November 20, 2017

Winterizing Your Houseplants & Patio Plants

What steps do you recommend as we transition our houseplants back inside and prepare patio plants for the winter?
-          Dan G., Bosque Farms, Valencia County, NM
Gazania flower in patio arrangement outside
Las Cruces storefront
(M. Thompson, Oct 2017)

Hibiscus flower in patio arrangement outside
storefront in Las Cruces
(M. Thompson, Oct 2017)

Three weeks ago, I brought in my container patio plants that can’t tolerate the cold. These included the huge spider plant that began as a cutting from my grandmother’s huge spider plant, various succulents, and my prized pineapple plant that I started from a pineapple top a few years ago. Other plants, like the octopus agave, geraniums, and purple heart, are hardier so they can stay outside longer, but I brought them in last week rather than risk it. I like to bring cuttings of various favorites inside for the winter and keep them in water. That way, if we get a severe cold snap, I have some plant tissue to rely on in the spring for replanting.
This week I’ll winterize the hardy patio plants so that they can stay safely outside all winter. The two rose of Sharon shrubs—named, “Althea” and “Althea-later”—are currently confined in pots, and so far they overwinter very well each year with occasional watering and plenty of mulch. Keeping container plants watered during the winter is crucial for the survival of their root structure. If temperatures get cold enough to freeze the soil in patio plants, water between plant cells may freeze too, so the cells cannot take in water and subsequently dry out and die. When a plant cell freezes, at even lower temperatures, the water inside the cell forms ice crystals. The cell then lyses, or bursts, thereby killing the tissue.
Plants have a few tricks up their stems for avoiding death by frost. One of these methods involves the accumulation of solutes inside cells, such as sugars and salts, which decreases the freezing point of water. This concept is also used when salting an icy road to keep the water in that slushy, liquid state rather than freezing solid. Plants also produce different proteins that may act as a type of antifreeze between cells or bind water within the cells to avoid drying out. Some root dieback may not kill a whole plant, but better to be safe than sorry. Depending on the type of plant, soil type, size of pot, amount of roots, and sunny position on the patio, you may need to water only once a month in the winter. A good practice is to stick your finger in the soil and check soil moisture every few weeks.
I recommend arranging patio plants so they’re easy to remember and easy to water during the winter months. I keep a handy watering can close by. If you use a watering hose, be sure to disconnect the tap end because ice in the line may ruin the hose or, worse, the water pipe itself.
Hold off on fertilizing until temperatures start to warm up in the early spring. Fertilizers can promote growth, but the goal right now is to slow growth and encourage your patio plants to go dormant. In dormancy, they’ll conserve resources and be ready for leafing out in the spring.
Now let’s go back inside with the houseplants. When you bring them in, be sure to carefully examine under the pot rim for black widows or other bugs that you do not want indoors. White, crusty rings around the edges of pots are caused by salt build up on the pot surface or in the terracotta clay. Those plants may need to be repotted with fresh, less salty soil. I will also repot if the root ball is especially overgrown. In general, though, I save repotting for a spring chore when I want to encourage vigorous growth.
Houseplants with variegated foliage.
Various houseplants with interesting
color and variegation
(M. Thompson, Nov. 2017) 

Bougainvillea flower
Bougainvillea flowers
(M. Thompson, Nov. 2017)
NMSU Extension Entomology Specialist and NMDA State Entomologist, Dr. Carol Sutherland, has these pointers for keeping pest populations under control on houseplants: 1) Clean the pot saucer for each plant you’re bringing indoors. 2) Check the drain hole on the bottom of each pot for obvious pests as well as for overgrown roots (time to repot?) or washed out potting medium. 3) Closely examine each plant and hand-pick obvious insects. Turn over those leaves! 4) Remove dead leaves and debris from the surface of the potting soil, where insects and their offspring might be hiding.
If you want to use a pesticide for insect control, here is Dr. Sutherland’s recommendation: “Pyrethrum—derived from certain chrysanthemums—is an irritant for many insects. Consider spraying each plant with a pyrethrin insecticide (read and follow all label instructions) a day before bringing them indoors. This treatment should send many potential hitchhikers packing, but not all. Check the label to determine if certain plants should NOT be treated with this active ingredient or a particular formulation. Those pests that are well hidden or immobile on the plant may escape detection. And those with bodies that are well protected by waxy coverings will survive most topical treatments.” So the best defense on those elusive pests is to watch for them throughout the year.
Every county in New Mexico has an NMSU Cooperative Extension office with Extension Agents who field questions about various horticultural problems. In the winter months, these questions largely involve houseplant pests. Check back next week for more on controlling these pest populations before they spread and get gross!

