Once plants are in the ground or in containers, the most important item they need for survival is water. However, due to chemical treatments and additions, tap water is not always the best choice for plants. Although there are more options for choosing the best water to grow plants indoors, there are options for outdoor plants as well.
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According to University of Minnesota Sustainable Urban Landscape Information Series, as long as it is not in an area with high air pollution, rainwater is one of the best choices for watering plants. In areas that allow for rainwater collection, this is an efficient and green method for watering plants. Companies such as Home Depot and the RainXchange have systems that you can buy, while Earth Easy has instructions on building a rainwater collection system.
Reverse Osmosis Water
If you have acid rain in your area, another option for watering your plants is reverse osmosis water. Reverse osmosis (R.O.) is a process that removes chemicals and contaminants from tap water by running the water through a series of filters. According to the University of Minnesota Sustainable Urban Landscape Information Series and the Water Quality Association, once you have bought the equipment, this is an inexpensive source for good water for plants. R.O. systems come in full-house units or units that fit under the kitchen sink. They also come as units that attach to the kitchen faucet.
Distilled water has chemicals and minerals removed. According to the Water Quality Association, using distilled water is one of the better choices for watering plants. However, according to University of Minnesota Sustainable Urban Landscape Information Series, "Distilled water has also been shown to be damaging to some plants."
Professor's House explains a simple method for removing the chlorine in tap water. Fill a container with tap water, set it outside and leave it for two days. This will allow the chlorine to dissipate from the water. Store the water in jugs. Make sure the water is at room temperature before watering plants to prevent shock to the plants' roots.