Irrigation Interval
(the time between waterings)

Knowing how many days to irrigate
Garden Tip
TB1120
Technical Bulletin Series
Watering too often will mean that the soil is already very moist. If your irrigation volume is set properly, a short off-interval will likely mean ponding or run-off.

The reason that moisture moves faster through a soil in the first hour of irrigation; IS THAT WATER IS REPLACING AIR. When the Interval is too short, there is insufficient air to replace. The water movement then slows towards the hydraulic conductivity rate [HCR].

Waiting too long before the next irrigation is far from the right way to reduce water expense. Our landscapes when healthy are beneficial to the air we breathe as well as our mental health and state of mind.

Think about the last time you drove down a residential street where the landscapes were filled with suffering plants and weeds. This impacts the health and happiness of those who live there.

An Irrigation Interval that matches the soil, plants and climate is the right way to have the best of both beautiful landscapes and watering efficiencies.

Phase 1
If, when the soil was irrigated; the rate and duration of application matched the soil - the Moisture pattern should look similar to the image above several hours after the irrigation finished.
For the purpose of this discussion , lets assume this was the case. Moisture has permeated to a good depth. Though the area shaded in lavender does not have sufficient moisture to support the plant, the other three areas do. As the soil dries (from both transiration and evaporation) during the "Irrigation Interval" period - this changes.


As the soil's moisture is used (during the Interval Period) the Moisture Profile looking at a crossection will begin to enter these next two phases.


Phase 2

Here two of the three moisture viable zones still hold enough water for the plant root system to be able to ustilize. Moisture percentage at "Wilting" (lavender) is between 5% and 35% depending on the type of soil. Clay (35%) holds a lot of water but does not release this moisture to plant roots easily.

As the moisture continues to diminish, the it's Profile will enter the 3rd phase, where less than 33% of the rootzone now has adequate moisture needed to supply the plant's needs.


Phase 3
If the climate at this time demands significant transpiration from the plant, and soil moisture is at phase 3; parts of the plant may soon begin to be sacrificed in order to keep the rest of the plant in a survival mode. Often you'll see leaves begin to "flag" or fail to hold their shape.

So, it is more critical during the warm season to avoid extending the Interval Period beyond phase 3. Usually this occurs when days are longer than 12 hours and temps are greater than 80 °F. When temperatures exceed 90 °F, you can be confident that transpiration demand is high.
Watering Too Soon
is really is NOT an answer
The root system needs to breathe.
respiration requires soil with
good oxygen content - DRYER

Getting the Interval Period Right is Important!

Here are some ideas:

A.
    As you look at the images reflecting the drying of the soil, you will notice that areas further away from a drip dry quicker than those that are closer. As you begin to learn the correct interval period, using a moisture meter to see how the soil is drying will give you clues as to when the soil moisture profile has reached stage three. Make sure your testing soil at least an inch deep. The surface dries very quickly.
B.
    Writing observations into a notebook or word document regarding various plant conditions (current month & schedule) will make sure that what you are learning - you can save and take advantage. After a season or two of observation and adjustments - you'll find the best Interval Period. Don't forget that an accurate Interval can only be achieved when the Moisture Profile following irrigation is correct.
C.
    Be Clock-Timer conscious. March - June & November are key times during the year to re-set your Interval Period. From One day a week to Four or more; if your not making an adjustment - how is it possible to keep the Interval Period accurate?

Begining an irrigation too soon not only wastes water but interferes with plant health. If your irrigation water application flow rate is aggressive, a short off-cycle will likely cause ponding and moisture profile distortion.

Begining an irrigation too late assures plant stress. The I&S Matching Calculators provide a recommended irrigation interval or frequency, and this does try to take soil composition and plant type into consideration.
NOTE
If you are begining the process of converting from short run-times and shallow watering - to Deep Watering; This should begin during early spring or late fall.
The root system will naturally be shallow due to the previous irrigation scheduling. It takes time for these roots to grow deeper into the new moist areas. Avoid beginning this process in the summer, as you need to increase the off-interval as you add run time. This will cause many of your plants stress and perhaps failure.
Evenly distributed moisture during an irrigation helps to assure the the Irrigation Interval Period is correct throughout the entire root zone.

Dripperline (with emitters fabricated right into the tubing) offers a simple method for assuring even moisture distribution. It comes in ¼ inch and ½ inch diameter. The ¼ inch has options of 6 or 12 inch spacing between emitters. The ½ inch diameter offers 12 and 18 inch spacings. Simply hook one end up to the feeder line and plug the other end. Space the rows appropriate for the size plant. Secure their position if you like and cover them with organic or rock mulch. Each emitter puts out approximately 1 gph.

Dripperline with "inline" emitters

Closeup of a single emitter in a dripperline
Spacing for large shrubs and trees
Typical spiral of dripperline around a tree

By creating a "spiral" of Dripperline around a shrub or tree, and spacing each row by between 6 and 12 inches, you will have created an excellent water and moisture distribution pattern all throughout the rootzone surface area. With the slow rate feed of these emitters, the water soaks deep, and total water use is minimized.
Ideal irrigation distribution
most closely

approximates rainfall!
Dripperline; economical and effective
STANDARDIZE!
All Emitters on the same zone should have similar flow rates!
When moisture is not evenly distributed there is little chance of setting the correct number of days during a week to water. One area's needs are different from anothers - because the amount of water they get per square foot is not the same.

Whichever type of emitter you select, you really need to stick with that type
throughout any single zone or valve. This is what is meant by
"Standardizing your Emitters"!

Have you taken a stroll through your landscape to inspect your drip irrigation?
As you try to set your clock to the best number of days per week, You should also check for adequate emitters count and coverage
as well as for plugged emitters.