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Friday - January 07, 2011

From: Austin, TX
Region: Southwest
Topic: Compost and Mulch, Planting, Soils
Title: What to do with soil left over from new driveway in Austin
Answered by: Barbara Medford

QUESTION:

When we had a new driveway put in in Mid-December, the topsoil and weedy grass was scraped into a low pile. I watered it and covered it with black plastic to kill all the plant matter. How long should I leave the plastic in place? Which native plants would grow well on this 'berm' which is somewhat shaded and N of the house?

ANSWER:

We understand what you are trying to do, and you are going in the right direction. Starting a new garden in a pile of leftover dirt is a "waste not, want not" technique that can be very effective.

However, (you knew there was going to be an "however," didn't you?) there are some basic tasks that need to be performed before plant selection. From this University of California at Davis website on Soil Solarization, we learned some of the things that are not going to work in the situation you describe. Read all of the article, but we are going to extract one portion which summarizes your first problem: (your attention is especially called, by us, to the words in bold type)

"Soil solarization, a nonchemical technique, will control many soilborne pathogens and pests. This simple technique captures radiant heat energy from the sun, thereby causing physical, chemical, and biological changes in the soil. Transparent polyethylene plastic placed on moist soil during the hot summer months increases soil temperatures to levels lethal to many soilborne plant pathogens, weed seeds, and seedlings (including parasitic seed plants), nematodes, and some soil residing mites. Soil solarization also improves plant nutrition by increasing the availability of nitrogen and other essential nutrients.

The area to be solarized should be level and free of weeds, debris, or large clods, which could raise the plastic off the ground. Transparent (not black or colored) plastic tarps or sheeting 1 to 4 mils (0.001 to 0.004 inch) thick are anchored to the soil by burying the edges in a trench around the treated area. Plastic tarps can be laid by hand for small farms or gardens or by commercial machinery for large farms. To prevent air pockets that retard the soil heating process, there should be a minimum of space between tarps and the soil surface."

Now, having told you the things that you are doing that are not doing you any good, let's go back and start over. You have a pile of dirt, you want to do something useful with it, like a berm. You need to plant shade tolerant plants there. Here's the thing-start with the dirt. "Topsoil" can mean just about anything. Just because it was the top layer of the dirt in your yard doesn't mean it started with any nutrients or a texture that will permit plants to flourish. While this article, What Is Topsoil?, from wisegeek.com has an inordinate number of ads, it also has some good information on what constitutes real topsoil. Topsoil is not necessarily the dirt scraped off the top of your ground. It may have been fill dirt delivered long ago by contractors for levelling the ground. It may once have had nutrients in it that have been consumed without replacement by grass and weeds. It may be clay that is such fine particles that it compacts, swells when it is watered, and permits little oxygen for tiny new rootlets.

We would recommend rethinking that pile of dirt. Is there any particular reason, other than it is already piled up, to have a berm in that specific spot? We would prefer to think in terms of a raised planting bed, using a perked-up dirt consisting of what you already have, with addition of organic materials, such as compost, to make it plant friendly. Since the dirt is already in a shady spot, and you probably want to do something with it before the hot summer months, select an appropriate spot, in the same area, if possible, to cut down on the amount of hauling you will have to do. Begin by clearing that spot of weeds, rocks and clods of clay. Apply several inches of compost to that area and start bringing the original dirt in and mixing it with the compost. Yes, this is work, unfortunately that crops up (pun intended) in gardening. You can keep adding compost if you wish, and get the dirt from the pile, compost and the dirt where you are building the raised garden all mixed up.

If the new raised bed is large enough, you might want to plant shrubs, which, since they are woody plants, should be planted now while they are still semi-dormant. Or, you may choose to wait a couple months and put in some shade-tolerant perennials or grasses. Whatever you plant, make sure it gets deep watering for the first season, and mulch the surface of the ground with a good shredded bark mulch. This will help keep weeds under control and, as it decomposes, will add organic content to the earth.

NOW it's time to talk plants for your newly constituted garden. We will go to our Recommended Species section, click on Central Texas on the map, which will give us 155 species of plants that are native to the Austin area. You can use the sidebar on the right-hand side of that page to select specifications for plants that will suit your planting area. For an example, we'll begin by specifying "Shrub" under General Appearance, and selecting both "Part Shade" (2 to 6 hours of sun a day) and "Shade" (less than 2 hours of sun a day, under Light Requirements. You can use the same technique to select trees, herbaceous blooming plants, vines, grasses, ferns or succulents. Adding more specific light and water conditions will give you a better, but shorter, list. Following each plant link to our page on that particular plant will provide you with projected size, bloom time, speed of growth and growing conditions.

Shrubs for part shade to shade in Austin:

Anisacanthus quadrifidus var. wrightii (Flame acanthus)

Callicarpa americana (American beautyberry)

Ilex vomitoria (Yaupon)

Leucophyllum frutescens (Cenizo)

Malvaviscus arboreus var. drummondii (Turk's cap or turkscap)

Pavonia lasiopetala (Rock rose)

Senna lindheimeriana (Lindheimer's senna)

Sophora secundiflora (Texas mountain laurel)

From our Native Plant Image Gallery:


Anisacanthus quadrifidus var. wrightii


Callicarpa americana


Ilex vomitoria


Leucophyllum frutescens


Malvaviscus arboreus var. drummondii


Pavonia lasiopetala


Senna lindheimeriana


Sophora secundiflora

 

 

 



 

 

 

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