What is Desert Horticulture?

Desert horticulture is a unique form of horticulture, very different from traditional horticulture in many ways. People who have gardened for many years in wet climates find themselves disoriented in desert climates. However, the laws of plant science have not changed, only how these laws are applied to the growing of plants.

Rainfall

Officially deserts should receive less than is of rainfall (4 cm) each year to be classified as a desert. However in a looser definition it has to do more with how much water plants could lose when irrigated. This involves both evaporation of water from nonliving surfaces combined with the transpiration of water lost from plants.

Irrigation is artificial rainfall. Desert plants respond to very quickly to rainfall and many times have evolved ways that keep them alive when rainfall is limited. When these rains come again, they spring to life. Drip irrigation is important in desert horticulture because it delivers this precious resource inexact amounts to plants and has the potential for conserving the most water of any form of irrigation.

When water is applied to native desert soils that have never been irrigated, rapid chemical and physical changes occur in many forms. Irrigated desert soils can damage plants, damage structures and cause many problems for the desert horticulturist.

Plants

Desert plants are uniquely tied to their climate. They respond to changes in their harsh climate that might threaten their lives.

Soils

Desert soils are unique because they develop under low rainfall conditions. There are oftentimes high in pH or alkaline in nature, salty, have very low organic content and contain a unique invertebrate and microbial community ready to flourish when water and organics are added.

Desert soils can be chemically and physically unstable when first irrigated because they have remained dry for so long. They are frequently rich in nutrients that have remained stable in this low rainfall environment for hundreds of years. Some of these high concentrations of minerals, like boron, sodium and chlorides can be toxic to plants. Others like sulfates can damage structures and other manmade contraptions in contact with this soil.

Sunlight

Sunlight intensity, the brightness of sunlight, in the desert can be quite strong. By some measurements, substantially stronger than sunlight in humid climates. Shade cloth that provides about 30% shade benefits many vegetables grown in direct sunlight.

Temperature

High temperatures are a hallmark of desert climates. It's one of the first things we think about when visiting the desert. The range in temperatures can be extreme in the desert, sometimes varying as much as nearly 75 ° F or more in a single day. Seasonally, temperatures might range as much as 100° F.

Shade is very important in desert climates. For humans and other animals, 100% shade can be extremely inviting on a hot summer day. But for plants, that rely on sunlight for a constant supply of energy, this is excessive. As an average, 30% shade is ideal for many flowering and fruiting plants. Plants that produce insignificant flowers and fruit may perform well under a higher percentage of shade.

Low Humidity

Low humidity does not sound like a big deal when growing plants. In fact, it is one of the reasons why plants grown in the desert have fewer disease problems than the same plants grown in a humid climate.

Salinity

Salts are common in desert soils and in some irrigation water.

Wind

Strong and erratic winds are a common feature of desert climates. Microbursts of winds approaching tornado speeds can be a common occurrence during cool spring weather hot summers.

Take Your Home Pest and Get Them Identified

Q. I spent days dealing with the green residue from a summer of thousands of midge infestation. We live near a pond, not sure if that is the reason for our problem. Wondering if they are in the plants in our yard and if we could spray something to stop them before the warm weather comes back in the spring. There are still a few holdouts even now.

A. I am curious about what you are calling “midges”. I know the word, but this term does not tell me which insect it might be. Without knowing exactly what this critter is makes it tough to understand what the effective control might be. Their exact Latin name is not necessary, but I think you need another set of eyes to identify them more accurately. There are some biological and organic controls that might work if its more accurately identified and I would hate to recommend a chemical spray if you don’t need it.

Proper Identification Leads to Using the Right Control Measure

            Catch some of these critters and put them in a jar with alcohol (clear vinegar will work in a pinch) and take them to the State Department of Agriculture in Las Vegas on East St. Louis. See if someone there has a more precise name for them. Phone ahead of time at (702) 668-4590 and see if there is someone in the office who might give you some ideas. While you’re at it, ask them what they would use for control. They have access to the state Entomologist in Sparks, Nevada, but it might take awhile to get it identified.

             Another place to take it is the Master Gardener help desk. Call 702-257-5555 and take them a sample in a glass jar and see if they can help get the right name for it and possible controls. Doesn’t need to be the exact insect ID name but close enough so we are on the same page.

