Two Different Types of Trees: Palo Verde and Crepe Myrtle

Q. I have a Museum Palo Verde and a crepe myrtle planted in the Fall of 2012. The  Palo Verde has grown well but the crepe myrtle hasn’t grown much in seven years. The roots of the Palo Verde are now lifting the stone ring I built around it. Can those big roots be cut out?

A. Palo Verde 

I will tell you about cutting the roots at the end but let’s talk about your situation first. You have two different types of trees; the Palo Verde is native to the desert Southwest and considered xeric (desert adapted) while the crepe myrtle is native to the wetter and richer soils of South and Southeast Asia. It is considered mesic and not desert adapted. You can’t grow them the same way, under the same conditions, and have good results.

            The Palo Verde can handle the poor growing conditions of our desert while the crepe myrtle must be pampered. Your crepe myrtle isn’t getting pampered that’s why you have problems with it. These two trees are planted, managed and irrigated differently.

            The Palo Verde will grow roots where water is applied to desert soils. If you water Palo Verde 2 to 3 feet deep and let the upper foot of soil dry between irrigations, the roots will  grow mostly 2 to 3 feet deep and not bother that stone ring lying on the surface of the soil. You can  control its rate of growth by giving it lots of water occasionally and let the soil dry out before the next irrigation. Every time it gets a deep drink of water, it will grow. When the soil is dry, it does not.

            The Palo Verde is more forgiving of desert soil even when it’s planted badly. I’m not saying it was or wasn’t planted correctly, but it tolerates bad soils better if they are amended before planting. The Palo Verde will look better if it’s planted in decent soil at the beginning, but how and where you apply water will dictate how fast it grows and where its roots are located.

            You can’t do any of those things with crepe myrtle. Crepe myrtle will not tolerate soils if amended badly at planting time. It won’t like the hotter and harsher locations in the landscape. But Palo Verde will. Crepe myrtle will grow much better if the soil is continuously moist and covered with 3 to 4 inches of woodchips. Palo Verde will like this too, but it can sail through desert hotspots and poorly amended soils better than crepe myrtle.

            If they are both on the same irrigation valve, then you will have a problem because the Palo Verde will get water when you think the crepe myrtle needs it. They should be watered separately. That would be too often for Palo Verde. It will grow shallow roots because its watered too often. If these trees are on separate irrigation valves, then the Palo Verde can be watered less often than the crepe myrtle. Using separate valves benefits both of them.

            Can you cut off the roots of the Palo Verde that are causing problems? You can remove a few of them each year for the next three years but start watering the Palo Verde less often and give it more water with each of its applications. Get its roots deeper and out of the way. Apply water to at least half the area under the canopies of these trees.

Crepe Myrtle

Readers crepe myrtle

On your crepe myrtle, I would put a ½ to one inch layer of compost 12 inches away from the trunk to about three foot away. You can get it bagged from Viragrow. I think you will need about ten bags or less. Concentrate the compost closest to irrigation water or drip emitters where it will get wet and decompose faster and move the leachate into the ground toward the roots. The tree will respond faster if you can auger as many holes as possible around the trunk to about 12 inches deep and fill them with compost. Spreading it on the top of the soil will work but it will take longer for the tree to get the benefits.

Cover that compost area with woodchips about three to four inches deep. You can get them free from the University Orchard in NLV (N. Decatur and Horse Drive) or their office complex on the corner of Paradise Rd and Windmill just south of the airport. Call their Master Gardener helpline at 702-257-5555 and make sure they have the woodchips available before you go.

Then I would prune back the height of your crepe myrtle and try to force some new growth lower on the trunk. Prune the top back as much as possible while still keeping the growth on top. I am hoping you will see some new growth lower on the trunk in the spring. This growth will help strengthen the trunk and I am hoping you will not need to stake it in a couple of years.