How to Water Sago Palm (Cycad) Once Each Week in the Winter

Q. Now that our irrigation is only allowed for 1 day, I was wondering if once a week is sufficient watering for my small sago palm. Should I hand water it on other days?

Not the readers sago palm (cycad) but it is small! It is in a container so it is more difficult to water than one planted in the ground. The soil in the container is more limited in size than one planted in the ground.

A. Hard to say. Depends on how much water your soil around the sago palm holds, where the drip emitters for it are located and the side of the house it’s on.

Hard to believe this cycad, or sago palm, is growing in Las Vegas. It was situated in the right location with the right care.

           Ideally the drip emitters are located between 12 to 18 inches from the trunk. For large sago palms I would suggest three emitters spaced in a triangle. Run the irrigation system long enough to water 12 to 18 inches deep. you can measure that with something long skinny and hard like a piece of rebar. If the sago palm is smaller, it may need only two emitters to wet the soil to the same depth. Smaller plants don't use as much water, but the system needs to run just as long.

Hard to accept this is the same plant as the one above. But this one is located in the heat of the sun and growing in poor soils.

           Plants on the south and west sides of the house or wall use more water faster than those on the east and north sides. A deep watering once a week should be all that is necessary for them in most soils and locations except the hottest.

Pines and Eucalyptus With no Water Growing in the Desert

Q. I have pines and eucalyptus that are not being irrigated at all. There is no irrigation applied but these trees are tall and healthy. What gives?

This eucalyptus has on applied irrigation in the desert. Sometimes trees can access underground water and survive.

A. Trees need water to survive. And large trees need more water than smaller ones. Some trees like your eucalyptus and many pines can grow deep roots. But trees need a minimum amount of water, or they won’t thrive otherwise deserts would be filled with tall, healthy trees like yours. They are getting water from somewhere.

Large trees use more water than smaller trees. Such is the case when the water was turned off to this mulberry.

           Plants are lazy, like us. Tree roots take up water where its easiest to survive. If they want to reproduce, then they need more than enough to survive. If the deep water is easiest to follow, then it will use it, if their roots can reach it.

           Tree roots don’t “seek” water in dry soil. They chase it. They “sense” water is there (compared to dry soil surrounding their roots) and grow best where water (and air) are abundant. If it can get lots of shallow water, like growing in a lawn, then that’s where tree roots grow abundantly as long as they can get air as well. If the water is deep, then that’s where roots grow if the soil is moist often enough to attract tree roots and they can “breathe”.

           Established pine trees grew “without water” at the El Rancho on the Las Vegas Strip after its fire. The property was abandoned, and the irrigation was turned off as well. Pine trees had to survive on only the deep salty water that their roots could get several feet below the Strip. This available water used to be considered a “nuisance” until developers saw its value in the desert. As Mark Twain used to say, “Whiskey is for drinking; water is for fighting over.”

           Both established pine trees and many eucalyptus have the potential to develop deep roots if given a chance and find a deep source of water.

Leaf Miner in Lemon Leaves

Q. My potted Myer lemon tree has “wrinkles” on the leaves. The nursery told me that the leaves have an insect living inside the leaves.  I used whatever product was sold to me last spring and the leaves still look wrinkled. This is my second year having the tree and it has 12 lemons on it.

Leaf miners on Myer lemon is usually a "greenhouse problem" with warmer temperatures.

A. The nursery was right. The leaves of your Myer lemon have leaf miners inside of them. But the product you were using was wrong. Leaf miners are usually small flies that lays eggs on the surface of the leaves. these eggs hatch and the “maggot” of this fly tunnels between the outer surfaces of the leaves. Previous to yours, the only place I ever saw leaf miner on Myer lemon were in another country grown inside a plastic tunnel they called a “greenhouse”.

Leaf miner infestation of tree leaves of Myer lemon is usually a greenhouse problem but here it was growing in a container. By the way, Myer lemon is a bit too large for container growing unless you use a very big container.

           There is no insecticide I would use to control them. The usual method used is to hand remove or pick the infested leaves and immediately destroy them.  Leaf miners found inside tree leaves are usually a minor problem when growing citrus. Their life cycle can be easily interrupted by removing infested leaves. Leaf miners are more of a problem in leafy greens like spinach and lettuce. 

