Plant Water Needs

How much to water your plants

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How much water do my plants need?


Determining plant irrigation needs can be a complex process that takes such factors as climate, rainfall patterns, plant size and type, density and microclimate into consideration. For the purposes of this workbook, we will use the ET method.


Estimating Water Requirements Using

Evapotranspiration Rates (ET)


Evapotranspiration, or ET is the combined effect of water evaporating from the soil and being used ( transpired) by the plants. The method assumes that all moisture by evapotranspiration will be replaced through irrigation.

You will need the following information:

  • The area of the plant in square feet.
  • The Species Factor of the plant.
  • The ET Rate for your area.


Area = π ( 3.14) x Radius squared = plant square footage. (rounded)   [example images below]


Plant species factor


The species factor indicates high (.8), moderate (.5) or low (.2) water use, and is used to account for differences in species’ water needs and broken down by plant type. In established landscapes, certain species are known to require relatively large amounts of water to maintain health and appearance (e.g., cherry, birch, alder, hydrangea, rhododendron), while others are known to need very little water (e.g., olive, oleander, pomegranate, hopseed, juniper). These numbers are highly variable, and detailed information on studies of 1,800 different species can be found at . These values are presented in Part 2 (WUCOLS III). Most garden books also provide this information and it is often represented with a water droplet: A full water droplet is high, half full is moderate, and an empty drop is low. Most fruit trees are moderate water users.


Plant Species Factors

  • Very low < 0.1
  •  Low 0.1 - 0.3
  • Moderate 0.4 - 0.6
  • High 0.7 - 0.9


For the purposes of this document, we will simplify plant types/ factors as such:

  • Low: .2
  • Moderate: .5
  • High: .8

Sacramento's Weekly ET Rates


Jan - .39

Feb - .56

March - .93

April - 1.28

May - 1.71

June - 2

July - 2.17

Aug - 2

Sept - 1.43

Oct - 1

Nov - .53

Dec - .39




Use this formula to calculate weekly plant water requirements:


.623 ( conversion coefficient to change inches to gallons) x weekly ET rate x plant species factor x plant square feet =


For Example: An 50 square foot peach tree’s weekly water needs in July.

.623 x 2.17 x .5 x 50 = 34 gallons a week (number rounded)

How to measure:


Measure from the middle of the truck to the furthest branches in the drip line.

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