September 3, 2018
By Kevin Vlietstra
Ground preparation is critical when installing any pool. For an inground pool, depth and grade, soil types, and water tables are some of the main concerns. After the area where the pool is being installed is excavated, several cubic meters of excess dirt waste soil is hauled away. For above-ground pools, the site is typically excavated to level the ground and clear away minor obstructions where the pool is being installed. In most cases, not much soil is actually removed. Instead, the majority of the soil remains on site.
After the ground is prepared and the dust is settled, it is time to install the pool. If there is any vegetation remaining on the ground, many believe it will die off naturally after the liner is installed due to the large surface area, tremendous amount of weight, and lack of light. These are all contributing factors that would typically wipeout all sorts of plant life and it is relatively sound logic, especially when considering what happens to grass after placing a large rock or landscape ornament on a healthy patch of lawn. That said, gone are the days of simply installing a pool liner on top of compacted level ground.
Enter nutsedge, or more commonly referred to as nutgrass. It is a perennial, grass-like plant that wreaks havoc on vegetation, crops, and other types of agriculture by taking away resources from other types of growth. When untreated or undertreated, this plant can poke through basic landscaping materials such as bark or plastic ornamental mulch. In some instances, nutsedge can penetrate asphalt, rock-covered blocks and beds, and even vinyl pool liners.
With respect to pools, nutsedge usually grows between the liner and the wall, and will eventually penetrate through the liner. It is also capable of growing directly under the liner, too. Regardless of its location, there are no viable recourses to remedy a hole in a pool liner caused by the growth of nutgrass.
Although nutsedge and nutgrass are often used interchangeably, it is important to know this plant is not a type of grass, but rather a sedge. Grasses typically have hollow, soft, and rounded stems that can be easily pushed to the ground and are capable of bouncing right back. On the other hand, sedges grow much more upright and have solid stems that are triangular in shape, which make them stronger plants. Additionally, sedges have much thicker and stiffer leaves compared to grasses.
Sedges tend to grow at a much faster rate, too. As a result, their stronger structure and vertical growth allows these species of plants to easily pierce through several types of surfaces and materials.
|Avoiding other nuisances behind the vinyl pool liner|
On the interior of the pool, the stains can be diminished for a period of time via super chlorination; however, they will eventually re-appear, as the source of the stain originates from the backside of the liner. Low lying areas or those with high water tables may be more prone to contain these micro-organisms. The installation of a polyethylene barrier between the vinyl liner and the walls and floor of the pool may provide a barrier that prevents this type of staining from occurring.
The portion of the dye that is exposed on the surface can be bleached by chlorine; however, new dye will migrate to the surface and it will appear as if the chlorine is having little or no effect.
The bacterial micro-organisms can become established on either the water side or backside of the liner. Growth on the water side may occur if free chlorine levels are allowed to fall below 1.5 parts per million (ppm) at the same time that organic matter and bacteria have accumulated in the water. Super chlorination at this stage will rid the water of the contamination, but if the dye has already penetrated below the surface, staining tends to linger indefinitely.
Growth on the backside may not take place directly on the liner, but rather on some other material in contact with the liner such as soil or a backing material like Styrofoam, felts, or taping. Even though an anti-microbial agent is incorporated into the vinyl formulation, the dye can migrate from unprotected components and stain areas well beyond the point of infestation. If there is a lot of pink dye visible on the backside or any backing material, it will certainly be the source of the problem.
If the liner is replaced, all contaminated materials must be removed and the entire pool shell (floor and walls) must be disinfected with a liquid chlorine spray or other suitable disinfectant.
Special problems are presented by locations that have high water tables which continually bring water loaded with micro-organisms to the backside of the liner. Using disinfectants at these sites may be ineffective, as they will be quickly washed away. A possible defense may be some type of barrier layer; either a plastic sheet, perhaps polyethylene between the pool shell and liner or a barrier coating of some kind applied directly to the pool shell.