A word of caution. These leaves must be destroyed soon after they are removed, or the leaf miner insect will emerge, mate and continue to infest other leaves.

           I guess if you were to use an insecticide for controlling leaf miner then I would spray a systemic insecticide like imidacloprid if it is labeled for food crops. But in your case the easier and safer method is to remove infested leaves and immediately destroy them and interrupt the insect’s lifecycle.

Palo Verde 'Desert Museum' Will Recover from Wind Damage

Q. I have a desert museum Palo Verde that was damaged during a windstorm. One of the branches blew off and damaged the trunk. It is an eyesore. Should I replace it?

Replacing a tree because of damage is a personal call. The tree will repair itself from that kind of damage. It may take three or four years if you can live with that kind of damage for the next year or two.

A. I would let the tree heal on its own but help it along its way. Healing takes two to three years if the tree is kept in good health. To do that, clean up the wound and apply management practices that encourage it to heal. Don't use any paint or “tree healer” as this was proven ineffective in past research and could actually slow the healing process. If you do paint the damaged area, use latex water-based paint. If there are any “splinters” resulting from the damage, remove them with a sanitized knife. Make the damage, and healthy areas surrounding it, as smooth as possible so the healing is faster and pleasant to look at.

Large tree wounds will heal if given time and you provide adequate water and fertilizer.  First they will compartmentalize their wounds and then start rolling in their cambium layer as it heals from site.

With that same knife remove the outer bark so that the edge is smooth and clean, and the damaged area is shaped like a vertical football. The damaged area will “compartmentalize” and the tree will “roll” over the area as it heals over the next couple of years. When the tree starts to grow this spring, make sure it gets adequate amounts of water and fertilizer. Good health practices help the tree to heal faster.

Early or late snowfall can damage trees with leaves still on them as this African sumac was damaged in Las Vegas. Repair of split limbs must be done very soon after the damage occurs or it won't work.

To reattach or repair a limb split, or otherwise damaged area from a tree during a windstorm, is usually a lost cause. If done successfully the limb must be reattached, or repaired, within minutes or even seconds after it is severed or broken. Time is very important so that the damaged area doesn’t “dry out” before it is repaired.

Century Plant (American Agave) Not Growing Well

Q. We have a century plant thriving over the past 10-12 years in our front yard. Towards the end of summer, we noticed lower leaves getting soft and tender, leathery even, eventually turning yellow. No new shoots are visible emerging from the core. It gets watered occasionally.

Century plant or American agave. A magnet for agave weevils.

A. Century plant is an agave weevil magnet! My guess is that's the problem. The only method I know to control agave weevil is to apply an insecticide around the base of all agaves in March or April of every spring. The insecticide is used to protect the plant from spring infestations. Sprinkle a granular insecticide at the base of the plant and lightly water it. The other option is possibly to drench the soil immediately around the agave with systemic such as imidacloprid (if the label permits). 

Warning sign of American agave that it might have agave weevils. Apply granular insecticide in the spring as a prophylactic treatment.

            The adult weevils have wings and can fly. They can fly from a neighbor’s plants to a different neighbor’s landscape. These adult beetles lay their eggs in the crotches of the agave in the spring. They don't use calendars but fly during the spring when it's perfect weather. That is why the timing for an application is sometime “in the spring”. The “grubs” hatch from the eggs and tunnel inside the core of many different types of agaves and cause their tunneling damage. Sometimes the damage is so severe it kills the plants outright or it might cause a smaller problem when plants are larger.

This is the type of problem agave weevil can do on established plants. Notice the base was rotten and the plant "collapsed".

            The other usual problem is watering too often. This can weaken or kill the plant. It doesn’t sound like that is your problem. I would caution you to water the plants deeply and not just a sprinkle them with a hose. Depending on the size of the agave it can take from 5 to 15 gallons of water varying from a small to a large American agave. If the American agave is large, then use three drip emitters located about 12 to 18 inches from the plant in a triangular spacing and watch for signs of stress. Apply water about three to six weeks apart during the summer.

Plant Found During Cooler Weather May Be Toxic

Q. Found a plant growing under my fig tree. It has sticky leaves, tiny white flowers, and clusters of small green balls. Any idea what it might be?

This picture was sent to me. I am guessing it is one of the nightshades.

