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Saturday - March 10, 2012

From: San Antonio, TX
Region: Southwest
Topic: Soils, Trees
Title: Tree to plant on rocky soil in San Antonio
Answered by: Barbara Medford

QUESTION:

I want to plant a tree in a particular spot in the yard but after digging down 10 inches I hit solid rock. I filled the hole with water and it took hours for it to go down. It is one of the higher elevations in the yard and a few feet away there is deeper soil (although the deeper soil is not a location I can plant a tree). It is not a spot that would get a lot of water unless I water it. If I build up the soil a little bit and dig a very wide hole and add decomposed granite would I be able to plant a bigtooth maple or texas ash tree there? (I know they both need good drainage but do well on rocky soil) If no, is there a tree you would suggest? I hope I have described the situation clearly enough that you could give me advice. Thank you.

ANSWER:

Trees all over Central Texas, especially on the Edwards Plateau, grow in exactly those conditions. Sometimes the seeds (in wild areas) hit an area, germinate and grow a few feet and then die, because their roots have hit solid rock, or they have received insufficient rainfall, or something ate the plant down to oblivion. That is why every plant creates so many seeds, to guarantee that at least some of them live. But you don't want to garden like that on your own property; you want to put it in the ground and then see it grow up to be a fine old tree.

One thing that makes this possible is that most tree roots are within about 12" from the soil surface, to permit the exchange of gases and better access to moisture. In terms of water, any newly-planted tree, as well as other plants, will need an adequate regular source of moisture for at least the first year of its life. On our website we have lists of plants that grow dependably in various ecoregions of Texas, based on average annual moisture and soil types. We believe that the list from the Edwards Plateau is the best one to look at for your purposes. This is not to say that a plant on the list would absolutely thrive in your area or a plant not on the list would suffer; neither is true. Your chances of success are simply increased when you choose plants that have proved themselves hardy in the same area. Similarly, an adjacent ecoregion, such as the South Texas Plains could yield some good results. Follow each plant link to learn more about prospective height, bloom color and time, as well as soil types. We have checked every plant we selected to make sure it is native in or near Bexar County.

So, to your specific question. First, read our Step-by-Step article on How to Plant a Tree. We would only add to those instructions by agreeing with your suggestion of a wider hole, and also to build it up by working compost and possibly some decomposed granite into the native soil. We will go through both lists, selecting on "Tree" on Habit or General Appearance. When you look at the lists, you might choose other characteristics, such as dry soil or part shade that apply in your situation. While Fraxinus texensis (Texas ash) appears on the Edwards Plateau list, Acer grandidentatum (Big-toothed maple) appears on neither. Acer negundo (Ash-leaf maple) appears on the South Texas Plains list.

Recommended Trees from the Edwards Plateau ecoregion list:

Chilopsis linearis (Desert willow)

Cotinus obovatus (American smoke tree)

Diospyros texana (Texas persimmon)

Fraxinus texensis (Texas ash)

Prunus mexicana (Mexican plum)

Ungnadia speciosa (Mexican buckeye)

Recommended Trees from the South Texas Plains ecoregion list:

Acer negundo (Ash-leaf maple)

Cornus drummondii (Roughleaf dogwood)

Zanthoxylum hirsutum (Texas hercules' club)