The other day someone asked one of the smartest people , I know about Willows, next to the pond. Bernie Kerkvliet is the owner of Skyline Ponds in Lake Arrowhead Ca. And his Knowledge is endless. Thanks Bernie,For allowing me to share this.Weeping willow’s are beautiful but are a pain if they are too close to water. They are a phreatophyte which means that they have to have their roots in a permanent water source. It is important to remember that roots follow water, they don’t seek it out. If a plants drip line is at the edge of a pond or will be as it grows, it will find it to be a reliable water source. Even if the pond doesn’t leak, water will condense on the outside of the liner which will make more water available to the plants.
Willow roots are especially aggressive and the can also sprout new roots thru its bark if it finds water.
With concrete cloth, you are thinking in the right direction but I would not use concrete cloth. Concrete will wick water through it and attract roots to it. It is the nature of concrete to crack. That is why expansion joints are used when pouring slabs and sidewalks. The purpose of the joints is to control the cracking in places where you want it to. Even if you can’t see the cracks, there are microscopic cracks in it. Hoover Dam still leaks water thru it as thick as it is. Microscopic root hairs con enter these cracks and eventually penetrate and crack it further.
A better solution is to buy a root barrier. They come in panels that slide together and come in different sizes. The panels attach together do you can make the barrier as large or small as you want it. When it is finished, it is like you planted the tree in a plastic container with no bottom. The idea is to have the roots grow horizontally until they hit the barrier, which is impenetrable, and then grow down.
When I do this, I will also dig a few vertical holes that are deeper than the bottom of the root barrier. The holes are between the barrier and the trunk of the tree. I the put a 4″ pipe in them and fill them with gravel to allow deep water penetration. I then put a bubbler or drip emitter in them so the water can go deep in the ground and the roots will follow. It is important to water heavily but not to frequently.
Have you ever noticed lawn trees and roots that come up to the surface? These are the roots that you hit with a lawn mower or trip over. Lawns are watered frequently because they have a shallow root system. Water seldom penetrates more than six inches deep. With so much water close to the surface, tree roots are encouraged to come to the surface to get it. That is where they will find a permanent water source. In a lawn area it is better to water more heavily and less frequently to encourage grass roots to follow the water down deeper and keep tree roots from getting to close to the surface. When done properly, this can save a lot of water because it reduces evaporation and provides more area for roots to collect water.
Trees such as willows, birch, alder, and poplars such as aspen, cottonwood, and the like are all phreatophytes and because of this, I don’t use them to close to a pond. If you are looking for a weeping or pendulous type of tree, try something like some of the weeping fruit trees. One tree I like to use is Camperdown Elm, Ulmus glabra “Camperdown”. It can get about 15′ tall with beautiful weeping branches that reach the ground. If you are in an area that is prone to Dutch Elm Disease, you may not want to use this one. There are also several weeping conifers available. Do your homework, you’ll find some nice substitutes.
I hope this help