watering plants in arid landscapes

Watering plants in the desert can be challenging: how often should you water? How much should you water at a time? We are here to help you understand this subject, but as with all subjects, there is no simple easy answer, so we will present all the elements you need to consider when answering questions about watering plants. You will want to pay attention to what kind of plant you are watering, what season it is, what your soil is like, and what method of watering are you using.

what type of plant are you watering?

First you need to consider what sort of plant are we talking about. Not all plants need the same amount or frequency of watering. The following categories are assuming that plants are in the ground and not containers. We will discuss container growing below.

general Landscape plants

Most of the standard, appropriate plants for landscaping in Arizona should be native, though a lot of people have non-native plants that are either adapted to our arid lands, or native to other aridlands elsewhere. We, of course, primarily recommend regionally native plants for your landscape. We are talking about the large trees, shrubs, ground covers, vines, and accent plants which will not need as much water. They are usually referred to as low-water or moderate water plants. In general, an established, xeric (dry growing) landscape should be watered deeply one every week or two (depending on the plants) in summer. In the winter, you can get away with every two to three weeks or even monthly deep watering. The amount of water you apply depends on your soil type and method of watering, but one should deliver the water slowly and soak the ground a few feet deep. If you have very clay soil, or a lot of caliche that doesn’t allow for good drainage, you will either need create drainage by breaking through the caliche when you plant, or you will have to adjust your watering habits to work around the situation, and install species of plants that can grow in such conditions. Succulent plants like cacti, agaves, desert spoons, yuccas, etc. can be watered once a month deeply. Drainage is even more important for succulent plants.

Perennials, annuals, and less xeric plants

There are many annual and perennial wildflowers that can bloom with little water, but most species prefer a little more frequent watering than larger landscape plants, often because their root systems are less deep than larger shrubs or ground covers. Wildflowers are often not low-water using plants, but plants that wait for the wet years in the right seasons to germinate and grow—the years we call “superbloom” years. Their method of dealing with our arid land is to skip the dry season, go to seed, and wait for the next wet year.

Other native plants that aren’t xeric are plants that occupy riparian areas and moist microclimates within our arid lands like canyons. These plants are important to wildlife since they are some of our most nectar-rich flowering plants, or produce seed or fruit that birds eat. They tend to be the pretty plants that we are also attracted to. Remember that if a plant spends a lot of energy on flowering and/or fruiting or seeding, it often requires more resources, like water.

When you plan a landscape, make sure you group these more mesic (water loving) plants together and avoid putting cacti or succulents in a position where they will get too much water from watering nearby plants. These type of plants may be watered every few days to a couple times a week, depending on the species. A FEW flowering perennials and annuals can grow in average landscape watering (discussed above) but may not live as long, or may flower less prolifically than if watered more. Don’t worry about nearby shrubs and trees. Although most native plants are drought-tolerant, that trait doesn’t mean they don’t like extra. Most native species will just grow faster and more vigorously with the extra moisture.

A note on plants that you water more—don’t ever feel guilty about watering plants. When you water a plant, you are not just providing for the plant itself, but the almost countless organisms that thrive because of that plant—from insects and birds to tiny microbes, all important parts of our ecology. Watering plants is one of the few times that our water use is benefiting nature. If you wanna feel guilty about wasting water, feel guilty about flushing toilets. That is a ridiculous waste of water, and in arid lands we should all probably have compost toilets instead of wasting a precious resource to dispose of our feces.

the vegetable and herb garden

An established vegetable and herb garden should be watered much like our flowering annuals and perennials, depending on what you are growing. You should be able to skip a day or two in the summer. But that is assuming you are watering deeply, that you have prepared your soil well, and that you are mulching. A layer of mulch at the top of the soil (without burying plants too deep) will really prevent a lot of water loss. Some people use pure compost, some use straw or woodchips. Some use a combination. But mulch will really help give some time between needed waterings. We do demand a lot more from our vegetable and herb plants, so you must provide them with more than the average plant. Well-fed garden plants are also much more efficient with water than malnourished plants. This goes for all species, but vegetables and herbs tend to become malnourished much quicker than native landscape plants.

