Water Considerations
for
Shrubs and Trees
Garden Tip
TB1110
Technical Bulletin Series
Proper watering or irrigation planning is one of the most important issues for plant health. For a salesperson or advising professional to give advice without adequate questioning of the customer, investigation into their soil composition, plant variety or current type of emitters, any answer provided has little chance of being correct.
HOW MANY GALLONS SHOULD YOU PROVIDE TO
YOUR SHRUBS and TREES?
(how many minutes / gallons)
Remember "how many minutes" varies with the emitter you use.
Most are rated in gallons per hour, so for convenience…
Please consult with a qualified professional to make certain
proper watering is selected for your landscape.
The size of a plant's root system is the critical factor in HOW MUCH water to apply during an irrigation, and if the roots are not large enough for the plant it will suffer. The Garden Calculator Pro-I can quickly answer this for any plant you have, and of course soil composition has a big impact too. As a rule of thumb, it will take about 0.5 gallons of water for every square foot over the primary root system. Example: a rose might have a 2 x 2 primary root system - therefore needs 2 gallons of water. The better this moisture is spread throughout the root area - the healthier the plant. The images below should help you get an idea of where the primary root system should be.

Water applied deep and wide

Water applied in multiple spots
Typical Irrigation Cycle
Watering Amount
Hard Slow Draining
minutes
gallons
Good Loamy Soil
minutes
gallons
Sandy Fast Draining
minutes
gallons
SMALL PLANTS
30 - 60 mins
0.2 - 0.5 gal
SMALL PLANTS
15 - 60 mins
0.3 - 0.7 gal
SMALL PLANTS
10-30 mins
0.5 - 1.0 gal
INTERMEDIATE PLANTS ¥
30 - 45 mins
1.5 - 10 gal
INTERMEDIATE PLANTS ¥
20 - 45 mins
3 - 18 gal
INTERMEDIATE PLANTS ¥
15 - 30 mins
4 - 23 gal
SHRUBS / TREES*
30 - 45 mins
1 - 8 gal
SHRUBS /TREES*
20 - 45 mins
2 - 15 gal
SHRUBS /TREES*
15 - 30 mins
3 - 20 gal
Drought Tolerant**
SHRUBS / TREES
2 per month
Drought Tolerant
SHRUBS / TREES
1 per week
Drought Tolerant
SHRUBS / TREES
2 - 3 per week
DESERT PLANTS
1 - 2 per month
DESERT PLANTS
2 - 3 per month
DESERT PLANTS
1-2 per week
TABLE 1
In order to determine runtime minutes, the above table is an estimation based on using slow rate "one gallon per hour" drip emitters. Hard soil absorbs water very slowly. The slower moisture is applied - the deeper it permeates into the soil. This provides larger root systems and longer lasting moisture.

If you are utilizing high flow rate sprays or emitters, the total gallons will remain the same as long as this does not cause puddling and run-off. The amount of time the emitters would run will obviously be changed according to the flow rate of the emitters used.

¥ A plant in an area slightly sunnier than recommended, or a higher water consumer. Here the amount of water needed is the same as a traditional plant, but varies according to soil type. The frequency that is needed for application is substantially different. See TABLE 1.
* The quantity of gallons for any SHRUB or TREE is also going to vary according to the size of its' CANOPY or SPREAD. Here the Canopy refers to the height and width (whichever is greater) of the Tree, and spread refers to the width and area of a shrub or groundcover type plant. This in turn determines the square feet area of soil under that plant needing irrigation.

** The quantity of gallons for any "DROUGHT TOLERANT" Shrub or Tree is also going to vary according to the size of its' CANOPY or SPREAD, but as this type of plant can tolerate a shallower root system the quantity of water per sq ft. is slightly less. The sq ft. area requiring irrigation can also be reduced.

For "Moderate" water use shrubs and trees consider this area to be about 75% the size of the canopy. For "Desert or Drought tolerant" plants consider this area to be about 40 to 50% the size of the canopy. Example: A drought tolerant tree with a canopy that spread 10 ft. in width would have approximately 50 sq. ft. root area.

Use the Garden Calculator Pro-I or check with a
Nursery Irrigation Specialist if you need help calculating.
With all your shrubs and desert plants larger than 1 foot in height, you should have a minimum of 2 to 3 drips on each plant. This makes certain that you have adequate coverage around the root system, and not just on one side.
Soil Composition type. Clay, sand and loam all have different water holding capacities and drainage rates. Most of the landscapes in the Desert Southwest are very slow to drain, but not all! Know the characteristic of your soil, and in particular the soil where you intend to plant. Slow draining soils should be watered 'less often'.
Using this Watering Guide
Remember that watering frequency and duration is based on many variables. Things like time of year, soil type (sand, clay or loam) and plant location (sun, shade or slope) all play a part in your decision. For best results, get in touch with your soil and experiment!

Try Using the Garden Calculator Pro I
Micro-Climate. Reflected sunlight, slope of the soil, partial sun and wind exposure are a few of many things that will affect water evaporation and well as transpiration. Though run-off is certain on a slope, water infiltration and drainage are still very poor in high-clay soil. Regardless of how quickly the moisture will disappear, the initial volume of water remains essentially constant.
Initial Irrigation Coverage and Volume. Solutions to complex problems like proper plant irrigation begin with first steps. This starts by making a plan for root zone area and depth coverage. Once this is set correctly the ongoing seasonal challenges can be met with success. Here is the point where you rely on your Nursery Plant Specialist to help.
Consider switching to dripper lines, or drip irrigation. Consult a Nursery Irrigation Specialist for specific information depending on your situation. In any event, applying water directly to the root zone (in comparison to sprays) and using surface mulches will reduce watering frequency.
If you decide on a drip irrigation system, you have made a great choice for the long term health and ease of maintenance for your plants. You will need to determine how many emitters and where to place them. In addition to this the run time for irrigation will need to be established in order to provide the right amount of moisture and adequate time for that water to permeate the soil. This decision can not properly be made without knowledge of the soil composition in the area.

The Garden Calculator Pro I
really helps make this easy
The Garden Calculator Pro-I (a free online tool) was designed to help you or your landscape professional evaluate your soil and the resulting emitter placement and emitter density.

This irrigation and soil calculator takes the necessary parameters into account and returns answers that will greatly assist in the long-term health of your plants.

If this type of investigation is beyond what you personally would be interested in doing, it certainly is well within the scope of any dedicated landscape professional to utilize and gain success from the application.