Among the worst enemies of a vinyl lined pool is nutgrass, a perennial grass-like weed that can penetrate right through the liner causing leaks. Nut grass seeks out the moist, poorly drained areas of a yard and grows faster in hot weather than does normal grass.
Nut grass is not actually a grass. It is a sedge, and extremely difficult to control. The leaves are thicker and stiffer than grass and the stems are solid and triangular. Grass stems, on the other hand, are hollow and round. It can also be identified by its flowers, which are commonly yellow or purple.
The plant is a perennial, meaning it comes back year after year, and it grows in the spring and summer.
The main distinguishing feature of nut grass is the root system. The roots, called rhizomes, can reach more than a foot underground. The roots also have small starchy tubers attached. These are known as nutlets, and give the plant its name.
The root tubers make the plant so difficult to control. They can remain active in the soil for years unless they are physically removed or treated. Buds on these tubers sprout and grow to form new plants.
The most effective way to prevent nut grass is physical removal. The soil beneath a vinyl lined pool should be dug up at least a foot and the nutlets removed. Remove about a foot of earth from the pool installation site and replacing the dirt with clean fill. Unfortunately, this is not a practical solution for many pool installations.
Chemical control has limited success.
Weed killers containing glyphosate as an active ingredient show some efficacy against the stalk and some of the root structure. Although this treatment will kill the plant at the surface, most of these treatment products show limited killing strength against the tubers. When the tubers are fully grown and intact, they will spawn new nut grass stalks. Glyphosate doesn’t usually move from the leaves through to the tubers. It is what is called a post-emergent herbicide Pre-emergent herbicides, which are intended to kill the tubers, are a little more successful.
Some users report success with products containing dichlobenil (Casoron). It is available in both dry and liquid forms. It is a selective herbicide, meaning that it will kill perennial grasses and not necessarily every plant it encounters.
To determine the amount required, it is necessary to know the size of the pool area. It is also necessary to go about three feet beyond the perimeter of the pool to account for the long root structure of the nut grass. Read the manufacturers instructions to determine dosage. For example, Casoron 4G, so named because it contains 4% dichlobenil, is effective for use beneath vinyl lined pools to control nut grass. With this strength, it should be applied at a rate of seven pounds per 1000 square feet. For most pool dimensions, that works out to between two to five pounds.
Immediately after the final grading has been achieved, use a tiller or a rake to mix in the top two inches of soil, and water with a fine mist from a hose or lightly sprinkle with a watering can. Using a hand-operated spreader to distribute the herbicide is ideal. The material should be raked to ensure that it gets well mixed with the soil. The treated area needs to be saturated with water to get the herbicide to dissolve. As it dissolves, it begins leaching into the surrounding area to form an herbicidal barrier. After pool installation has been completed, twelve to eighteen inches of perimeter should be cleaned of grass and the soil treated. It is recommended that the perimeter be treated annually.
Tubers underneath this herbicidal barrier will continue to attempt to produce new nut grass plants.
However, these new plants will die off as they germinate into the herbicidal barrier.
Herbicides on the market to remedy nutgrass.