Certainty

• Only charismatic hucksters who promise to seed clouds and do a dance for a fee ever guarantee they can make it rain in a certain location at a certain time. Everyone else is subject to complex worldwide weather patterns that may or may not produce rain. For the most part, gardeners can depend on having a sufficient amount of tap water to give plants on a schedule that suits the plant. Tap water allows gardeners to grow water hungry plants in regions where rain does not provide enough water.
  
  

Collection

• Gardeners have the option of saving rain in containers. The methods of collection range from simply leaving open containers out in the rain to sophisticated drainage systems. Some municipalities impose restrictions on residential rain collection, a consequence of the intricate water rights agreements among states. Gardeners also collect household tap water via greywater, which is any household water initially used for any purpose any other than the toilet. Greywater is not as versatile as rain, but it will extend the use of tap water for the garden.
  
  

Chemicals

• Municipalities treat tap water in some manner to make it potable and to provide health benefits. Typical water additives are chlorine, chloramines and fluoride. Tap water may pick up chemical contaminants as it travels through pipes to the faucet. The concentration of chemicals and minerals in tap water is too low to adversely affect the majority of garden plants. Chemicals may cause a change in the soil pH levels, although you may monitor and compensate for the changes.
  
  

Cost

• Watering your garden with rain as it falls is free. The out-of-pocket costs of tap water are whatever fees and taxes residents pay. In some instances, residents pay a flat fee for water and in other instances, residents pay for water based on metered use; some pay nothing. Additionally, municipalities may restrict residential use of tap water for gardens and impose fines and penalties on anyone who violates the restrictions. The cost of rain water increases with storage and disbursement schemes. Costs to collect, store and distribute tap water from its source are shared by a large segment of the population.