Install an Underliner

• 1). Install an underliner when having your pond put in. This is an extra barrier in addition to the plastic liner that serves as the bottom and sides of your pond. You can use sand, gravel, old carpet, newspaper or commercial felt liners---all work well but commercial felt liners are the most effective.
  
  
• 2). After you line the bottom of the hole with the underlining material of your choice, place the liner in the hole on top of the underliner. The easiest and most reliable liners to use are prefabricated ponds, but you can design your own pond using a garden pond kit, which usually contains an EPDM plastic liner. Form-fit the liner to make sure it reaches into every nook and corner of the hole and the underliner; add some water to help weigh it down as you're working.
  
  
• 3). Fill the rest of the pond and then install edging around the top to help keep the liner in place. You'll need to use heavy materials such as rocks, tile or brick to ensure the liner doesn't shift. Depending on the material you use, the installation of the edging will vary. For instance, you will need to grout tile. If you're unsure about how to implement this step, consider purchasing a garden pond kit that includes the edging so that you will have specific instructions to follow.
  
  

Install a Root Barrier

• 1). Purchase a linear root barrier. These are panels that you install in the ground near the tree to help to direct its roots away from your pond. Root barriers are best installed while you are constructing your pond, but they may be added at a later point, too.
  
  
• 2). Determine the width and depth of the barrier. Root barriers are typically between one and two feet deep; you can have custom barriers made by companies such as Espro if you think you'll need something deeper. Consult an expert to determine the width of the tree's canopy once it is fully mature. If the tree has been there for 100 years you can assume it is mature and use that width to help you determine how much barrier you will need. Add two feet to the width of the canopy and purchase as many panels as you'll need to construct a barrier that wide.
  
  
• 3). Drain the pond using a shop vacuum if possible. If the pond already contains fish or other wildlife, consider covering it with a plastic or canvas tarp to protect it from contaminants while you work.
  
  
• 4). Assemble the barrier panels. Simply slip the male panels into the female panels until they click together. Then, position the barriers at least six inches to a foot away from your pond liner and dig out the installation area.
  
  
• 5). Lower the barrier into the hole with the top of the barrier inclined towards the tree. Make sure the ribbing on the panels is facing the tree and are in a vertical position. Leave the tops of the barriers exposed above the grade just a little to help prevent root overgrowth. Refill in the holes around the panels.