A. I could not make out the plant from the pictures you sent so let me take an educated guess. Toxic black nightshade is a common weed that grows when the temperatures begin to cool off. It has 3/8 inch in diameter white flowers that resemble tomato flowers and produces immature green fruit that are ball shaped that vary from 3/8 to ½ inch in diameter. These green “balls” of fruit turn darker and sometimes black when they mature.

I am guessing the weed is toxic black nightshade which is a common weed during the cooler fall weather. If it's sticky it might have aphids. Regardless it doesn't belong near fruit trees. Get rid of it.

Toxic black nightshade found growing in the Orchard at Ahern.

            If the plant attracts aphids, then it can get sticky due to aphid “poop” from sucking the internal juices of the plant. Even though the plant is poisonous, aphids do like this plant. Like any weed growing near fruit trees, pull it when the soil is moist and get rid of it. Otherwise, the aphids will ride the winter out at about soil level as it gets colder and spread to your fruit trees in the spring. I thought I saw rocks around the fig tree. Make sure you rake the rocks back, put a thin layer of compost down and water it in to enrich the soil.

Nightshade with small green balls (fruit)

Cure Yellowing Eucalyptus Tree

Q. I have a 20 plus year old eucalyptus tree. The leaves are very yellow. What is the cause of this?

Its not a simple answer when it comes to yellowing eucalyptus. Could be anything. Judging from the plants around it and recent water restrictions I am wondering if it is water related.

A. Yellowing leaves can mean many things. It could be a nutritional problem, watering issues, or damage from insects or disease. I need more information as well as leaf and tree pictures. Make sure it gets an iron application to the soil in the spring. To be on the safe side, apply an iron chelate such as EDDHA.

This is the iron chelate called iron EDDHA. Because of its chemistry it performs well in all soil pH if applied in the spring.

            Check for a watering issue or a leak that floods the soil in that area. Eucalyptus is “mesic” in their water use so an application frequency for large trees should be used. When water is applied it should be deep and wide or large shrubs that receive water should be located under its canopy. Once a tree is chosen its increasing water use as it gets bigger has also been chosen.

Why Not Encourage Winter Planting in the Warm Desert?

Q. Why do you encourage people to avoid planting during the winter months? I can understand it in Minnesota but why southern Nevada?

Oleander winter freeze damage during a particularly cold winter.

A. Simply because I don’t know what is going to happen. I tell you spring and fall are the best times to plant. If it is a warm winter. then planting during the winter is okay. But if it is a cold winter, such as the lower teens at night, then be careful! Plants need soil at least at 45 -50F to grow roots. Otherwise the roots just sit there until the soil warms. 

Winter Freeze?

Cold or freeze damage to myoporum (locally known as Australian racer).

Will it freeze this winter or not? If the plant is winter tender, like bougainvillea or most citrus, then you are playing Russian roulette with the winter temperatures. It is best to plant those (that might freeze during the winter) in the spring for that reason.

Winter freeze damage to bougainvillea.

Watering Yucca in Winter

Q. Should I turn off the water to my yuccas during the winter?

Yucca rigida with golden barrel cacti beneath it. Y. rigida is a native to the southwest and, along with golden barrel another native cacti to the southwestern deserts, can be watered less often than yucca not native to our deserts. Examples of Yucca not native to the soutwest include Y. gloriosa and Y. filamentosa.

A. It depends on the yucca and where it came from. Some yucca come from dry regions and others come from wetter regions. I would not irrigate as often yucca native to the southwestern US. Examples include Yucca rostrata sometimes called Adams Yucca or Beaked Yucca, Y. schidigera and Y. elata as well as others.  I would water them once during the winter months of December and January. These are xeric in their water needs. That is the time to give them a good soaking. Confused yet? Ask your nurseryman which yucca it is before you buy it. Put Yucca native to the deserts of the southwest together so you can irrigate them at the same time. 

Yucca elata, soaptree yucca, is also native to the deserts of our area. Plants surrounding it, golden barrels, are also native to this area. The landscaper did a good job of grouping plants together with similar watering needs.

But yucca native to the southeastern US (such as Yucca gloriosa sometimes called Spanish Dagger and Y. filamentosa sometimes called Adams Needle as well as others) should be irrigated once every two or three weeks during the winter. Water these as you would any other landscape plant. They are mesic in their water needs. Put non desert yucca together so it is easier to irrigate them because they should get watered more often.

