Understanding Irrigation

Observation, Not Memorization

One of the most common questions I hear is, "How often should I water?"

I wish there were a simple answer, but there isn't.

Every landscape is different. Soil type, weather, season, sun exposure, plant size, and irrigation system all influence how quickly the soil dries and how much water a plant needs. Even your own goals matter. Some gardeners are happy to let native plants naturally slow down during the hottest months, while others prefer a greener landscape and are willing to use more water to maintain it.

Rather than trying to memorize a watering schedule, learn to observe your garden. Seasonal schedules and smart irrigation controllers are valuable tools, but they are only a starting point. Your soil and your plants will tell you whether your irrigation is doing its job.

What Are We Growing?

The first step in understanding irrigation is understanding the plants you're trying to grow. Different plants have evolved under different conditions, so they naturally have different water needs.

Native Landscapes

Most native plants have evolved to survive long periods between rainfall. They generally benefit from deep, infrequent irrigation that encourages roots to grow deeper into the soil.

Many native plants can also tolerate periods of water stress. Some gardeners are happy to let their landscape naturally slow down or become less lush during the hottest months to conserve water. Others choose to provide additional irrigation to maintain a greener appearance. Neither approach is right or wrong—it simply depends on your goals.

Vegetable Gardens

Vegetables are different. They grow quickly, produce flowers and fruit in a single season, and many have relatively shallow root systems. They need more frequent irrigation and more consistent soil moisture to produce a healthy harvest.

Unlike many native plants, water stress in vegetables doesn't just affect the appearance of the plant—it often affects the quality of the food we eat. Lettuce may become bitter, root crops can become woody or split, and fruiting crops may produce fewer or lower-quality vegetables.

This is one of the tradeoffs we make as gardeners. We're often willing to invest more water in our vegetable garden because the reward is fresh food. That investment is worth it to many of us.

Although the details are different, the basic principles remain the same. Every plant should be watered deeply enough to reach its active root zone, then allowed to dry as much as that particular plant can tolerate before watering again. Understanding those principles is far more useful than memorizing a schedule.

Choosing The Right System

Once you understand what your plants need, the next step is choosing the right irrigation system for the job. Different systems deliver water at different rates and are designed for different purposes. Understanding how they work will help you choose the right system, set appropriate run times, and avoid many common irrigation problems.

High-Volume Irrigation

High-volume irrigation delivers a large amount of water in a relatively short period of time. Traditional sprinklers, spray heads, and rotors are all examples of high-volume irrigation. These systems are designed to quickly wet a larger area.

Low-Volume Irrigation

Low-volume irrigation delivers water slowly over a longer period of time. Drip emitters, inline drip tubing, bubblers, and many micro-irrigation devices are all examples of low-volume irrigation. By applying water gradually, these systems allow moisture to soak into the soil where the roots can use it.

Using Both Systems

Many landscapes use both high-volume and low-volume irrigation, and that's perfectly okay. The important thing is that they are generally operated on separate valves (watering zones) because they deliver water at very different rates.

A high-volume system may only need to run for a few minutes, while a drip system may need to run for an hour or longer to apply the same amount of water. If they're connected to the same valve, one system is likely to receive too much water while the other receives too little.

Understanding GPM and GPH

One of the biggest sources of confusion is the difference between gallons per minute (GPM) and gallons per hour (GPH).

LOW-VOLUME IRRIGATION

2 GPH

💧💧

2 gallons
in ONE HOUR

HIGH-VOLUME IRRIGATION

2 GPM

💦💦

2 gallons
in ONE MINUTE

The only difference is the unit of time. Drip irrigation is measured in hours because it applies water slowly. High-volume irrigation is measured in minutes because it applies water much more quickly.

The goal isn't to water longer or shorter. It's to deliver the right amount of water to the root zone.

Different irrigation systems simply get there in different ways.

How Much Water Do Plants Need?

It's one of the most common questions gardeners ask, but it's difficult to answer because plants don't measure water in gallons or minutes. What matters is whether the water reaches the roots.

The goal isn't to apply a certain amount of water. The goal is to wet the entire root zone, then allow the soil to dry as much as that particular plant can tolerate before watering again.

Water The Roots Not The Plant

The goal of irrigation is to deliver water to the soil where the roots can use it. The goal of irrigation is to wet the entire active root zone. The deeper the roots, the deeper the water needs to penetrate.Whether you're growing vegetables or native plants, focus on thoroughly wetting the root zone rather than simply making the surface look wet.

