Crop Coefficients and Plant Type

How do you know what crop coefficient should be used for each plant type? I have a 1 zone wonder with shrubs and trees on the same zone. I have my crop coefficient set at 70%?

Why is 50% recommended for shrubs and 75% for trees? They are similar plant types. There is a grey zone between tall shrubs and small trees.

Shouldn’t crop coefficient change throughout the year? I thought Deciduous plants and Evergreen plants have different water needs relative to each other based on the season.

You’ll want to look up the crop coefficients if you have the time. In Rachio they’re in Percent but everywhere else they’re not (times by 100 for input to Rachio advanced settings). You’re right deciduous trees have vastly different needs especially in the winter. The coefficients do change over the year but Rachio simplifies it with defaults to be something like annual averages for the USA. I have been known to set reminders to change crop coefficients for my stone fruit trees-spring, summer, late summer, fall, winter. Your university extension may have crop coefficients or plant factors for your local area; these can be different depending on your climate.

Here’s an example extension website with some annual average crop coefficients/plant factors:

Here’s an example of monthly crop coefficients for turf grass:

A search of the Internet will give you decent values for Crop Coefficient for most plants. The sites quoted above should help.

Basically, that just says that trees, with their deeper roots and high foliage, require 50% more water than shrubs. And yes, there’s a lot of gray area here.

Some do change throughout the year, others are fairly constant. Remember that Crop Coefficient is not an absolute value for water required; it is a multiplier of the value of FRET (Forecast Reference crop EvapoTranspiration), which basically is the amount of water evaporated out of a shallow tray per day or week which varies with the temperature, humidity, sunlight, wind speed, etc. So much more is used in summer than winter, meaning CC can be relatively constant and still result in variable rain.

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Crop Coefficient = Kc

This is a quick reference from my Cal Poly notes for PF Plant Factor which is used for a guide.

Plant Type Plant Factor
Tree, Shrubs, Vines, Groundcovers

(woody plants)|0.5|
|Herbaceous Perennial Plants|0.5|
|Desert Adapted Plants|0.3|
|Annual Flowers & Bedding Plants|0.8|
|General Turfgrass Lawns, cool-season (tall fescue, Ky. bluegrass, rye, bent)|0.8 2, 3|
|General Turfgrass Lawns, warm-season (bermuda, zoysia, St, Augustine, buffalo)|0.6 2, 3|
|Home Fruit Crops, Deciduous|0.8 2|
|Home Fruit Crops, Evergreen|1.0|
|Home Vegetable Crops|1.0 2|
|Mixed Plantings|PF of the planting is that of the plant type present with the highest PF|

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