Probably Yucca rigida, a desert native. Probably watered too often.

Iceberg Roses Have Holes in Their Leaves

Q. I am growing iceberg roses and have noted holes in the petals of the flowers. I have taken this problem to nurseries, and they are stumped. Do you know what causes this?

Iceberg roses with holes in the flowers.

A. The shape of the damage to the petals can give some clues. If the shape is circular and smooth, or near circular, then it is probably damage from a leaf cutter bee. These bees cut holes from flowers that are nearly circular and about the size of a dime. They are great pollinators when they are active.

Holes Due to Leaf Cutter Bees

            Since these are solitary bees and the female cuts these circles for egg-laying, they start showing up when it gets warm, in about April and then through the summer. That’s why they make good pollinators for apples, pear or other late or continuous bloomers. You will not see these near perfect circles show up until that time. If the holes have jagged edges around their outside, then most likely it's a chewing insect or possibly snails/slugs.

Leaf cutter bees make nearly perfect circles or near circles in leaves and flowers of plants like this pepper leaf.

Holes Due to Slugs or Snails

            You will have to do some detective work if the holes are not from leaf cutter bees. Snails and slugs are the easiest to detect because they'll leave a slimy trail on leaf undersides when they do their damage. If you think its snails or slugs, then spread out wet newspaper or cardboard just before nightfall. They like darkness and moisture. so, the dark and moist undersides make perfect traps. Squeamish fingers will pluck them from this underside for execution.

Slugs or snails can leave holes in leaves but usually leave a slimy trail behind. The browning of these lettuce leaves is wilting damage as well.

Sticky Traps

Sticky trap in greenhouse for insect trapping.

            If it’s not snails or slugs, then make or buy sticky traps. These can be made by hanging yellow sticky 4” x 6” business cards close to the damage and covering them with a sticky substance. Smeared Vaseline jelly will work but not Vicks!

Get Bottle Trees out of Containers and Into the Ground

Q. I am growing healthy bottle tree in a very large concrete container in Queensland, Australia. The lower branches have been turning brown and then falling off. Is this typical behavior as this tree grows taller or a water problem? I am watering maybe 2-3 times a week using a handheld hose.  We are currently moving into our summer with very warm to hot days.

A. All bottle trees (Brachychiton populneus and B. rupestris) growing in containers or pots presents its own set of problems (primarily heat, soil, and water management) that growing them in the ground does not. I would encourage you to think about planting in the ground if possible. Our bottle trees get big! So do Australian-type bottle trees!

Bottle Trees in Australia are Different Than Ours

            In Australia they sometimes call Brachychiton rupestris a bottle tree. In the US, Brachychiton populneus is called the Australian bottle tree. Both are natives to you but rupestis gets the typical bottle look when younger while populneus gets a bottle shape that is not as exaggerated. Dropping of lower limbs may be natural with that species if they get eight years old. Hard to know since it depends on how old it got and if the trunk was starting to swell.

Be careful when putting bottle trees in lawns. These trees don't like lawns because lawns are watered too often. They are native to Queensland and Victoria, New South Wales, Australia and prefer soils that have good drainage and are not watered frequently.

Heat Management of Containers

I would suggest growing them in double pots. The outside container creates shade and acts like a heat protector for the roots. When the sun shines on the pot directly it can damage or kill about 50% of the roots due to heating up of the soil.

            The surface of dry containers, when the air temperature is 105F, is about 175F. Plant roots can handle temperatures at about 135F. I tell people to water their plants just before the heat of the day. As trees get older their roots occupy more of the container and this can be a significant problem during the summer. Root growth occupies space.

Bottle trees, Brachychiton populneus, can sunburn on upper branches because of its thin bark. It is easily vandalized as well.

Soil Management of Containers

Soil in containers “wear out” (depletes) in about five years and should be changed or amended every three to five years. I would discourage you from growing a large tree (bottle trees mature at about 50 or 60 feet) in containers. Most containers are not large enough to handle tall trees and their roots as they get larger. Containers are better suited for smaller plants like limes, calamondin, and finger limes.