If you're using an overhead irrigation system, avoid watering during the heat of the day. Much of the water is lost to evaporation before it can soak into the soil. Watering in the evening or early morning gives the soil time to absorb the moisture and makes your irrigation more efficient.

Plant

Trees

Large Shrubs

Small Shrubs

Perennials, Ground Covers, Cacti

Vegetables

Approximate Root Zone

3-6 Feet

2-3 Feet

18-24 inches

12-18 Incches

Varies by crop and stage of growth

Newly planted trees and shrubs have much smaller root systems than established plants. Water to the depth of the existing roots, not the size the plant will eventually become.

Irrigating Vegetable Gardens

Vegetables follow the same principle as every other plant: water the soil, not the calendar. The difference is that most vegetables have shallower root systems and require more consistent soil moisture to produce a healthy crop.

Germination

Seeds have no roots yet. The goal is simply to keep the top inch of soil from drying out.

During hot weather, this may mean watering several times a day for only a minute or two. You're not trying to soak the soil. You're keeping the seedbed consistently moist until the seeds germinate.

Seedlings

Once seedlings emerge, begin encouraging the roots to grow deeper by thoroughly wetting the upper six inches of soil before allowing the surface to begin drying.

Mature Vegetables

As vegetables grow, so do their root systems.

Leafy greens (lettuce, spinach, etc.): about 6 inches
Root crops (carrots, beets, radishes): about 12 inches
Tomatoes, peppers, squash and other larger vegetables: 12 inches or deeper

Covering the soil with mulch helps retain moisture, keeps soil temperatures more consistent, and reduces evaporation.

Just like any other garden, don't guess. Check the soil.

Check Before You Water Again

About 30 minutes after irrigating, push a piece of rebar, a long screwdriver, or a soil probe into the ground to the depth you intended to water.

Here in the desert, our rocky soils don't always make it obvious where the wet soil ends. Instead, I use what I call the cake test.

Think about checking whether a cake is done. When you insert a toothpick, a clean toothpick means the cake is finished. If batter sticks to it, it needs more time.

The same idea works for your soil.

If the rebar or screwdriver comes out with damp soil clinging to it, there's still moisture in the root zone. If it comes out mostly clean and dry, it's probably time to irrigate again.

Your soil - not the calendar - should tell you when it's time to water. As you become familiar with your soil and irrigation system, you'll begin to recognize how long your landscape holds moisture during different times of the year.

Build Proper Tree Basins

Trees benefit from wide watering basins whenever possible, allowing water to soak slowly into the soil over a much larger portion of the root system instead of running away from the tree.

Water at the Right Time

Whenever practical, irrigate during the evening or early morning.

Avoid overhead irrigation during the hottest part of the day when evaporation is highest. Watering when temperatures are cooler allows more moisture to soak into the soil before it is lost to the atmosphere.

Automatic Irrigation Is a Tool

Automatic irrigation is a tool, not a replacement for observation. People often tell me they don't want an irrigation system because they enjoy watering by hand.

That's great...until life happens.

Illness.

Travel.

A busy week.

An automatic irrigation system provides a safety net. You can always turn it off and water by hand when you want to, but it's there when you need it.

Properly programmed automatic irrigation systems also tend to use less water than hand watering because they apply a consistent amount of water and reduce the temptation to overwater.

Smart irrigation controllers take that one step further by automatically adjusting watering schedules based on weather conditions. They still need a good starting schedule and occasional adjustments, but they can help reduce water use while keeping your landscape healthy. Begin by determining how often your soil dries to the desired depth using the rebar test. That becomes your baseline schedule. Many smart controllers can then make seasonal adjustments based on weather conditions, but it's still a good idea to check your soil occasionally and make changes if needed.

Your irrigation controller is a tool. Your soil is always the final authority.

Don't have an in-ground irrigation system? That's okay. Hose-bib timers, including Wi-Fi models, can automate a simple drip system connected to an outdoor faucet. They're an easy way to add the convenience of automatic irrigation without installing a new irrigation system.

Before You Buy

Many local water agencies offer rebates on smart irrigation controllers, drip irrigation supplies, hose timers, and other water-saving equipment.

Before you purchase anything, check with your local water provider or visit CV Water Counts to see what rebates are available in your area.