Cedar, more accurately known as juniper, can be tricky to control. It is a very aggressive woody plant that grows here in Texas as well as in other places. It is often considered invasive because, if left unchecked, it can quickly take over a pasture, choking out any grass growth.
Along with quickly taking over a pasture, it is a very thirsty plant. They can soon suck a spring dry leaving you with no water. I knew a rancher near here that when he started removing cedars on his place, he had a spring start producing water again.
While controlling cedar is possible, it is not a one-and-done type of thing. It will take many applications to get this little pest under control.
