rod for chastisement. The birch rod however was not called "ferula" but "virga".
Many plants of the Ferula genus, especially F. communis, are referred to as giant fennel, although they are not fennel in the strict sense.