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Lack of water causes established trees to die back

Q: Around town and in my neighborhood, many midlife trees are dying. In beautiful large trees, there are dead branches and sometimes just a fully dead tree. Some trees are planted in gravel and some in a yard. Do you notice that also?

A: Yes, there are a variety of reasons for this happening. The bottom line is stress, but the bottom, bottom line is probably poor tree health primarily because of a lack of water. Collectively, several factors have played key roles.

n Drought. Older landscapes converted from lawns to desert landscaping using drip are more water efficient, but that might take its toll upon larger trees. Trees growing in lawns extend their roots a distance equal to about twice their height. Drip irrigation in desert landscapes applies water within a few feet of the trunk. That leaves many of the roots of larger trees outside this wetted zone from drip irrigation.

Older desert landscapes frequently don’t increase the amount of applied water to account for trees growing larger. Just increasing the minutes is not the complete solution. The irrigation water must be applied to a greater area under the tree. The only solution is to increase the number of drip emitters under the tree canopy.

n Disease. Three major diseases are responsible for dying trees and dying limbs here: Sooty canker, Aleppo pine blight and ash decline. Sooty canker invades many kinds of trees under some sort of stress, oftentimes water stress. Aleppo pine blight affects only Aleppo pine trees, but there are many planted throughout the valley. Ash decline infects most ash trees and causes limbs to die and eventually the tree.

n Borers. Borers are insects that attack and feed on many kinds of trees in the Las Vegas area and all through the valley. The insects hone in on trees that are stressed, sunburned or not getting enough water. Infested trees first show dying limbs, and eventually the tree dies.

As water becomes more expensive, conserving water and applying it correctly will slowly push us toward more desert landscaping and planting trees appropriate for desert environments.

Q: I would like to eliminate 1,400 square feet of grass in my backyard and take advantage of the SNWA rebate. Could you advise me if I can get free or low-cost landscape design ideas from anyone? I plan to do the conversion myself.

A: If you do some digging online, you can get plenty of ideas regarding the design. Mostly through images. Key elements in desert landscaping to remember when conserving water is to minimize the number of plants used, keep these plants small and use plants that originate from desert climates.

A big problem in hot desert climates is the cost of air conditioning during the summer months. In our climate, cooling is far more expensive than winter heating costs.

For this reason, focus on shading the south and west walls and windows with deciduous plants, plants that drop their leaves during the winter. Shading the walls and windows has more impact on cooling than shading the roof during the summer.

A third element important in desert designs is the application of water in the landscape. Become familiar with two terms regarding desert design found on the internet: hydrozones and the mini-oasis concept.

You will find a lot of information on the internet about how to create hydrozones. There is less information about the mini-oasis concept. This concept is explained in detail with some plant and design suggestions in a book you can purchase called “Plants for Dry Climates.” You can buy this book on Amazon.

The last important step you should contemplate is the correct installation of the irrigation system. Once the trees, shrubs, ground covers and vines have been planted, then select someone competent to install the irrigation system. They should have a clear understanding of hydrozoning and how to implement it.

If it interests you, I will offer several low-cost classes on each of these steps for do-it-yourselfers in January. Sign up on Eventbrite or contact me for more information.

Q: I want to prepare planting holes for my landscape plants now rather than wait until spring. I want to dig the hole, get the soil ready with a good compost such as ViraGrow and install the irrigation. Will I lose the benefit of compost by waiting until spring?

A: No, you will not if the soil in the planting hole is not watered. Actually, there is some benefit in preparing the amended planting holes now and waiting later to plant.

The value of any compost is twofold: improving the physical structure of the soil when mixed with it and adding nutrients or fertilizer to the soil. Good compost will always improve the structure of soil when mixed with it. The amount of nutrients added to the soil depends on the compost. Some composts are richer than others.

The ViraGrow compost you mentioned is now called 166 compost and is rich in all major nutrients including nitrogen. Very little nitrogen is lost during the winter if it is not watered. The soil will remain improved, and nearly all the nutrients will stay locked up in this soil mix because it is kept dry.

Prepare the holes three times wider than the planting container. Use this excavated soil and mix it in equal volumes with compost. Fill the hole with the amended soil and water to settle it.

There is no need to add more compost or soil amendments at planting time. Plants growing in this amended soil with rich compost will not need any additional fertilizer for about two to three years.

The benefit I mentioned is from the aging of the compost and the chemistry of this soil mix. During the winter months, the composted soil mix will have time to chemically settle down and build the microorganism activity more than using fresh compost as a soil amendment.

Q: My ornamental milkweed has a shiny substance on lots of its stems and leaves. From a distance, it looks like someone just sprayed water on it. It’s shiny and glistens. The rocks directly below also have this shiny substance on them. The rocks feel dry, but the plant feels a little oily.

A: Milkweeds are notorious for attracting aphids, sometimes called “plant lice.” These insects suck plant juices from the leaves and stems of this plant and then excrete a sugary waste that attracts ants, bees and other insects.

These aphids are on the bottoms of leaves. This sugary secretion, called honeydew, gets all over everything including the ground and rocks beneath the plant as well as lower plant leaves. This sugary substance leaves a shiny, somewhat sticky, residue.

They are particularly troublesome during the cooler months of spring and fall. Aphids reproduce quickly and don’t need a mate for this. The female aphids give birth to living young without a male partner.

At this time of the year, aphids are mostly interested in surviving through the cold winter months. As the temperatures drop, aphids travel lower and lower on the stems of plants to a location where they are protected through the winter.

During the coldest part of winter, they are found hibernating slightly below the soil surface but still attached to the stems of soft, succulent weeds and other plants.

When temperatures warm in the spring, they migrate up the plant, where they give birth to living young on the soft, succulent leaves as they emerge.

Ants like them because of the sugar and move them from leaf to leaf on the same plant and to new plants.

How to get rid of them? Spray the undersides of the leaves with any kind of concoction that kills aphids. That can include soap and water sprays, neem oil, pyrethrins and conventional insecticides bought from nurseries and garden centers.

It is your choice on how you want to control them, but if you don’t, they will get out of control and may eventually kill the plant in the spring.

Bob Morris is a horticulture expert and professor emeritus of the University of Nevada, Las Vegas. Visit his blog at xtremehorticulture.blogspot.com. Send questions to Extremehort@aol.com.

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