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Keep Trees Off of Your Pond Dams
The cornerstone of artificial ponds and reservoirs is the dam. This structure impounds water, manages discharge, and can even be a signature aesthetic feature. Proper sizing and construction of the dam is obviously important, but so too is maintenance to keep them functional, safe, and strong. One aspect of maintenance that is often overlooked is keeping shrubs and trees from growing within the footprint of the dam.
A misconception of earthen pond dams is that they are waterproof. Even well-built dams with good clay soil still pass groundwater through them, they just slow and redirect the groundwater to move underneath the base of the dam. Core trenches are designed to contain highly packed clay through the center of the dam extending at least 12 inches below the foundation of the dam. This core provides the least permeable barrier within the dam and encourages percolated ground water to travel underneath the dam’s foundation. This helps slow water loss, but also keeps water from moving along the seam between the original ground level and the foundation of the dam.
One of the most important steps in prolonging the integrity of a new pond dam is establishing and maintaining a healthy growth of grass. Each plant pushes roots into the soil anchoring the plant to the slope and subsequently reinforcing the soil against runoff and other forms of erosion. Once the grass is established, it is important to mow the grass regularly to prevent woody plants like shrubs and trees from getting started.
The biggest dangers posed by woody plants, especially trees, is that their roots dig deeper into the soil. This can introduce routes of water flow through the core, increasing water loss through the soil and the chance of developing leaks. Especially if a tree dies, its roots decay leaving an even larger void for water to move. Shade produced by trees can also reduce grass growth on dam slopes, increasing erosion of exposed soil during rain events. The worst-case scenario with large trees on dams is a strong storm with high wind striking the dam after the ground is saturated from heavy rains. Trees blown over during storms often tear large holes in the soil from their roots being pulled from the ground. Sometimes the damage is only superficial and can be repaired at the next convenience, but downed trees in sensitive areas around spillways, drains, and the crest of dams could result in dam failures during extremely heavy rain events.
If a pond dam has already started growing trees, there is a window of opportunity to correct the problem. Trees less than about 6 inches in diameter at breast height (4.5 feet from the ground) can safely be cut down. Roots from shrubs and trees up to this size usually are not extensive enough to pose significant risk to the integrity of the dam. Cut the trunks as close to the ground as possible to allow mowers clearance over them. Some trees, especially willow, are difficult to kill and can regrow from cut trunks. For these resilient species, following trunk cutting with an herbicide (use an aquatic version if applying on the water-side of the dam) applied to the stump can increase the chance of fully killing the tree. Any trees larger than 6 inches at breast height should be left uncut, just prevent any more trees from establishing on the dam. Cut trees and brush can be used for firewood, creating fish habitat, or be mulched.
Trees are magnificent. But they, like aquatic plants, fish, animals, and people, become a nuisance when they go places they are not supposed to be. In this situation, removing trees is just a practical matter of protecting a critical structure from potential damage. Be sure to stay ahead of them before they get too large to safely remove.
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