Send gardening questions to Southwest Yard and Garden - Attn: Dr. Marisa Thompson at, or at the Desert Blooms Facebook page (@NMDesertBlooms)
Please copy your County Extension Agent and indicate your county of residence when you submit your question!

For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms and the NMSU Horticulture Publications page at

November 12, 2017

Cover Crop Considerations: Meeting Your Gardening Needs

Southwest Yard and Garden |  by Marisa Y. Thompson

Cover crop mix of Triticale, oats, and white
clover. Planted in the last week of October
at the Los Lunas Ag Science Center
(photo by M. Thompson, taken 11/9/17)

We want to prep our garden area for the winter. What cover crop species do you recommend?
-        Jeff G., Belen, Valencia County, NM
All of the NMSU Extension researchers I invited to give advice emphasized that the first step when considering a cover crop is to identify your management priorities. Are you planting this to reduce soil erosion, improve soil structure, build organic matter, or suppress insect pests, weeds, and diseases? There are many benefits to cover crops, and your selection(s) will depend on your end goal. 
For single cover crop species, Dr. John Idowu, NMSU Extension Agronomy Specialist, offered, “We have had success with wheat, rye, and barley cover crops. Oats are also a good option, but they may winterkill in northern parts of NM, depending on the severity of winter. In our research, we have seen effective ground cover and weed suppression with the winter cereals. They also produce considerable biomass, which can improve the soil organic matter when the plants are terminated and returned back to the soil.”
Dr. Stephanie Walker, NMSU Extension Vegetable Specialist, pointed to the use of legumes, such as vetch and clover, which can increase the nitrogen content of the soil, especially if they too are terminated and worked back into the soil before flowering and producing seed of their own.
NMSU Extension Weed Specialist, Dr. Leslie Beck, explains how cover crops can help control weedy take-over: "The use of cover crops can be very effective in suppressing weed germination and population establishment. The more dense and healthy the cover crop, the more it is likely to outcompete small germinating weeds for space, water, nutrients, and light... survival of the fittest! However, it is important to try and establish these cover crops as quickly as possible, and ideally prior to the ideal soil temperature and moisture ranges that trigger weed seed germination in the fall and the spring. If the cover crop is trying to establish from seed at the same time as summer or winter annual weeds, most likely the weeds will be able to outcompete the cover crops. As a result, you probably won't get the levels of weed suppression that may have been one of the primary reasons for planting a cover crop in the first place." 
Cover crops with penetrating root systems, like some of the brassicas (mustards), will help improve the permeability of hard, crusted soil. Cover crops can also serve as a catch crop, meaning they take up and hold nutrients left in the soil following a vegetable crop. Cover crop mixes, where seed from legumes, brassicas, and/or cereals are mixed together, may also be used so that beneficial impacts of the different types of cover crops can be combined.
NMSU Sustainable Crop Production Extension Specialist, Dr. Kulbhushan Grover, says, “Hairy vetch and winter rye is a popular cover crop mix with farmers. There is also a cocktail (sold as ‘fall mix’) that includes hairy vetch and winter rye, plus field peas, ryegrass, and crimson clover.”
Dr. Walker warns, “Caution should be taken that cover crops do not become a weed issue themselves, especially in warmer parts of the state where many will easily overwinter. For optimum impact, particularly in cooler parts of the state, cover crops must be planted early enough in the fall so that there is enough time for plants to grow to a helpful size. Brassica cover crops reduce germination of weed seed, but may also hurt vegetable seed. The use of vegetable transplants should be considered following a brassica cover crop.”
NMSU Extension Plant Pathologist, Jason French, suggests, “It is important to know what disease problems have occurred in the past. You do not want to plant a cover crop that is going to exacerbate your disease problem. Knowledge of past problems or common problems in a certain area will help you make the best cover crop selection.”
French added, “When soil conditions are poor, soil-borne diseases can be particularly damaging. Soil compaction, inadequate drainage, low organic matter, and poor fertility can predispose plants to infection. Cover crops can be used to improve these soil conditions and decrease the incidence and severity of the disease problems. Cover crops can also suppress plant pathogenic fungi and nematodes. For example, certain brassica cover crops contain high levels of glucosinolates, which act as natural antimicrobials when tilled into the soil. Still another use for cover crops is the promotion of biodiversity in the soil. Many of these organisms will compete with plant pathogenic microorganisms for resources. This competition can, over time, reduce pathogen populations.” 
Many cover ‘croptions’ are available. Drs. Idowu and Grover published a helpful Extension Guide, titled “Principles of Cover Cropping for Arid and Semi-arid Farming Systems” (Guide A-150), that details the reasons for cover cropping, challenges in our environment, and species selection.
Cover crops are a great tool for controlling weedy species, reducing erosion, and enhancing soil health in large- and small-scale gardens. Thanks for sending in an excellent question for this week’s column.
Send gardening questions to Southwest Yard and Garden - Attn: Dr. Marisa Thompson at, or at the Desert Blooms Facebook page (@NMDesertBlooms)
Please copy your County Extension Agent and indicate your county of residence when you submit your question!