Water Management

The usual problem with bottle trees is poor drainage and watering too often. The roots will rot if they get a continuous water supple and the soil stays wet. Digging them up when roots are rotting is usually accompanied with a putrid smell as the roots rot. When watering by hand about one fourth of the applied water should come out the bottom to remove salt.

            If you can’t control the water in the soil (drainage) or the application of water, then plant the tree on a “hill” that is at least one foot higher than the surrounding soil and six feet in diameter. Cover the soil with a 2-to-3-inch layer of surface mulch, either rock or woodchips.

Chosing One Product for Pest Control

Q, If you were to buy one product for insect control on plants, what would it be?

Organic castile soap for mixing in an emergency. What you shoot, is what you get when using soap and water sprays!

Probably the single most important insecticide all season but it is applied in the winter as a precaution. Insurance insecticide.

A. Probably soap and water sprays or an oil but not a Neem-type oil. Soap and water sprays are deadly to all insects whether the insect killed is a good guy or a bad guy. With soap and water sprays “what you spray is what you will kill”. Be careful when you use soap sprays and spray only what you intend to kill. In many ways it's like a gun.

            The advantage of industrialized pesticides is that they stick around longer after you spray them. Soap and water sprays must be repeated more frequently to protect plants from undesirable insects but are perceived as more environmentally friendly. I always carry with me a bottle of soap for mixing with water in case I see an insect problem that needs my immediate attention.

            In a pinch you can make your own soap spray by adding about one to two tablespoons of dishwashing soap to a gallon of water. I prefer using a pure Castile soap, that I am comfortable about, to mix with water.

            The oils I'm talking about are the “horticultural oils” or “dormant oils” made from paraffin or mineral oil and not from the Neem plant. These types of oils have been proven to be very effective on soft bodied insects like scale, aphids, mites, and whiteflies. Follow the label directions when making an application. Don't add soap to the concoction if it is already "homogenized" and has something in it that already mixes the oil and water together.

Horticultural oils (don't use Neem oil) comes in smaller and larger quantities.

Bottlebrush With Open Canopy and Yellow

Q. I have a bottle brush shrub that is open and getting yellow. What to do?

A. The Australian bottle brush plant is sold as a shrub but can reach heights of 25 feet so it can also qualify as a small tree as well. Bottlebrush varies from dwarf types to trees and flower color from red to white to pink. 

Flowers of the red bottlebrush.

Later the flowers produce this which are seed capsules.

Dwarf Types

Just don’t use dwarf types like ‘Little John’ if you want a larger tree sized plant. In tree form it can be used on single story homes for shade. To be on the safe side plant it away from hot walls. There is a weeping form and a non-weeping form as well as dwarf varieties. Also flower color can range from yellow to red and shades of red to pink and white.

One of the dwarf types, don;t know which one, with freezing damage.

Yellowing Leaves

            It oftentimes has a problem. Yellowing of the leaves oftentimes occurs when the soil is low in nutrients as well as its organic matter content. Fix yellowing by applying a landscape fertilizer every year in the spring and combine it with an annual application of chelated iron. Enriching the soil with organic matter is done when you used small rocks by raking them back and applying a thin layer of compost to the soil, wetting it, and raking the rocks back. Large rocks may not need raking. The compost and fertilizer will just wash through it to the soil.

One of the iron chelates, EDDHA. I prefer this type of iron chelate because it is stable regardless of the soil pH. Other iron chelates like EDTA and DTPA iron chelates become ineffective at soil pH above about 7.6. Apply it early in the growing season when you make a fertilizer application.

Increase Density of Canopy

            To improve the density of its canopy, make sure it gets enough water. The plant grows best if treated as a “mesic” plant rather than a desert or xeric plant. It grows well when surrounded by lawn. Water should wet the soil about three or four feet in diameter to a depth of about 18 to 24 inches deep each time. Water it as you would any mesic plant such as ash, bottle tree, African sumac, and others. As this plant approaches ten feet tall then wet the soil about five to six feet in diameter. Another possibility might be low soil organic content. So rake the rock away and put a thin layer of compost on top of the soil, water it in and replace the rock.

Pruning to Tree

            To get a small tree out of this, remove the lower limbs when it is 3 or 4 four feet tall so that the canopy occupies about 2/3 and the trunk is about 1/3 of its height. Plant it at least five feet away from hot walls or it will fry. Also in the desert remove lower limbs slowly as the lower limbs touch the ground. Otherwise the trunk may get sunscald.

