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How (and How Much) to Water

Too much water is the most common cause of decline in landscape trees and shrubs, either through directly killing plants or making them more prone to disease. Too much watering can result in fertilizer runoff into storm drains and pollutes waterways. Maintain plant health and protect water quality by watering correctly.

> Water properly
> How much to water
> Factors that impact water needs


Water properly to promote plant health and avoid runoff

Use an irrigation system controller. A controller (aka "timer") hooked up to your irrigation system helps you save considerable water and money. It is the most useful tool for determining how much and when to water. Controllers also make it easy to increase or decrease water when needed. 

Water earlier rather than later. To avoid the heat of the day, water plants during the early morning rather than during the heat of the day. Water before 9 am or after 7 pm.

Use drip irrigation to deliver water only where needed. 

Set multiple start times on slopes to avoid runoff. For example, water for 10 minutes at 6 am and then again at 7 am, rather than 20 minutes all at once)

Check soil moisture before watering. 

Water at the drip line. The majority of tree roots are in the top 2 to 3 feet of soil, and tree roots generally spread 2 to 3 times beyond the drip line (more when water is available; less when water is restricted).

Water deeply and infrequently. In the absence of rain, most trees and shrubs benefit from a once-a-month, thorough watering.

Create hydrozones. Place plants with similar water needs in the same "zone" (or valve). Conventional sprays and drip cannot be on the same valve. Here are common hydrozones: 

• Lawn – pop up sprays 
• Low water plants – infrequent water except for long hot spells
• Mixed perennials and shrubs – depends on needs of plants
• Trees – deep, infrequent water
• Vegetable beds – needs change year to year depending on crops grown
• Orchard – deep, infrequent water
• California natives – infrequent water (some prefer no water once established)
• Succulents – infrequent water
• Potted plants – daily or twice daily water in heat of summer
• Hillside  – multiple drip irrigation waterings per day to allow each to soak in


How much to water

Many people turn on their irrigation systems full tilt as soon as the rainy season ends. But over the course of the summer, a landscape's water needs will vary depending on what's growing, weather conditions, day length, soil, and other factors. By adjusting your irrigation schedule weekly to give your plants just what they need, you’ll save water and money while maintaining a healthy garden.

Adjust the amount of water based on what plants really need
This can be helped by the use of smart irrigation controllers and rain shut-off valves.  

Newly planted trees and shrubs need more frequent watering for up to two growing seasons until they become well rooted.

Established plants can be weaned
to tolerate less frequent watering. Proper weaning develops deep roots and helps plants better tolerate drought.

Don't overwater. More trees are lost to over-irrigating than probably to any other cause. Over irrigation combined with poor drainage especially leads to tree death.

Notice the signs. Most plants show signs of moisture stress.  
• Too little water: leaves may appear smaller than normal, wilted, brittle, folded, misshapen, or dull gray-green. (Important: plants can sometimes show these signs when they are over-watered.)
• Too much water: leaves may drop, appear limp, or become wilted or yellow (this could also be a nitrogen deficiency). 
• Learn more about solving plant problems

Do the math. Some plants, especially edible crops, come with specific irrigation instructions such as "provide one inch of water per week." One inch of water is 0.68 gallons per square foot. To add 1 inch of water each week using a drip irrigation system, calculate irrigation based your emitter capacity/spacing and factor one gallon of water per square foot. For example, if using a 1-gallon per hour drip emitter in a 12 inch pot, you would run that drip for 1 hour per week by dividing the run time into 3 irrigations for 20 minutes each. 

NATIVE PLANTS: Water native plants sparingly. Because they evolved in California’s summer dry climate, once established, most natives prefer little or no water and can be harmed by overwatering. Learn how to water and care for native plants.

EDIBLE OR FOOD GARDENS: This depends on what you are growing, the size of your vegetable garden and other factors. Use this worksheet to calculate how much water you need for your food garden.
Edible garden water calculation worksheet

Sign up for the Marin Municipal Water District’s (MMWD) weekly watering schedule e-mail.


Factors that impact water needs in the landscape 

In order to keep your plants well-watered and minimize your water usage, it is helpful to understand the factors that determine how much and how often to water your plants.

Soil: The type and quality of your soil can have a significant effect on the amount of water it can hold. Generally, the coarser the soil, the less water it will hold. Soil that contains healthy quantities of organic material can hold more water. Learn more about soil.

Planting site: The proximity of buildings and other heat retentive surfaces to the plant can speed up moisture loss, while areas under trees or other shade structures may produce an environment with higher humidity. 

Weather: Temperature, humidity and wind: Weather conditions play a role in the rate of water loss in plants. The higher the air temperature and wind velocity, the more rapid the water loss. The higher the relative humidity the more slowly the rate of water loss. Learn more about managing weather extremes.

Sun exposure/light intensity: The more hours of sun each day, and the greater the intensity of the light, the greater the chance of increased water loss.

Mulch: A layer of mulch over the soil will significantly reduce the rate of water loss.

Plants choice and spacing:
• Gardens with native, drought tolerant, and other no-water and low-water plants use considerably less water than gardens with lawn or other thirsty plants.

• The more closely plants are spaced, the greater the overall plant density. The higher the density of plants, the more quickly the ground can be depleted of moisture.

• Generally speaking, the larger the plant, the greater the amount of leaf surface, the greater amount of water loss (transpiration).