Container plants and raised beds

Again, it depends on what you are growing. But plants in containers are generally more demanding of water because pots dry out a lot faster than your soil profile. Some raised beds can retain water well, and the wider the raised bed, the more efficient at holding water it should be. All pots and raised beds should have drainage holes at the bottom. Some raised beds are continuous with the soil below and that is fine. Most plants will rot in a pot that doesn’t drain, as the soil becomes anaerobic—it is not healthy for plants if water stays logged in the bottom of the pot. Cacti & succulent plants are great container plants. They can grow well in restricted soil conditions, and will need feeding seldom or never.

Consider the season

Even if you stick to regionally native plants, some species want less water when they are dormant. This is especially true of cacti and succulents. Some plants are warm-season growers, some are cool-season growers, and some grow year round. What determines these habits are the genetics of the plant, as influenced by the climate it adapted to and evolved in. Plants from Baja California, southern California, or western Arizona (in the region we call the Mojave Desert) evolved depending on the winter rains, and go dormant in the summer. Many of these species don’t want a lot of water while they are dormant, and some may not want any water at all during that time. In the Chihuahuan Desert, which is in eastern Arizona, New Mexico, Texas and the eastern half of northern Mexico, the rains are concentrated in summer, and winters are dry. So many plants from this part of our region want little to no water in winter, and do all their growing in the warm season. Right in the middle is the Sonoran Desert which gets both winter and summer rains. But even within the Sonoran Desert you will find plants that are more active in either the cool or warm season. Know your plant’s growing season. The nursery that you get your plant from should be able to tell you when it is. If you are growing a non-native plant, like a Euphorbia from Africa, you should try to find out if it is a warm-season or cool-season grower. Most Euphorbias are warm-season growers, but there are exceptions, and those exceptions are from areas dominated by cool-season rain.

Most of the time, if you maintain normal landscape watering for these plants, and your soil drainage is good, the plant will be fine if you don’t decrease the watering—especially if you are watering ideally; less often but deeper. But a few plants (mostly succulent species) will want no water during their dormant periods. Adeniums, for example, really do not want water in the winter. The combination of cold weather plus moisture is not something Adeniums are adapted to, and they will rot. These are specialty plants, and most of the time these specialty plants are not sold as regular landscape plants. Many of them are grown in containers. If you don’t have the time to pay attention to these plants that require more specific care, we suggest you avoid these species in favor of the plants that are more common and fine with regular landscape watering.

What method of watering should I chose?

How you deliver the water to the plant depends on your available time and resources. We highly recommend setting up an irrigation system. You don’t have to get an expensive installation. There are timers that hook up to hose bibs and are battery-operated (available at most hardware or irrigation supply stores). They aren’t hard to assemble or program. And irrigation timers are more consistent and dependable than human beings who may forget, or go away for vacation. With a timer, you just need to have it set correctly and make sure that if it runs on batteries, that the battery is not running low (most timers that have batteries show how much life is left in the battery).

If you water with a hose, you will have to do so slowly, and use basins and berms so that the water goes where you want it to and doesn’t run off. We suggest making basins whether or not you use an irrigation system because when it rains, you will want the rain water to collect where the plants are. Rain water is SO MUCH BETTER than our tap water which is very alkaline and salty. And who doesn’t wanna save some money by using less tap water?

Some people use soaker hoses which deliver water slowly by soaking through either tiny holes or a spongy membrane. These systems are ok, but can get clogged and ruined quickly by our alkaline water.

No matter what your method of watering is, you want to water the whole root profile, which is about 1-2 feet deep—deeper for larger landscape plants, more shallow for smaller, flowering perennials and annuals. The deeper you water, the deeper established roots will reside. Also, deep watering pushes the salts that accumulate past the root zone, so we suggest watering about two feet deep to keep those salts from building up in the root zone of the soil.

