Water containing heavy metals, dissolved minerals, and chemical residues can damage plants and irrigation systems. The mineral content of water can alter fertilization requirements, and water pH can alter soil pH.

Location, weather, environment, crop type, and time of year will all influence a grower’s choice in a water source. Zoning and water conservation ordinances may limit water supply in some regions. Reliable water supply is vital to planning irrigation systems for greenhouses.

The amount of water needed will vary depending on the growing system (pots, beds, etc.), environmental factors such as rainfall, and on the growth stage of the plant. The water amount needed can be estimated by calculating 0.3 to 0.4 gallons per square foot of growing area per day.

Drilled wells may require permits and be subject to state regulations, deep wells are ideal. Groundwater quality is highly variable so quality should be evaluated in detail.

Municipal water is a high-quality greenhouse water source but it is costly. It commonly contains fluoride, chlorine, and sodium that may harm plants if not treated. Calcium content of municipal water can alter the fertilization needs and must be considered so plants are not given excess amounts.

Rainwater is a very high-quality water source when collected before contacting the ground. Greenhouse gutters make it possible to collect rainwater that flows off of the greenhouse roof. Debris needs to be filtered from the water, as with most sources, but overall rainwater is very clean and tends to be soft water with few dissolved solids. 1” of rainfall collected from greenhouse gutters can amount to up to 27,100 gallons (greenhouse square footage multiplied by 0.4 to estimate).

Drainage ponds commonly have chemical and fertilizer residues as well as algae necessitating reconditioning and careful quality and microbial testing.

Surface water diverted from rivers and streams can be cost-efficient, but these sources tend to have more issues with water quality. Surface water commonly has high sodium and chlorine levels that necessitate filtration. Road salt, industrial and agricultural chemicals, algae, and pathogens are major contaminant risks to plants irrigated with surface water. The quantity of water will also depend on rainfall.