Some common weeds can be removed by tilling, scooping, or simply pulling them out; however, these methods are non-effective at eradicating nutsedge. This is because a small bulb will develop at the end of the plant’s root structure, deep beneath the soil. These bulbs are often referred to as the ‘nut’ of the plant (hence its name), but are actually called tubers. Though typically small, depending on the type of nutsedge, tubers can grow as big as 13 mm (0.5 in.) in diameter. It is the tubers that make nutsedge removal difficult. Whether they are attached or separated from the plant structure, they will fully develop into a new plant.
Tubers can retreat deep into the soil, upwards of 0.3 m (1 ft) below the surface. At this depth, the difficulty in removing nutgrass and its tubers by hand from the area where a pool is going to be installed is practically impossible.
The most effective method of eliminating nutsedge in areas where the plant is established and actively growing is to remove the soil from the pool installation site and replace it with clean fill. Unfortunately, however, this is not a practical option for most homeowners or pool builders.
The next best option is to chemically treat the ground using an off-the-shelf weed killer that contains glyphosate as the active ingredient.
When used as directed, these herbicides are doused over the plant and will kill the stalk and some of the root structure. Though this treatment will kill off the plant above the surface, these treatment products may only minimally impact the plant’s tubers, especially if they are fully grown. When tubers are intact, they are free to grow and spawn new nutsedge stalks. To help prevent future growth—after removing what has grown above the surface—products containing dichlobenil should be used as a ground treatment.
Available in dry and liquid forms, dichlobenil is a ‘selective’ herbicide, which means it will only kill off perennial grasses and weeds, rather than all vegetation it comes in contact with. More specifically, the herbicide is only applied to the top layer of soil, so established shrubs, trees, and other plants with deep root structures will not be affected by its application when used in accordance with the label directions.
Knowing the size of the pool area is important to determine the amount of dichlobenil that will be required. It is also important to factor in an additional 0.6 to 0.9 m (2 to 3 ft) beyond the pool perimeter. This additional treatment around the area where the pool is being installed will account for the long root structure of the nutsedge.
After following the manufacturer’s directions with regards to the application rate, the material can be uniformly scattered. Using a hand-operated spreader is ideal. After it is applied, the material should be raked to make sure it gets mixed well into the soil. When using the crystalline version of dichlobinel, the treated area needs to be saturated with water to start eroding the dichlobenil crystal. As it erodes, the crystalline eventually leaches to a small surrounding area to form a herbicidal barrier. A liquid application of this herbicide will also provide this barrier, but its effects are not as long-lasting.
Keep in mind, tubers, deep in the soil, will start to produce new nutsedge plants. Once the barrier is established at the soil surface, the new plants coming up though the soil will die off as they germinate into and through the herbicidal barrier.
Using a ground preparation treatment should be considered if the bordering area of the property, or even the surrounding neighbourhood, is known to have nutsedge infestations. Reapplication should be performed around the pool perimeter on an annual basis to ensure new growth does not permeate through the ground.
The most common types of nutsedge are yellow nutsedge (cyperus rotundus) and purple nutsedge (cyperus esculentus). They are recognized by many agriculture organizations as an invasive species. In fact, nutgrass is a weed in more than 90 countries and is one of the world’s most invasive weeds based on its distribution. Nutsedge thrives in moist, nutrient-rich soil and is a highly adaptable plant. When given enough time to germinate, it can tolerate drought conditions, poorly maintained soil, and even mountainous areas. It is native to North America, but of sub-tropical origin and has spread north into temperate regions. Though prominent in the southern U.S., some genera of the plant have started to develop in the northern states, along both coastlines, and into Canada. Other minor types of nutsedge plants are already present in several Midwest and northern states.
With files from the University of California Agriculture and Natural Resources (ANR), the Department of Botany University of Hawaii at Manoa, Alberta Invasive Species Council, and the United States Department of Agriculture. For more information, visit http://ipm.ucanr.edu/PMG/PESTNOTES/pn7432.html, http://www.botany.hawaii.edu/faculty/carr/cyper.htm, https://abinvasives.ca/wp-content/uploads/2017/11/FS-YellowNutsedge.pdf, and https://plants.usda.gov/core/profile?symbol=cyro#.
Kevin Vlietstra is the technical director and regulatory specialist with Haviland Pool and Spa Products. He has been working in the recreational water industry for more than 20 years. He can be reached via e-mail at firstname.lastname@example.org.
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