            Once you find out what it is and get some ideas about how to control them, contact me again and let’s go from there.

Delay Heavy Pruning of Fruit Trees Until Winter

Q. My new trees (apple, plum, peach) were planted a month ago and appear healthy. I watched several YouTube videos and am nervously ready to trim these trees. Being nearly May and heat coming on, should I wait until next year or just do it now?  

Delay major pruning of any tree to the winter months. It is safer for the tree, it reduces sun damage, the form of the tree (if its deciduous) is easier to see.

A. I usually prune right after planting in January or February. My rule of thumb is 90% of my pruning is directed towards improving the structure of the tree and only 10% is concerned with production; the second year focuses 50% of my pruning efforts on the structure and 50 % toward  production and by the third year only about 10% focuses on the trees structure and 90%  on production.

Tree Architecture is Easier to See

            Each year the trees structure is evaluated but most pruning focuses on entering production no later than the third year of growth in the ground. Some older varieties may take longer than that. Do I keep a few fruit to savor earlier than this? Of course! But only a few. I’m encouraging as much good growth as possible and getting rid of unnecessary growth as early as possible.

Always Carry Your Hand Shears

            I always carry a sanitized and sharpened hand pruner on my walks through the Orchard any time of year. I never know when I’ll see something that needs my attention. Better to remove small amounts of unnecessary growth early in the season than wait until December or January and remove a large limb.

This is a locust tree that had too much pruning. The bark of this tree burned in the strong Mojave Desert sunlight. Be careful when you remove limbs from trees growing in the desert. Removing too much or removing at the wrong time of year can result is sunburn of the trunk and limbs of trees.

            Light pruning can be done now or anytime but in the Mojave Desert be careful of removing too much which can contribute to sunburn of the limbs and fruit. Since we are in late spring, wait until December or January to do any major pruning. You can  lightly prune fruit trees all year long if they need it but don’t remove too much. Summer pruning is done about now to older trees to keep them smaller but that’s for the experienced fruit tree grower.

Try "Wiggle" Trick After Planting Fruit Trees

Q. My newly planted fruit trees you told me to, "Check for root problems by bending the tree over and see if it wiggles in the soil."  How hard should I push?  When I push my tree, it does move in the soil, but I wouldn't say it “wiggles”.

A. The tree should be solid in the soil, and not “wiggle”, after it is planted. Staking helps but doesn’t replace proper planting. Moving the top of the tree back and forth (like a strong wind is blowing) says a lot about potential problems during the first year and future establishment.

"Wiggle" the Tree After Planting

            After the soil has settled from planting and watering, move the top of the tree back and forth (wiggle it), before staking, to see if it was planted too deep . Once staked, a tree is tough to “wiggle” and for you to make that determination.

            If your tree never leafed out or, if it did, the leaves promptly died, the tree was most likely planted too deep. A tree should not be planted more than ½ inch below the surrounding soil.

After planting this tree, it was "wiggled" or the stem moved back and forth before staking. Wiggling the tree shows where the roots are planted and how deep the tree is in the soil. If the tree is more than 1/2 inch too deep then it must be removed from the hole and replanted. Planting a fruit tree too deep can result in the tree dying from trunk diseases.

Plant the Same Depth Where it was Growing in the Container

            Plant the tree at the same depth it grew in the container or (if bare root) the same depth it grew in the field when harvested. When no soil is attached to the roots (bare root) look for a color change in the trunk separating below ground and above ground parts. A metal stake (or stakes), driven deep enough so it is in solid ground, and tied to the tree with stretchable, plastic tape a few times keeps the trunk and roots from moving.

All plants should be planted the same depth they were in the container. If they are "bare root" trees, there is a change in color on the trunk that will show you the depth it was growing in the field. It should be planted at the same depth. Not deeper or soil piled against the trunk. That will kill the tree!

Mistakes Made at Planting Time

            Mistakes I see frequently at planting time are planting trees too deep, planting too shallow (so roots are exposed after watering), not staking, and digging the planting hole too deep when there is no good reason for doing that. Some people don’t stake trees because they think its not necessary. Well, staking shouldn’t be necessary when the tree is small and there is no wind.