For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms and the NMSU Horticulture Publications page at

November 6, 2017

Girdler Bug Attack: No Treatment Necessary

Southwest Yard & Garden

Our mesquite trees are experiencing what appears to be a girdler bug attack. This is the first time we have experienced this. How harmful are they, and what can be done to rid these pests?
-        Jack D., Doña Ana County, NM

Close up photo of mesquite girdler
Mesquite girdler. (photo credit: Dr. Salvador Vitanza,
previously with TexasAgriLife, El Paso, TX)
Submitted photo showing mesquite girdler damage
Whiteish band on stem near middle of the
photo was caused by the mequite girdler.
(submitted photo)

Submitted photo showing mesquite girdler damage
Mesquite stem die back caused by the
mesquite girdler. (submitted photo)

Submitted photo showing mesquite girdler damage
Girdling visible even though the
stem is out of focus. (submitted photo)

In order to answer your question, I enlisted the help of NMSU Extension Horticulture Specialist and NM State Entomologist, Dr. Carol Sutherland. Here is her response:
Yep. The culprit here is a type of long-horned beetle known as the “mesquite girdler” (Oncideres rhodosticta).
Adults are about a half-inch long and are several shades of dark gray, with dots or patches of rusty brown, especially on their forewings. Their most obvious feature is the very long pair of antennae, which is characteristic of most beetles in the Cerambycidae family.
The mesquite girdler is generally found from Baja California probably into southern California, and from Arizona to West Texas and south into Mexico, wherever potential woody hosts occur, including mesquite, acacia, mimosa, palo verde, and ebony. Honey mesquite may be the beetle’s preferred host and is where I have seen the most activity.
Many of the beetles I’ve seen have been resting on utility poles where security lights are located. On that kind of background, they are far easier to spot than on mesquite twigs, where they blend in well.
In natural settings, males can be actively patrolling mesquite twigs where females are located. Females girdle the host twigs before laying their eggs. I would think it makes more sense to lay eggs below the girdle than above; above the girdle, everything is drying up, probably too soon for the egg to hatch and larva to develop. Incubation and larval feeding should take several months, at least. New adults should appear next year.
Females are the ones that girdle host plant twigs. These twigs will usually be about the diameter of one of your fingers. When the beetle is viewed from the side, the axis of the body is at right angles to the beetle’s head. The jaws are perfectly in position to bite the bark; with each bite the female side-steps, going all the way around the twig of choice. This cuts the vascular system leading to the end of the twig, causing the leaves to soon fade and dry. As that happens, the girdled twig dries such that if you pull on it, the twig breaks off cleanly at the girdle mark. Broken twigs become part of the ground litter.
You can see this type of damage on road-side mesquites from Las Cruces to Deming and beyond as well as from Deming almost to Silver City. I-25 has had a nice display again this year from Las Cruces almost to Truth or Consequences. There are probably other areas where damage is noticeable, too.
Like other native insects, populations of this Oncideres go up and down from year to year. This year seems to be one of the “showier” years for beetle activity. Next year, the mesquites will grow again, putting on new twigs and foliage. Mesquites are very tough desert survivors with a variety of insects that feed and/or reproduce on and in them.
No treatments are recommended or needed for Oncideres in New Mexico. The beetles do not stay in an area very long; flights and infestations are “hit or miss” from one year to the next. Land managers hoping for mesquite control by these beetles are disappointed. People with ornamental mesquites see the damage and fear the worst, but, again, this is hit or miss from one year to the next. Developing larvae are well protected by wood and bark.
As for these twig girdlers attacking orchard trees, other species of Oncideres do that in different parts of the country. In the eastern US, another species of Oncideres girdles various fruit and shade trees. In extreme south Texas, still another Oncideres, the “huisache girdler,” is associated with native acacias, both wild and landscape trees.
Special thanks to Dr. Sutherland for answering the question for this week’s column. 