How to Reduce Water Use in a Condominium HOA; Part 2.

 In How to Reduce Water Use in a Condominium HOA; Part 1 you determined what plant water use category you are in; 

Landscape water use categories

0-2 feet of water covering the irrigated landscaped area (nice job!)

3-4 feet of water covering the irrigated landscaped area (not bad)

5-6 feet of water covering the irrigated landscaped area (too much water used, scale back!)

Cranking the Water Use Down

If you are not happy with your landscape water use, or want to save even more water than you have, first consider eliminating trees above the roofline of the condos or at least pruning them lower. Are large trees important? Yes they are but perhaps not nearly as important as the cost of the water they need to survive.

These pine trees are planted too close but the shade they produce can't be denied. As these pine trees get larger, the cost of watering them may be worth more than the shade they produce. It will be up to you but consider removing them after replacement trees have been established.

Big Trees Use More Water than Little Trees

It may sound comical but people buy trees that grow 50 feet tall and only need a tree that is 15 feet tall (single story homes). Examples are pine trees. Sure pine trees may not use as much water as a mulberry but it still uses water. The bigger it grows, the more water it needs. Plant or keep trees that shade the walls and windows on the south and west sides of a building.

This hot, west facing exterior wall has four small shrubs that create no shade on the wall but still require water. Their distance apart (about 10 feet) would provide the water needed to plant and locate three small trees or shrubs (ten feet tall) that would shade this hot wall. As the plants we chose to replace them got larger, more water should be added. Add one more small tree (patio tree) to provide shade for the patio area walls and door.

Selecting "Backbone" Trees and Large Shrubs

Selection of "backbone" trees and shrubs should be hardy, deciduous, and survive temperatures to at least 20F during the winter. What are backbone trees and shrubs? These are trees and shrubs you don't want to lose to winter freezes. The trees and shrubs should be deciduous because we want the sun to warm this condo in mid-winter. If you want to save even more water, use small deciduous desert (xeric) trees and water them separately from the more frequently watered mesic plants. 

Landscape "Negative Space"

The fewer plants used and the smaller they are will, for the most part, determine how much water your landscape uses. Deserts use less water by NOT growing plants. I call not planting.... "negative space". Where are plants needed? Where are plants NOT needed? Leaves these spaces open. Creative use of negative space is challenging. In the wetter, eastern US lawns were used to occupy negative space. In the desert we must be more creative; boulders, changes in rock size (texture), changes in elevation, artwork, wall paintings...all can be important when not using water.

Dry washes can use textural changes along with plants. Rocks don't use water.

Open spaces don't use any water and the plants used were desert in origin (xeric) so they aren't watered very often. I would substitute small trees near the windows.

Textural change in the mulch and wall paintings add color and interest to an otherwise somber area and don't use any water. What is the water use here?

Planting in Desert Soil

Locations of ten foot tall trees or shrubs should be 4 to 5 feet from the home. Taller trees and shrubs can be planted further away. 

Soil amendments should be lightly mixed with the soil at planting time along with water. 

Planting holes should be wide and not necessarily deep unless there is a clear drainage problem. If a slight drainage problem exists, plant on a mound of amended soil rather than digging deep.

Water should be applied away from the foundation of the home. In the desert, plant roots follow where water is applied.

Trees and shrubs should be fertilized once in the spring. Two times at most. Showy plants are fertilized three or four times a year and when they flower best.

Trees use water. Bigger trees use more water. Lots of big trees use lots of water

How Much Water Do Pine Trees Use?

Q. We have on our street in Las Vegas pine trees that are over 20 years old and 50 feet tall. We obviously want to ensure their survival as we try to conserve water. Approximately how many gallons of water would each tree require each month?

A. The exact water requirement for pine trees is going to be difficult because water use of pine trees has never been quantified in our desert climate. Even if they have, those amounts are "ballpark amounts" and close approximations. Every site is different. Every year is different.

Mature pine tree about 35 to 40 feet tall and 50 to 60 feet wide. The area to water is at least 25 feet in diameter to stabilize the tree and spread the water it needs out. Can a smaller tree do the same job? It may use less water.

