Question
If roundup is used on the knees, how long would it take before it would kill the tree?

Answer
. If you want to get rid of the cypress trees then apply a herbicide called Roundup to the fresh cut knee and the roots will transport the herbicide to the tree and other roots and kill the tree. I would get an old paint brush and paint the stump with the Roundup straight from the container. Throw the brush away after you finish.  

The time will depend on the size of the tree. IF you want to speed this up cut a slit around the trunk of the tree cut into the wood and put the round up in the slit. This will be picked up by the tree and the death will be much faster. I would apply the roundup this spring as the new growth starts thie will be moved up into the tree faster in the early spring.