Watering from below

Some people experiment with devices or containers that water from below or from the side. They might use pipes that inject the water deeply, or porous ceramic ollas that are buried in the soil and filled up. The issue to be careful about with any of these methods is salt accumulation. When you deep-water from above, you push accumulated salts past the root zone. If you water from below, you are pushing those salts up toward the plants. So if you use these methods of watering, try not to depend ONLY on these methods and water from above occasionally (especially with some gypsum) so those accumulating salts are pushed back down and/or dissolved and broken up.

Soil types

Soils in our region vary depending on the geology of the area. What your soils is like will determine how often, and how deeply you water. For a deeper discussion on the fertility of soil, see our info page on that subject.

Sandy soils

Near riparian areas like washes, you may have sandy soil which drains well, but needs to be amended often with organic material because sand doesn’t hang onto nutrients very well. Sandy soils are great at holding moisture but without drowning or choking the plants. This is because the particles are large, so they allow for oxygen to be present in the soil. Sandy soil dries quickly at the surface, but can stay very moist below. The biggest challenge with sandy soils is managing nutrition: you must feed (organically) or add compost often to sandy soils. If you add fill dirt that is more silty (silt is smaller particles than sand, but larger than clay particles) you can improve the nutrient-holding capacity of the soil. If you water frequently, you will deplete nutrients faster in sandy soils.

Clay soils

Clay soils are composed of tiny particles that cling together. They can hold nutrients but often also lock up those same nutrients in alkalinity. Clay soils also lack a good amount of air, an essential need for most plants in the soil. Clay soils will need water less often than sandy soils. They dry out faster and deeper than sand or silt. That is why deep cracks appear in clay soils: the water is pulled by the sun’s heat from the top of the soil, and there are no air pockets to block the water from being pulled deeply out of the soil. In sandier soils, the air blocks the heat from pulling the water deeply—like insulation. Gypsum is very beneficial for clay soils—it helps create air pockets in the profile which allows for better nutrient absorption. It also acidifies our alkaline soils in those pockets.

loamy soils

The best soils are loamy, which is heterogenous—loamy soils have a mix of sand, silt, and clay. The healthiest loamy soils have organic material in various stages, from freshly deposited organic material, to humic acids which are fully broken down and important for healthy plant growth. Loamy soils hold water well because the sun has a harder time pulling the water out of heterogenous soils. They also hold air in the soil well. Moist loamy soils with lots of oxygen in the soil profile are what most plants really like—a consistent supply of moisture, but without drowning or smothering the plant roots.

rocky soils

Often in places like the foothills of our mountains, you encounter rocky soil which is composed of silty soils mixed with rocks. It can be difficult to dig for planting, but surprisingly most rocky soil types are really great for plants. Plants can navigate their roots and follow the silty soils. The rockiness of these soils allows for lots of oxygen in the soil. You may need to feed often and mind using lots of organic material, since the rocky parts of the profile act much like sand, not holding nutrients well.

caliche

Caliche can show up in many soil types. It is the accumulation of calcium carbonate—solid sedimentary rock. Caliche may show up in harmless chunks that are annoying to dig up but don’t disrupt plant growth, or it may appear in large planes that block water drainage in the soil, and take up room where a nice silty soil would be preferred. They are in areas that have experienced a lot of erosion, where the top soil has been washed away (or bulldozed). If you have caliche dominated soil that doesn’t drain or has little arable soil, you may have to bring in fill dirt, and you might consider raised beds in the most difficult patches.

fill dirt

If you bring in fill dirt to remedy the soil you have, know its source. Often fill dirt can have a bad history—perhaps an old automobile leaked oil for years into the dirt. Or maybe construction waste was dumped and mixed in with the fill. Perhaps someone used toxic chemicals…you never know until you know. So ask about the history of any fill dirt you find. It can be found at various quarry supplied landscape material companies, or offered by people on Craigslist.