Send gardening questions to Southwest Yard and Garden - Attn: Dr. Marisa Thompson at, or at the Desert Blooms Facebook page (@NMDesertBlooms)
Please copy your County Extension Agent and indicate your county of residence when you submit your question!

For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms and the NMSU Horticulture Publications page at

October 30, 2017

Planting and Zoning: Knowing What Not to Try

Southwest Yard & Garden | Marisa Thompson, PhD

I have a spot with southern exposure in my yard where I would like to plant a quaking aspen (5,792 ft elevation).  Is quaking aspen suitable for this environment?
-        John R., NE Albuquerque, NM
I traveled all over New Mexico in the past few weeks, enjoying the striking fall colors. It is no surprise that you are feeling inspired to plant such a sensational tree.
Spoiler alert: quaking aspen (Populus tremuloides) prefer the cooler climates offered above 7,500 ft. Some say they can be planted above 6,000 ft, but only with very special care and only in a very cool spot along the north or east side of a building where the soil remains mostly shaded.
Even if you babied your tree by giving extra water, it would likely suffer from heat stress every summer. Retired NMSU Extension Horticulture Specialist, Dr. Curtis Smith, says he has seen just that in the NE heights, where a small stand of aspen trees in a south-facing residential landscape is watered daily, and although it is surviving the clump of trees is not thriving at all. Right now it is showing some fall color, but the edges are all browned.
Another reason to resist the urge to for urban plantings of aspen trees is described by Tom Zegler, NM State Forester for the Socorro District. Zegler warns that “their biological strategy is at odds with what we want from a tree. As an aggressive pioneer species with a short lifespan, they budget energy for height growth and root suckering at the expense of defense and maintenance of individual stems. In fact, above-ground biomass is a dispensable part of the organism. This is why aspen in a horticultural setting – even a setting with suitable environmental conditions – are dependable for nothing but eventual disappointment.”
When selecting plants for our gardens, the number one concern is usually cold-hardiness, which refers to the minimum temperature a plant can be exposed to and still survive. The USDA developed cold-hardiness zone maps of the US to help gardeners match the cold-hardiness of a particular plant with the zone in which they live. In NM, cold-hardiness zones range from 4b in which average annual extreme minimum temperatures are -20 to -25°F (way up in Rio Arriba county close to the Colorado border) to 8a, with minimum temperature from 15 to 10°F (down in southern NM closer to El Paso and the Mexican border).
USDA Plant Hardiness Zone Map of New Mexico from
USDA Plant Hardiness Zone Map of New Mexico

If you look up the USDA plant profile for quaking aspen, you will find that they can withstand minimum temperatures of -70°F, which is off the US charts.
However, cold-hardiness is not the only limiting factor when deciding what to plant and where to plant it. Heat tolerance also needs to be considered. The American Horticultural Society created a tool akin to the USDA cold-hardiness zone map. This Plant Heat-Zone Map defines zones based on the average number of days per year above 86°F.