Approximate Water Need

I can approximate. We do know that as the tree gets larger it needs more water to grow healthy. Its annual water use is probably between 4 to 5 feet (48 in. to 60 in) of water applied under its canopy and surrounded by bare soil or mulch. This is approximately what it might need when its not surrounded by grass. Grass of mixed landscapes makes it cooler and it uses less water BUT not good for pine roots and its health.

Increase Water Estimates: Plant Need + Salts + Water Lost When Irrigating

If the tree is being watered by city water (because city water, not most well water, has salt in it) then it will need to be increased by about 20% to compensate for mixing in Colorado River water with it. Then, on top of all that, you must figure in losses due to imperfections in the irrigation system. If you are using drip irrigation, that water loss will be about another 5 to 10% beyond what it needs. 

                                                       Water needed by the plant

                                                                            +

                                          Las Vegas' salty Colorado River water (20% more) 

                                                                            +

                                  Water lost by the irrigation system (varies from 5 to 45%)

I will just guestimate what pine trees need. Add another 25% on top of that to compensate for Colorado River salts and another 5 to 10% if using drip irrigation. The water lost each month depends on the weather so I will give you approximate values. If it is a colder than normal month, the trees use less. If it is hotter and windier than normal, the trees use more.

Irrigate Following the ET Curve

The water use will follow a bell-shaped curve like the one below. These amounts relate best to tall fescue lawns.

Annual water use of adequately watered tall fescue in Las Vegas, Nevada. (Month 1 = January, 12 is December). Water needed by green and growing tall fescue lawns is about 1/10 of an inch of water each day in both December and January. This adds up to about 75 to 85 inches (7 to 8 feet) of water use per year. Pine trees don't use as much water as this tall fescue lawn so its water use is about 4 to 5 feet of water (48 to 60 inches) applied under its canopy each year. Water use is in inches of water use per day when the sun is shining and very little wind. The larger the tree, progressively, more water is needed by the tree. As trees get bigger, they need more water.

Monthly Water Use

January tall fescue lawns in full sun use about 0.1 inch per day;  so pine trees may use about 2 inches in 30 days if applied evenly under the canopy.

February a tall fescue lawn uses 0.14 inch per day; so pine trees may use about 3 inches

Following the same reasoning:

March 0.19 inch per day; 4 inches

April 0.28 inch per day; 6 inches

May 0.36 inch per day; 8 inches

June 0.41 inch per day; 9 inches

July 0.40 inch per day; 9 inches

August 0.35 inch per day; 8 inches

September 0.28 inch per day; 6 inches

October 0.22 inch per day; 4 inches

November 0.15 inch per day; 3 inches

December 0.1 inch per day; 2 inches

Watering a Mixed Landscape

First of all don't. Mixed landscapes of trees and grass are difficult to water.... physically. Second, trees are better off NOT growing in grass. Their roots are better off and their roots will grow deeper. When trees are NOT in a mixed landscape, the trees can get the right amount of water and the water can be applied more easily. 

Trees interfere with applying water to grass as well. They get in the way. Whenever possible separate trees and lawns. Trees and lawns should be on separate valves anyway. Water in our area is just going to get saltier and saltier.

Salt accumulation from the irrigation water on pine trees on this golf course slowed the growth of this tree on its lower limbs (about halfway up). The hybrid bermudagrass (growing beneath it) was fine.

Mixed Landscape of Grass and Trees

In a mixed landscape of grass and trees, lawns probably uses about 6 to 7 feet of water (a bit less due to shade and cooler temperatures of grass) on top of the pine water use. It depends on the temperature, sunlight, wind and humidity it receives but having two plants receiving water for the same area, although not additive, does make for higher water use than either plant alone. 

Mixed landscape in southern Nevada.

Grass in full sunlight uses 7 to 8 feet of water. Pine trees use perhaps 4 to 5 feet more water if pine trees alone are  grown in the same area (grown in bare soil or mulch). Grass is cooler than bare soil in the summer so water use will go up when it is removed (to maybe 6+ feet) but still not as much as needed by the grass alone (8 to 9 feet). Water use of plants in mixed landscapes can be like whack-a-mole.

What To Do?

Use fewer and smaller plants to do a similar job.

Follow the ET calendar when watering.

Give more water to bigger plants. Taller plants are watered wider, deeper, and less often.