Beginners Guide: Using Plant Species Factor For Watering

Different plants placed in the same landscape may have different water requirements. This is easy to understand when comparing a cactus to a fern. But when comparing African daisies (Osteospermum) to Lavender (Lavandula) it is probably not so easy to tell. Plants use water differently and they all don’t respond to weather the same way. As a result, before selecting plants for your landscape it is important to use a plant species factor to understand how much water we need and also how best to group our plants to maximize water use.

Plant Species Factor

The easiest way to understand evapotranspiration is to think the opposite of rain. ET is the amount of water that evaporates from the soil and plant surface plus the amount of transpiration through the plant.

Temperature, solar radiation, humidity and wind velocity all affect ET on a daily basis. Knowing when plants need water and knowing when they are full is the first step to a healthy thriving landscape. ET is almost always discussed in terms of a reference crop, alfalfa or grass. When it is reported today’s ET was .2 or this weeks ET was 1 inch consider this is for alfalfa ETr or grass ETo. The plants in our landscapes may need more or less water depending on several factors one important factor being the plant species.

Reference ET is a measurement of water use for a specific grass or crop. The plants we plant often times will respond differently to weather and water than alfalfa or grass. This is where the plant species factor becomes important.

Knowing your plant species factor allows watering for a specific plant. By multiplying the reference ET by the plant factor we determine the water requirement of the specific plant.

Here is a link to a site that has the most comprehensive database of plant factors for California landscapes. It even breaks it down by zones in California. If you have information on websites providing this information for other locations in the United States please share them in the comments section. You would think this would be easily found, but the information is not readily available.


One of the best water conservation strategies we can use is grouping plants with similar water requirements in the same areas of our landscapes. This is called hydrozoning and it is important because we set our irrigation controller run times based on the water requirement of the plant needing the most water. Going back to the original example – if we had a cactus and fern growing in the same area of our landscape being water in the same zone, the cactus would receive way too much water in order to keep the fern alive. 

One of the many benefits of drip irrigation is the ability to water a variety of plants on the same zone by running tubing through the landscape and using different emission devices for each separate plant. In this example you could use a 0.5 gph emitter for the cactus and a 4 gph emitter for the fern and not overwater as much. You can still practice this same concept when establishing hydrozones. Precision irrigation methods are developing more each day as we understand the value and cost of water. In addition, precision irrigation helps create healthy beautiful landscapes.

There are many factors to consider to determine how often and how long to water your landscape. At times the amount of information to evaluate seems overwhelming.

Four times a week for 20 minutes will probably keep everything alive, but the financial cost of water and what this does to the landscape or worse yet, your paying customers landscape is just not worth it. Taking extra time to carefully calculate and consider all the factors will pay back in both dollars saved and a more aesthetically pleasing landscape. After all, the landscape is our signature on our homes.

If you’d like to read more helpful articles about drought tolerant landscaping and water conservation please follow me @H2oTrends on Twitter.


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