Bareroot trees have no soil attached to them. Container plants are easier to plant because you can see exactly how deep to plant. If you are not sure, look at the roots. The roots are growing immediately below the soil and grow deeper. Barely cover the roots when finished planting.

            When trees are planted correctly, and before they are staked, they should feel like they are solid in the ground when moved back and forth. Then stake them so the lower trunk and roots don’t move later on.

Wiggle, Wiggle, Wiggle

            “Wiggling” the tree from side to side (when it’s not yet staked) visually exposes trees that are planted too deep. Look down. The tree trunk makes a larger hole in the soil when its moved back and forth. That’s a sign the tree just may have been planted too deep.

            You know that old saying, “Dig a $50 hole for a $5 tree,” is still a valuable adage even if the money is wrong. 

Peach Tree Dropping Fruit

Q. I have a well-established, prolific peach tree. About this time every year it loses some of its immature fruit and I know that is normal. However, this year it's losing an abundance of fruit. I'm literally picking up 50-100 peaches a day. I have checked the irrigation and it seem to be fine. (drippers, 15 min per day, twice a day.)

A. I don’t like that you are applying water every day, twice a day in fact! Is there any way that can be changed to less often? That irrigation frequency sounds like watering a lawn or vegetable garden when it’s hot and windy out! I don’t know about the amount you are applying, but you are applying water way too often.

Peach trees will probably set less than 5% of the fruit that results from flowers. If you don't remove the fruit, the tree will do it for you. The end result is, what are the size of the peaches when you harvested? And how did they taste. Far more important to consider than fruit drop. Also, letting the soil become too wet or too dry will encourage fruit drop and loss. Change your irrigation practices!

What's Happening? 

Fruit trees should have water applied to them twice a week in May when growing alone in the desert and in most soils. The applied water should wet the roots to a depth of about 18 inches each time its watered.  

Sometimes people will ask me...how much mulch or woodchips should I have around my trees? Its like asking how far should you keep weeds from growing near a plant. The futher, the better for weeds. For mulch, at least to the dripline of the tree. But further is better!

For fruit trees growing in the desert, I like to see a layer of woodchips on the soil surface 3 to 4 inches deep. These woodchips provide a layer that protects the roots from getting too hot, conserves water, prevents most weeds from growing and helps keep the soil dark and rich.

Only a few months after applying wood chip mulch to the soil surface in the irrigated area you will see the color of the soil become darker. This is from the decomposing wood chips covering the soil. It decomposes and adds organic matter to the soil and you see worms flourish in those areas.

            I imagine the tree roots  are only growing about 2 inches deep if watering is this often. They should be growing 18 inches deep to anchor the tree in the soil. Roots like this are cycling back and forth between too dry, too hot or too wet. Roots can’t grow deeper because they are drowning (if you are watering a lot) or getting too hot and dry (if you aren’t watering enough). I need to know how many gallons you are applying, not the minutes, and where it’s being applied.

What to do? 

Cover all the soil under the tree’s canopy with 3 to 4 inches of woodchips. Apply water to the soil 12 inches from the tree trunk all the way to the edge of the canopy. This can be done by constructing a donut around the tree trunk 6 to 8 feet in diameter.

Irrigation donuts or moats around trees when they are small, are smaller. As the tree increases in size, the donut increases in size as well.

            Fill the inside of this donut with a 1-inch layer of compost with woodchips on top. Fill the inside of this donut with water once a day. At the end of August, begin watering every other day. At the end of September water every third day. By December you should be watering once a week.

Growing Tropical Trees in the Mojave Desert

Q. We want to plant some tropical trees and shrubs in our garden. I am wondering if I should turn the soil in the whole flower beds over and amend the soil OR I should only dig holes where the plants go and fill the holes with amended soil.

A. A lot depends on the soil that you have, but I  think you can amend the existing soil where these plants will grow and won’t need to amend the entire area. Keep tropical plants grouped together for planting and irrigating ease.

Organics Added to the Soil are Important

            Cover the area where these plants are growing with a thick woodchip surface layer (mulch) when you are finished. You will rely on the woodchips decomposing to improve the soil where roots will be growing over the next few months.