According to NM State Climatologist, Dr. Dave Dubois, “Average temperatures in the Albuquerque heights have been increasing over the years. Not only has the overall average been increasing, but the average high temperatures have also. This is from a National Weather Service cooperative observer's weather station positioned at an elevation of 6,270 feet in the Sandia foothills that started collecting data in 1991. This follows the trend all over NM of increasing temperatures due to climate change.” Dr. DuBois can be found using the twitter handle @NMClimate.
Moreover, water needs are a major concern when selecting the appropriate species for NM. A plant may survive in a location, but not look so great because it is constantly struggling no matter how much care is provided. Take Russian sage, for example. In northern NM, Russian sages thrive and are easy, low-water, landscape plants with long-lasting, lush purple blooms. Whereas, in southern NM (at least in my yard) they tended to looked piqued all season. This is what I am afraid of with the aspen idea. The tree may technically live, but be an unhappy resource hog that performs at a level far below expectations.
Depending on the desired tree qualities (e.g., low water requirement, fall color, shade, low maintenance, etc.), here are some helpful online resources for species selection in our area:
If only we could grow our favorite plants wherever we wanted. I know what I would plant: Bougainvillea, in a heartbeat.

Send gardening questions to Southwest Yard and Garden - Attn: Dr. Marisa Thompson at, or at the Desert Blooms Facebook page (@NMDesertBlooms)
Please copy your County Extension Agent and indicate your county of residence when you submit your question!

For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms.

October 21, 2017

Salty Roses: Identifying Rosebush Problems

Southwest Yard & Garden | Marisa Thompson

Can you tell what seems to be plaguing the various rosebushes in my backyard? 
-          L. Peters, Sandoval County, NM

Compound leaf of rosebush with browned margins on leaflets. Compound leaf of rosebush with blackened margins on leaflets.Compound leaf of rosebush with dark margins and white build-up on leaflets.

Thanks for sending such great photos with your rosebush question. These photos depict rose foliage with varying degrees of browning edges on the leaflets and some white crusty buildup on leaflet surfaces, especially at the margins. Some of the leaflets have a burned look with black edges.

First off, the leaf margin damage looked to me like salt burn, and I asked Dr. Curtis Smith, retired NMSU horticulture specialist, to weigh in. Dr. Smith agreed that the tissue necrosis at the leaflet tips and margins might be due to salt burn. Salt accumulation at these points is caused when dissolved salts are brought up to the leaves from the roots during transpiration. This could be due to salts in the soil, in the water, or just accumulated after a long, hot, windy summer. As pure water transpires from tiny pores in the leaves, the salts are left behind.  As they build up there at the tips, they can cause that burned appearance, as well as the crusty, white build up, especially on the older leaves.

These symptoms may be alleviated by watering more deeply, but with longer intervals between irrigations. In the chapter on roses in the 1967 edition of her book, “Southwest Gardening”, Rosalie Doolittle recommends watering deeply at five- to seven-day intervals during the hottest months and then less frequently in the winter. She warns, “More failures in rose growing in this area result from overwatering than any other cause… Light and frequent irrigation causes feeder roots to grow close to the surface and they are easily injured from heat and wind.” Salt build-up is also more likely to occur in the top few inches of the soil profile with light waterings. Contact your county NMSU extension agent to find out how to get your soil tested.

Both Dr. Smith and Ms. Doolittle recommend mulching to conserve moisture and moderate temperature in the soil. Dr. Smith also noted some wind damage, especially on the stems, and the possibility that these rosebushes are getting a little too much shade, based on shadows in the photos. Powdery mildew may also be present on the leaf surfaces as evidenced not as much by the white film of spores spreading across leaf tissue, but by the residual angular darkening on some leaves. Refer back to a September Southwest Yard and Garden column for more on powdery mildew identification and control.

Finally, Dr. Smith suggested deadheading, which is the act of cutting spent flower stalks down far enough to reach the leaves with five leaflets during the summer. Major pruning can wait until later in the winter. Depending on your growing zone, that will likely be late February to late March (or later!), when the buds begin to swell. Rule of thumb: Prune your roses when the forsythia are in bloom. Idea for next week’s column: “How to identify a forsythia plant.” Perhaps I will save that one for February.

Roses offer bright doses of color and perfume to our desert gardens. My mom bought me a 5-gallon hybrid tea rose when I first moved to Las Cruces, and it has performed spectacularly with very little help from me (even after I tortured it for several years by keeping it in the container while moving from place to place until finally planting it next to a superhot cinderblock wall). By “very little help,” I am referring to the fun garden activities we covered: pruning, deadheading, and thorough, but not too frequent, waterings. Of course, there is way more to discuss on rose care. Check back in the spring for tips on fertilizing and the joys of spraying aphids off rosebuds with a garden hose.

