Even gardeners in very wet regions find they need to irrigate garden plants during dry spells. Most home irrigation systems are connected to wells or municipal water supplies. Although city and well water is perfectly adequate for plants' needs, well water is often hard and can leave salt residue in the soil. City water usually has chlorine in it, which is harmful to plants and kills beneficial microorganisms in the soil. Rainwater is healthier for plants because it is relatively free of impurities and hard water minerals.
According to the University of Edinburgh's Institute of Cell and Molecular Biology, rainwater is a rich source of nitrogen that is soluble through the biological processes of plants and soil. During a thunderstorm, lightning interacts with moisture to fix atmospheric nitrogen into the water. Rainwater also collects nitrogen from particles in the air caused by industrial pollution. The North Carolina Department of Agriculture & Consumer Services points out that nitrogen is the element plants have the highest need for. Nitrogen helps foliage grow lush and green because it is the basis of photosynthesis, and it is an essential component of each stage of plant growth and development.
Water that contains a lot of calcium and magnesium is considered hard water. These minerals are collected by water as it passes through the ground. Hard water causes a white or yellow crust to form on the soil surface, and it can make a fine coating when sprayed on plant surfaces, according the University of Saskatchewan Extension Division. Plants' leaves can become discolored from too much hard water, and it can cause their leaves to drop off. Hard water also negatively affects root growth and development. Rainwater does not contain these minerals, and it helps wash them out of the soil as it soaks in. In the garden, salts get pushed down out of the root zone, and the increased root development makes plants more drought resistant. Even houseplants benefit from an occasional watering with rainwater or melted snow to flush excess salts from the soil.
One downside of rainfall is that it can cause a flush of weeds, particularly in gardens that are watered with drip systems or soaker hoses. Weed seeds sitting on the soil that don't get water from irrigation can easily germinate when the ground gets wet in a rainstorm, especially in the spring and fall when most annual and perennial weeds start growing.