Watering your landscape seems like a science, but you will quickly discover it’s more of an art. That is why the best answer to the question “How much should I water my roses?” is – That depends. The reason for this is the variables used to determine how long and when to water. For the engineering minded gardeners, it seems simple. You just need to complete a few calculations like these, and you will know how to program your controllers. The best weather-based controllers today allow for the water manager to make adjustments to the schedule and if it doesn’t, you need to find a different smart controller.
Be aware of these five variables, and you will be on your way to a more beautiful and healthy landscape.
Watering Your Landscape
Soil Type
Different soil types have different water holding capacity and different abilities to accept water. Clay soil, for example, can hold a lot of water, but because it is slow to absorb water it has a higher probability for runoff.
Sandy soil has excellent drainage and accepts water quickly. It’s easy to waste water in sandy soil because water moves past the root zone quickly. Short run times and more frequent watering is needed with sandy soil.
Loam soil is known as a gardener’s best friend because it is an excellent mixture of sand silt and clay soils. A loamy soil accepts water well and stores it well. This type of soil will stay most true to your calculations. However, there are additional variables to consider.
Read an excellent article about soil by Michael Derewenko here.
Slopes
Slopes in the garden can be a fantastic visual enhancement. However, they provide an extra degree of difficulty for irrigation. As the severity of the slope increases so does the challenge to water correctly. The potential for runoff is higher due to the slope. The soil comes into play here as well. The good news is selecting the correct emission device along with the proper run times for your controller makes this a solvable issue.
Take advantage of the cycle and soak feature if your controller has one. Cycle and soak automatically breaks the total time of water up into separate watering schedules that allow water to soak in. For example, if the total run time is 12 minutes your controller (based on the severity of the slope) breaks up the 12 minutes into three cycles of 4 minutes allowing an hour between cycles, so the water to soak into the soil.
If your controller does not have a cycle and soak feature, you can always break the total watering time up to 3 separate start times with shorter run times. You manually provide the cycle and soak for your controller.
Sun Exposure –Shade/Sun/Solar Radiation
Solar radiation, direct sun, and shade dramatically change the temperature of an object that it hits. Typically the difference in temperature between sun and shade stays in the 10 – 15-degree range. Reflective heat can get much more extreme. Ever stand next to artificial grass on a sunny day. They heat you feel coming off the grass is hot and impacts the water use of all the plants around.
Consider the sun exposure your plants receive. Northside or south side exposer is easy. Take the next step and see what shade your plants receive from trees, buildings, or any other overhead structures that throw shade. Then consider any reflected sunlight on your landscape. Windows, artificial turf, parking lots are a few that come to mind right away. You will have to make some adjustments to your runtime calculation once you consider these important factors.
Root Depth
A deep-rooted plant has access to a greater amount of soil moisture than does a shallow-rooted plant, usually allowing it to go longer between irrigations. Below are some root depths of some common garden plants.
| Type | Example | Root Depth |
|---|---|---|
| Cool Season Turf | Bluegrass, Fescue, Rye Grass | 4-6” |
| Warm Season Turf | Bermudagrass, Zoysiagrass | 4-6” |
| Combined Turf | Warm and cool season combined | 4-6” |
| Flowers | Most seasonal color | 4-6” |
| Trees | 12-18” | |
| Shrubs | Roses, Star Jasmine, Escallonia | 6” |
That should give you some help and for more details check out this root depth chart.
Also please remember, newly planted transplants or seeds need more frequent watering because it takes time for the plant’s roots to grow.
Effective Rain Fall
Landscapes are often watered with a combination of rain and supplemental irrigation. Even if you live in Seattle, Washington most years you will need some type of supplemental water for your grass during August and early September. Effective Precipitation (EP) is the amount of precipitation that is actually added and stored in the soil.
If the rainfall is high, a relatively large part of the water is lost through deep percolation and run-off. Especially in sloping areas, heavy rainfall will result in a large percentage of the rainwater being lost by surface run-off.
Another factor to consider is the variation of the rainfall over the years. Especially in low rainfall climates, the little rain that falls is often unreliable; one year may be relatively dry and another year may be relatively wet.
As you can see the answer of “that depends” when it comes to watering plants is a critical answer. Consider the variables above when it comes to watering your landscape or garden, and you will be well on your way to a healthy and productive landscape.