Here are some great rose resources for New Mexican growers:

And check out these mini-videos from the Southwest Yard & Garden series with Curtis Smith and John White:

How To Choose The Right Rose To Grow In New Mexico

How To Prune Roses In The Summer

How to Fertilize & Deadhead Roses

October 14, 2017

Knowing How Much to Water: #itsSTILLcomplicated

How much should I be watering my trees? 
-  Multiple Gardeners from All Over NM

Answer (Part 2):
In last week’s column, we learned about how the type of soil in your garden affects tree water requirements. Now we will focus on other considerations, such as rates of water movement, tree species, age, canopy size, and seasonal fluctuations in water needs.
Plants take water up from the soil through their roots all the way to the leaves where it is released into the air. Transpiration is the process by which a plant loses water, primarily through pores in the leaves called stomata. This is a necessary process that involves the use of about 90% of the water that enters the plant through the roots. The other 10% of the water is used in chemical reactions, like photosynthesis, and in plant tissues. Transpiration is necessary for mineral transport from the soil to the plant tissues, for the cooling of the plant through evaporation, for moving sugars and plant chemicals, and for the maintenance of turgor pressure. The amount of water lost from the plant depends on several environmental factors such as temperature, humidity, and wind or air movement.

Here's a video I found on YouTube that provides a helpful visual of transpiration in action. PLEASE NOTE: the cartoon depiction of the tree roots in this video are NOT accurately drawn!

Mary Irish and Judith Phillips include a simple table on how much and how often to water landscape plants in their book, “Arizona & New Mexico Getting Started Garden Guide.” In this table, trees in New Mexico are divided into two categories: cool desert-adapted and high water use. The recommended number of days between waterings differ depending on the time of year, partly because tree water needs increase when temperatures are higher. In winter they recommend watering established, cool desert adapted tree species once every 45-60 days. Intervals of 14-30 days between waterings are suggested in the spring and fall and shorter intervals of every 7-21 days in the summer. More water may be needed for trees that are newly planted (less than three years), especially in the hottest, driest times of the year.
Irrigation scheduling table from gardening book (see caption)
Irrigation scheduling table from "Arizona & New Mexico Getting Started Garden Guide" by Mary Irish and Judith Phillips, page 225.
Slow soaks at long intervals between irrigation events are best for tree root systems. Remember that for mature, happy trees the roots are more concentrated in the top two feet of soil depth and extend out beyond the canopy, even two to four times the height of the tree. So be sure to water deeply enough and not just at the base of the tree trunk. For younger or neglected trees, it is most important to apply water both where you know where the roots are and where you want them to grow.
Drip irrigation is a great method for watering trees, but the placement of emitters will need to change each year to accommodate root and canopy growth until the trees are established. Similarly, low berms of soil can be used to build a basin around the dripline of a young tree if watering with a hose, but these too will need to be expanded as the tree grows.
Certainly, some plant species are bigger water guzzlers than others. Here are some resources on recommended trees for New Mexico and how to grow them:

Shade Trees for New Mexico NMSU Extension Publication Guide H-326

Selecting Ornamental Trees for New Mexico NMSU Extension Publication Guide H-328

Fruits and Nuts for New Mexico Orchards NMSU Extension Publication Guide H-310

NMSU offers a wide selection of other helpful publications. Check them out here:

Several counties offer lists of recommended trees and other plants for their specific climates. If you know of another reference you think might be useful please share it in the comments section below (or on social media!).

More and more communities in New Mexico are adopting incentives for water-wise gardening, like rebates (aka “tree-bates”) for the inclusion of low water use plant species and installation of drip irrigation systems with controllers.

Cover image of the Albuquerque Bernalillo County Water Utility Authority Xeriscaping handout
The Albuquerque Bernalillo County Water Utility Authority printed this great handout as a guide for converting to a water-wise gardening strategy and getting rebates on your water bill!

