Question
QUESTION: What chemical can I use to kill the roots or knees of bald cypress trees that are ruining my foundation and watering system.  Will the chemical kill up to the tree or if I cut down the trees will the roots stop growing

ANSWER: There is no chemical (herbicide)that will kill the roots or knees that will not also kill the tree. You can install barriers along the foundation to stop the roots but it will be difficult or impossible to install barriers for the watering system. Sounds like the tree is too close to the house. Bald cypress can grow large  50 to 70 feet in height with roughly 30 foot spread with the roots spreading to about 45-60 feet from the trunk. The "cypress knees" usually only develop when grown in or near water for most of the year. In wet soil conditions, the bald cypress will form sections of root growing above ground to gather oxygen. Since they grow in water the roots will seek water and grow toward a good water source.
If the roots are ruining your foundation I am sorry your best bet is to remove the tree. The roots will sprout from a cut tree. I would treat the stump when you cut the tree with a herbicide called Roundup. The Roundup will be taken up by the stump and transported to the root system killing the system. That way there will be no new sprouts coming from the roots. Roundup can be found at any nursery/ garden type store. Roundup will kill any plant so make sure you do not get the mixture on any leaf of a plant you do not want to kill. I would get an old paint brush and paint the stump with the Roundup that way a spray will not be carried by the wind to adjacent plants. Throw the brush away after you finish.


---------- FOLLOW-UP ----------

QUESTION: Would it be just as effective to use Roundup on the roots?
We did cut down one tree but the root system is still growing. the tree stump was grounded up and is not longer there.  In other words could the process be effective in reverse?  

Answer
The Roundup will work if you can get the herbicide into the roots. If the roots are above ground you can cut them and apply the Roundup. Or if the roots are sprouting new growth you can spray the new sprouts foliage and they will transport the Roundup to the roots. It is not as quick as treating the freshly cut stump but it will work. You more than likely will have to treat new sprouts from time to time since all of the roots will not sprout at the same time. OR if the area is part of your lawn you can just keep mowing down the sprouts and over time the roots will use up their food supply and die. (this will take some time) Beware if you spray the sprouts keep the spray off any green plant not wanted to be killed.