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Water quality plays a critical role in healthy landscapes, yards and gardens, especially during high temperatures, which can lead to plant stress and vulnerability. In peak summer and fall heat, the way in which water is applied to gardens and landscapes can drastically impact chemical interactions with plants. For instance, soil compaction can reduce the holding capacity of water applied directly to the surface of the media, which doesn’t allow for vital minerals and nutrients to reach the soil. Sufficient volumes of water should be able to leach through the plant media, especially in potted plants, to reduce the accumulation of soluble salts that may be in your water source.
It is important to understand the factors that can impact your plants, including the type of water, which influences parameters such as salinity, pH and alkalinity. Each water quality parameter can have different effects on foliage and flowering plants. Many different types of water can be used, including:
Tap water: The majority of city municipal water comes from either well or surface water sources. This water can vary depending on the region, with mineral accumulation being the most important parameter. The Southern Hills Aquifer System in Baton Rouge is known for its “soft” water, which provides high water quality. Other areas may have more “hard” water, which refers to slightly elevated calcium or magnesium that can cause excess accumulation in soil.
Rainwater: This is the most desirable water for plants due to very few natural impurities or contaminants. This naturally occurring water can impact some outdoor landscapes and gardens with minimal effort, but in areas of limited rain, it can also be collected for use. Though rainwater harvesting can be tedious, there are simple designs and user-friendly guides to installing systems at residential or commercial spaces for collected rainwater runoff.
Reverse osmosis water: This water is produced by a membrane filtration process where salts and contaminants are removed, but trace minerals are able to pass through. These systems are a cheaper alternative to water filtration and provide valuable water for plant watering.
Distilled water: This is the result of water that is vaporized into steam and then cooled before collection. The condensation process results in reduced salts and contaminants. This water is typically more expensive and is usually not recommended for plants because of the lack of micronutrients.
Sprinkler spray head for nursery production systems. Photo by M.P. Hayes
To fully understand what elements are impacting the water quality of your current irrigation method, a complete water analysis by an accredited laboratory is recommended. Many university labs, including LSU Soil Testing and Plant Analysis Lab or LSU AgChemistry, provide comprehensive water analysis packages to show parameters that can impact plants and landscapes. This analysis should be done to establish a working baseline and can be done as frequently as needed. After the first analysis, it is a good idea to keep simple water quality test kits on hand for more periodic sampling. This can be done at home on a regular basis to keep a record of water quality fluctuations. Homeowners can monitor irrigation water and log data in a notebook to help detect changes over time in plant health. Four water test parameters are recommended: pH, alkalinity, hardness and electrical conductivity.
pH: The pH of water is a measure of how acidic or basic it is on a scale from 0-14 in pH units. A pH of 7.0 is considered neutral and indicates pure water, while solutions less than 7.0 are acidic and greater than 7.0 are basic. In most media and soil-based systems, pH is an important driver for plant growth because of the impact on the availability of fertilizer and nutrients. Testing water for pH is very easy, with tools being readily available online or at your local pet, hardware or pool supply store. The simplest, most inexpensive tool is a test strip that can be dipped in a water sample and compared to a color chart to determine the pH. These strips can have a broad pH range (0-14) or a more precise range (5.5-9.0), so be sure you are getting the ones appropriate for your water. Additionally, there are pH probes or data loggers that can be used as a handheld device. These are typically more expensive and require calibration for accurate measurements. The ideal pH of water for greenhouse and nursery production is around 5.4-7.0, while specific landscape and garden plants may tolerate different ranges. It’s best to check the targeted pH for specific plants you are growing. Acidic or basic adjustment solutions or fertilizers can always be used to alter the pH to more ideal conditions for your plants.
Alkalinity: The measure of water’s ability to neutralize acids or the capacity to change or resist a change in pH of the growing media is alkalinity. This establishes that a buffering capacity is valuable for plant roots to resist sudden changes due to the pH of fertilizer or media. If alkalinity is too high or low in water, it can impact nutrient availability by reducing pH impact. This is normally due to carbonates, bicarbonates and hydroxides in water. Similar to pH, there are at-home test strips that can be used to measure a range of alkalinity. More accurate and scientific methods for this test include titrations with compounds or digital handheld units, which can be more expensive and time-consuming. Often, high alkalinity water has a higher pH and may require adjustment. The recommended upper limit for alkalinity for both greenhouse and nursery production is 100 mg/L (100 ppm), while other garden and landscape plants should be checked for more specific ranges.
Hardness: In water, the dissolved calcium and magnesium drive the hardness of tap and irrigation water. Lower concentrations of these in water make it soft, while elevated concentrations are referred to as hard. Though not always the case, hard water can correlate to higher alkalinity due to the presence of calcium carbonate. If either calcium or magnesium is too high, it can block the plant’s ability to take up the other critical micronutrients. If residual buildup can be seen on soil media, it may be due to accumulation of these compounds, which also can be present in residue on plant leaves and clogged irrigation equipment. These parameters can also be measured with at-home test strips that are inexpensive and color-indicated. Higher-caliber alternatives like digital handhelds can be purchased for more accurate measurements. Hardness in water for greenhouse and nursery use should not exceed 150 mg/L (CaCO3).
Electrical conductivity (EC): This is the measurement of the concentration of dissolved ions, such as salts, acids or bases, that conduct electrical current in a solution. Testing for this can provide an estimate of the number of ions in irrigation water and, in many cases, is linked to dominant sources like fertilizer. This measurement is reported in millimhos per centimeter (mmhos/cm), millisiemens per centimeter (mS/cm) or decisiemens per meter (dS/M), which are all equivalent units. Other similar parameters (though slightly different) are total dissolved solids (TDS) and salinity. TDS includes all the following minerals, salts, metals, cations, or anions dissolved in water, not just dissolved ions like EC. TDS is also reported in parts per million (ppm). Salinity is the measure of all salts in water, which includes chloride, sodium, sulfate, magnesium, calcium and potassium, and is reported in parts per thousand (ppt). Electrical conductivity is the most practical way to measure salinity or TDS of water, and the preferred method and scale for landscape, nursery and garden settings. This is because higher dissolved ions can affect plant cell membranes by increasing salts and other minerals in the media or soil. This can drastically reduce growth and lead to stress or decrease plant quality over time if not flushed with low-dissolved-ion water, like rainwater. The best method for measuring EC is with a digital handheld meter that is easily calibrated with a two-point solution: pure water and a saltwater solution. Most meters purchased online or in the store will come with a solution for calibration and detailed instructions. Overall, EC greater than 1.0 mmhos/cm may indicate a high salinity and potential hazard for some plants.
The LSU AgCenter has additional information in the form of factsheets for horticulture tips and water quality impacts for various environmental systems. For more information on the water quality specifically, check out the LSU Water Quality Extension Lab homepage at https://faculty.lsu.edu/hayes/index.php or email mhayes@agcenter.lsu.edu.