It is tricky to figure out when to water, how much, and how to deliver water to the trees in your garden. Try not to get discouraged. Selection of species that grow well in our climate with minimal extra water is an excellent first step. Mulches and groundcovers are also great tools for conserving soil moisture so that less water is evaporated directly from the soil into the air, but I will save those topics for another week.
Send gardening questions to Southwest Yard and Garden - Attn: Dr. Marisa Thompson at, or at the Desert Blooms Facebook page (@NMDesertBlooms)
Please copy your County Extension Agent and indicate your county of residence when you submit your question!

For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms.

October 7, 2017

Knowing How Much to Water #itscomplicated

How much water does my tree need? 
- Multiple Gardeners from All Over NM

I wish I had a simple answer. Since starting my position as NMSU’s extension horticulture specialist two months ago, I have heard this question in nearly every community I visited. Additionally, I have been pondering this question for years in my research as well as in my own garden.
In order to address the complicated question, I split this column into two weeks. This week I describe how the type of soil around your tree plays a big role in determining water requirements. Next week I will cover several other considerations.
Even if you know the species of your tree and try to look up water requirements, you will likely find a complicated answer. For example, in Judith Phillips’ revised New Mexico Gardener’s Guide the watering directions for desert willow are as follows “Until it reaches the desired height, water to a depth of two to three feet every two weeks when temperatures are above 90 degrees Fahrenheit, especially when the tree is flowering. Water once a month late spring through early fall, and every six weeks or less in winter…” Notice, she does not provide specific amounts of either water quantity or irrigation timing for each irrigation event. This is partly due to the variability of water output based on type of irrigation equipment and water pressure, but also due to the huge variability of water capacity in different types of soil. Phillips is also giving away a clue about general root structure, but we will learn more about that in a minute.
In the book Second Nature, Michael Pollan provides an interesting comparison of water holding capacities based on soil type: “100 pounds of sand will hold 25 pounds of water, 100 pounds of clay will hold 50 pounds of water, and 100 pounds of humus will hold 190 pounds of water. A soil rich in compost will need less watering, and the plants growing in it will better withstand drought”. This is not to suggest that soil amendments, like compost, are beneficial when planting trees in New Mexico – in fact, many arborists recommend zero soil amendment in the planting hole in order to discourage roots from growing only in that small, amended area. Using Pollan’s soil comparisons, imagine what will happen to 5 gallons of water if allowed to sink into sand versus clay. The water will infiltrate deeper in the sandy soil and wider in the clay soil. So how much water you need to apply around your desert willow, as well as the rate you apply it, depends on the soil type. The USDA Web Soil Survey is a great online resource for finding your exact soil type. CLICK HERE for a quick, very homemade, tutorial on how to navigate that website.
When I first read Phillips’ water recommendations for different species, I wondered how I could figure out how deep the water is moving in my soil. One way to do this is to get a t-shaped rebar tool and push it down into the soil. The rebar tip will move easily in moist soil and stop when it reaches dry soil. Many gardeners use a screwdriver to test soil moisture up to the length of the blade, so that is one easy place to start. Judy Nickell, a veteran Master Gardener in Bernalillo County, recommends the long, narrow bladed shovel for moving a wedge of soil just enough to check soil moisture depth when you are establishing your watering routine and when you suspect a problem.

This is key: roots will not grow through dry soil to find a water source. You read that right! Roots grow where soil is moist. Therefore, when you are irrigating, I believe it is crucial to know both where the roots currently are in the soil as well as where you want them to grow.
Somewhere along the way, most of us picked up the wrong idea for the shape of tree root structures. That image of the root mass mirroring the shape of the tree canopy is not accurate. In fact, root systems often extend two to four times the height of a given tree. Moreover, 90% of mature tree roots grow in the top three feet of soil and 75% in the top foot.
Diagram of shallow and extensive tree roots
Tree roots are shallow and extensive! (diagram source:

Next week, check back for a discussion on irrigation needs based on tree species, age, canopy size, and more.
USDA Natural Resources Conservation Service. 2013. Web Soil Survey [Online]. Available at
For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms.
Send gardening questions to Southwest Yard and Garden - Attn: Dr. Marisa Thompson at, or on social media: @NMDesertBlooms

Please copy your County Extension Agent and indicate your county of residence when you submit your question!