Basins, or donuts, around trees can be important to capture water if they are not on drip irrigation.

            After planting, surround each plant with a “donut” of soil about 3 to 4 inches deep. This “donut” acts as a water reservoir to get the applied water deeper in the soil. Smaller plants have smaller “donuts”. Bigger plants have bigger “donuts. Water daily the first few days after planting and before turning it over to an irrigation system.

Tree Selection for HOA Communities

Q. I am on the Board of Directors of a local HOA and we need to replace our plum and pear trees. We have lots of wind. Our landscaping company recommends a pistachio tree to replace the ornamental pear. Will that do well planted in the strips along our entrance and throughout the community?

Pistache like this 'Red Push' is a good mesic tree for HOAs with two story homes.

A. The Chinese pistache is a good choice in the desert as a general landscape tree, street, or lawn tree. The pluses: It has fewer problems than the pear tree and uses about the same amount of water. It handles wind better than pear or plum trees.

Chinese Pistache Negatives

            The negatives are possibly its size and water use. Chinese pistache is not a big tree and it’s not a small tree. It’s in between. It is mesic in water use, grows to about 30 feet in height and the same size in width. Don’t get me wrong. It doesn’t use as much water as the large Mulberry with its big leaves and fast growth.

Chinese pistache has great fall color but it is not a "small tree" suitable for single story homes. Remember, big trees use more water than small trees.

Single Story Homes Should Use Small Trees

            In terms of its size, it is best suited for a subdivision that includes two story homes. If your HOA has all single-story homes, then consider smaller mature trees in the 20-foot “at maturity” height range. Smaller trees growing among smaller homes is visually more appealing. Smaller mesic trees use less water than medium-sized mesic trees when mature. Smaller xeric trees use even less water when irrigated less often!

These trees were small when they were planted but now are out of scale with the single story home.

Grow a Mixture of Trees

            A word of caution when you are selecting replacement trees. Select a mixture of different kinds of trees in a community landscape. Having a mixture of trees reduces potential costly  outbreaks of diseases or insect pests that could sweep through a community of identical trees in the future.

Losses from tree diseases like ash decline can be minimized with a diversity of trees grown in an HOA.

            If you are having trouble selecting a plant, look online at the plant database found at SNWA. Cross reference your selection with Arizona State University’s plant database in Tempe Arizona. Select important trees used in your landscape that survive a low winter temperature of 20° F. Use a minimum temperature of 25° F for plants that you can afford to lose every few years.

Livestock Added to Organic Trade Between Japan and the US

U.S. and Japan Expand Organic Trade Opportunities Livestock Added to Arrangement Effective July 16, 2020, the U.S.-Japan organic trade arrangement adds livestock to the list of products that may certified to either country’s organic standards for sale as organic in both markets. “Opening new markets for America’s organic farmers and ranchers continues to be a priority for USDA,” said the U.S. Department of Agriculture (USDA) Marketing and Regulatory Programs Under Secretary Greg Ibach. “Japan is already one of the top export markets for U.S. organic products. This agreement opens additional opportunities for everyone involved in the international supply chain for livestock, from farm to table.” “Japan is a key international partner in the organic market sector,” said U.S. Trade Representative Chief Agricultural Negotiator Gregg Doud. “This expanded arrangement protects and increases access for American organic farmers, ranchers, and businesses to the third largest U.S. organic export market.” Equivalency arrangements reduce required certification costs, fees, inspections and paperwork for American organic farmers, ranchers, and businesses across the supply chain. See the Japan International Trade Policies web page and the USDA press release for details.

Organic Integrity from Farm to Table. Consumers Trust the Organic Label. Organic Insider Archive

Roses Don't "Sleep" in the Summer

Q. I read that summertime is when roses “sleep” in the desert heat. The question popped into my head a few days ago, “Can I prune my roses bushes in June and let them grow and flower through the winter months?”

A. I wouldn’t. Roses don’t really “sleep” or become dormant during the summer months, but they “struggle” instead with the heat and intense sunlight. Therefore, they might stop flowering and look bad. The ideal temperature for most roses is around 65F and “comfortable growth” can range as high as 82F. Temperatures higher than this range produce increasing plant stress.

