When it comes to trees and ponds, one of the most commonly associated species is the weeping willow. Its scientific name, Salix babylonica, might suggest a Middle Eastern origin, but it actually comes from near-tropical parts of Northern China. Today, the weeping willow can be found growing all across America, and in many other countries as well. They have long, slender and wispy branches that are densely covered with thin, flat leaves. They get their common name, 'Weeping Willow', from the sound rain makes as it drips off the tree's many leaves, as if the tree is crying.
These trees love water, and they have very thirsty roots. It is for this reason that they are so strongly associated with ponds and are often found growing near bodies of water. As a non-native species, it is quite possible that many weeping willows you have seen were deliberately planted, rather than propagating through natural means. They can be quite useful when it comes to strengthening shorelines, as their roots form extensive networks and may seem drawn to water like a magnet. This can help hold a sandy lakeside together and prevent it from being washed away into the water during storms.
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Despite their beauty, these trees can have a downside. They can be troublesome if they are too close your liner-bottom pond. The problem is that their roots are strong enough to penetrate through your pond's liner and cause your pond to leak, and unlike other causes of leaks, this kind doesn't have a fix-and-forget type of solution. This is because weeping willows are a type of tree known as a phreatophyte, which means they need to have their roots in a permanent water source. So even if you cut away the offending roots and repair or replace the liner, the problem is going to happen again, and it will likely happen sooner than you might expect.
When keeping a weeping willow on your property, It is important to understand that their roots follow the water, they don't seek it out. If a plant's drip-line, meaning the perimeter of its branches, comes to the edge of a pond, it will find it to be a reliable water source. Even if the pond is outside the drip-line today, that line may eventually extend beyond the edge of the pond as the tree grows. You could end up having to choose between the tree and the pond. Even if the pond doesn't leak, water will condense on the outside of the liner which, making the water readily available to the plants, and promoting the likelihood that the roots will continue growing right through the liner.
Another thing you should aware of is that willow roots are especially aggressive. These trees can sprout new roots thru their bark if they find water. There is also a lot we don't completely understand about plants in general. Do they have a memory? Do the roots communicate with the leaves? Experience suggests that they do, even if we may not be able to explain how it works.
So simply digging up the roots and trimming them way back from the pond is not going to eliminate the likelihood of the tree just regrowing them back and piercing your fresh liner like they did the first time.
What about a barrier, you may be wondering. A solid object that the roots cannot penetrate should keep our liner safe from the tenacious roots of the hydrophilic willow, shouldn't it? Perhaps something like concrete cloth...
Well, you are thinking in the right direction, but I would not use concrete cloth. Concrete will wick water through it, attracting thirsty roots. Not to mention that it is in the nature of concrete to crack. After all, that is why expansion joints are used when pouring slabs and sidewalks. The joints are to confine and control where the cracking occurs. Even if you can't see them, there are still going to be microscopic cracks in it. Don't think you can just make the concrete super thick, either. Even at 660' thick, Hoover Dam still leaks water! Luckily, there are no willow trees on the other side of the dam, because microscopic root hairs can enter such cracks and eventually penetrate the concrete 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 so 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 impenetrable barrier, then grow down.
When doing this, it is also wise to dig a few vertical holes that are deeper than the bottom of the root barrier. The holes go between the barrier and the trunk of the tree. Sink a 4" pipe in them and fill them with gravel to allow deep water penetration, then install a bubbler or drip-emitter in there so the water can go deep into the ground; the roots will follow. It is important to water heavily, but not too frequently.
Have you ever noticed lawns with trees that have roots coming up to the surface? Perhaps you've hit these roots with your lawn mower or tripped over them in the dark. The reason this happens is because lawns have a shallow root system, and must be watered frequently. Seldom does this water penetrate lower than six inches into the ground. With all that water so close to the surface, tree roots are encouraged to come up to the surface to get it; that is where they will find a permanent water source. It is better to water lawns 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.
Willows are not the only tree that can cause problems for a pond owner. Other trees such as birch, alder, and poplars including aspen, cottonwood, and the like are all phreatophytes. Because of this, we don't use them too close to a pond. If you are looking for a weeping or pendulous type of tree, try some of the weeping fruit trees. Another good choice is one of the several weeping conifers that are available. Do your homework, you'll find some nice substitutes. If you're not sure or would prefer to consult an expert before making a decision, give us a call and we'll be happy to help!
Landvista Aquascapes provides Pond & Water Feature Design, Installation & Maintenance -Repair services for South New Jersey Homeowners