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Dealing with nutgrass: Proper ground preparation can help avoid vinyl liner issues

By Kevin Vlietstra

Ground preparation is critical when installing any pool. Thanks to plants like nutgrass, gone are the days of simply installing a pool liner on top of compacted level ground.
Ground preparation is critical when installing any pool. Thanks to plants like nutgrass, gone are the days of simply installing a pool liner on top of compacted level ground.

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.

Rearing its ugly head

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.
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.

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
It is highly likely that pink blotches, which can appear on liners of all colours, are caused by an indelible pink dye that is excreted by bacterial micro-organisms.
It is highly likely that pink blotches, which can appear on liners of all colours, are caused by an indelible pink dye that is excreted by bacterial micro-organisms.

Grey/black staining
Grey/black staining can occur as a result of micro-organism activity on the backside of the pool liner. These micro-organisms can produce dyes that are soluble within the plasticizers used to make the vinyl liner pliable. The microbial dye becomes visible on the pool side (or inside) of the liner as it wicks through, creating an unsightly, irregular-shaped blotch.

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.

Pink staining
It is highly likely that pink blotches, which can appear on liners of all colours, are caused by an indelible pink dye that is excreted by bacterial micro-organisms. Similar to grey/black staining, this dye is highly soluble in the plasticizers used in flexible polyvinyl chloride (PVC) pool liners that it can easily migrate through the entire thickness of the liner.

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.

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