Don't Prune Roses in Summer

             Intense direct sunlight causes considerable damage to previously shaded stems if roses are pruned and opened up in the summer.

            Allowing intense sunlight inside the plant canopy is the major reason I wouldn’t prune roses during the summer. If you were living in Seattle or San Francisco I would tell you to go ahead and prune in the summer

Growing Palms in Containers

Q. I have 11-year-old, 4-foot tall palm trees that are in 25- or 30-gallon pots in my backyard that get direct sunlight every day. What kind of watering cycle should they be getting during the hot summer and throughout the year? Also, what kind of nutrients should I be adding to them and how often?

Even though not really a palm, sago palm or cycad makes a great container plant.

A. Containers will dry out faster on the south and west sides if they are in full sun. Those sides of the container are notorious for getting super hot and killing roots. Shading the container or putting the existing container in a second, but larger container, prevents that.

Note: windmill palm and pygmy date palm make great container palms!

Watering Plants

            Always water in the morning before it gets hot. How much and how often to apply water depends on the soil in the container. Buy an inexpensive moisture meter used for monitoring houseplants to gauge when to water again. Push the tip of the moisture meter in the soil about four inches deep. Do this in three different spots so you can get an average reading. When the average reading is near 5, water again.

Inexpensive moisture sensor for houseplants does a pretty good job telling you how wet or dry the soil is.

Fertilizing Plants

Fertilize container plants more often than plants growing in the ground. The application frequency varies from once a month to three or four times a year. The same amount is applied over the course of a year, but the amount applied each time is split into monthly or quarterly applications. The best way to judge when an application of fertilizer is needed is using leaf color and plant growth as indicators. Plants with enough fertilizer are dark green and have strong growth.  

Controlling Soil Salts

            When you water, a little bit should always come out the bottom of the container. This helps remove salts that might otherwise accumulate in the soil. Apply fertilizer after every 40 waterings.

Remove Seeds From Vitex?

Q. My Chaste tree bloomed beautifully once in early spring but nothing since. Now it just has seeds where the blooms were. Do I need to cut the spent spikes off?

This is a mature vitex tree. It can grow quite large when it gets all the water and fertilizer it wants.

A. Vitex blooms once in about June. Most vitex varieties have blue flowers, but some are white and some rose colored. The flowers turn to seed. The seed germinates anywhere there is water. Some think these seed pods are unsightly. If you agree, cut them off but it is not necessary.

This common vitex (blue flowers) survives in rock aka desert landscapes. Planted too close to that driveway btw.

Read what Dr. Chris Martin at ASU says about this tree.

            Vitex isn't considered a desert tree (xeric) but mesic instead, is small and grows very nicely in the heat of the desert if it gets water. Make sure it gets lots of water the month prior to flowering and the flowering will be spectacular. The rest of the year water as you would any other plant in your landscape.

Some people dont care for how vitex looks in the winter. BTW this tree was pruned too high.

Air Layering Peach Tree

Q. I am interested in air layering my peach tree. What time of year would it be best to do air layering? Also, would it be bad to try during the hot summer?

Air layering...sometimes called marcottage...is used mostly on plants that are difficult to propagate from cuttings. Here I would put this in the interior shade of the tree and not in full sun.

A. Air layering, also called marcottage, is a way of growing roots on a stem while it is still attached to the mother plant. Air layering is used frequently when plants are difficult to propagate from stem cuttings. The best time to air layer plants is the same time as planting them; spring and fall months.

This video on air layering is pretty good.
But I would use black plastic and not aluminum foil in the desert. And remember to sanitize your knife!

            You can also propagate peach and nectarine from stem cuttings. The size that works best is pencil diameter and taken during the winter months after leaf drop. These “hardwood” cuttings grow roots best if a rooting hormone is applied soon after the cutting is made and “stuck” into a propagation medium.

This is an apricot rootstock (bottom) grafted on to a Blenheim apricot. The rootstock imparts some very important characteristics on to the apricot that is lost if grown from cuttings.

            When propagating peach and nectarine either by air layering or stem cuttings, the important advantage of a grafted rootstock is lost. The rootstock on peach and other stone fruits can play an important role. It can control the tree’s eventual size, how soon it starts to bear fruit, resistance to some diseases, and even affects the fruit flavor.