Site Preparation and Soil Layer Building

OK! So you made it to Site Preparation. Congratulate yourself! You are over halfway there (learning your native and making a naturescape plan are usually more time-consuming). And recognize that now is when the real fun begins – you get to be outside and maybe even get dirty!

The site-preparation you will do (or have done) will depend significantly on your site. It may entail removing trees, killing grass, removing invasive plants (such as English ivy, Himalayan blackberry, kudza, vinca, or other), and amending/restoring damaged or compacted soil, etc. We will focus first on transferring from a grass lawn to a naturescape because that is the situation in which most people find themselves. We will then discuss some of the other situations.

From Turf Lawn to Naturescape

In transferring from a turf lawn to a naturescape it is important to realize that you are not just removing grass and planting natives, but you are restoring the soil or soil “layers” at your site.

To help appreciate what this means, we would like to distinguish between mineral soil and organic soil. We will use the term mineral soil to generally refer to soil that has a low organic component – we often call this “dirt” and it tends to be rather hard when dry. We will use the term organic soil to refer to soil that is comprised largely of decomposing leaves, needles, and other organic material – it tends to be soft and moisture-absorbent.

Can you remember the last time you went for a walk in the woods or other natural area? What type of soil did you find (assuming you actually looked)? You probably found an organic soil layer of 3-4 inches (generally 1 to 6 depending on conditions) which gave way to a mineral soil layer. You may have also noticed that the herbaceous plants (the non-woody plants that often grow close to the ground) tend to grow in the organic soil layer, while the shrubs and trees tend to grow in the mineral soil layer.

Contrast the local woods or an undisturbed site with a turf lawn. Have you ever driven a shovel into a turf lawn? What soil layers did you find there? We suspect that you found only mineral soil layer. That is because prior to installing a turf lawn, the
organic soil layer is typically stripped off.

While some herbaceous plants can support being planted directly in mineral soil, particularly sun-liking perennials such as grass, etc., most herbaceous plants, particularly the partial-shade and full-shade liking ones (those found among trees and shrubs) need an organic soil layer. Accordingly, if you will have anything other than a full-sun meadow (or sand based “desert-scape”), it is preferable to have an organic soil layer. And, regardless of whether you are planting a full-sun meadow or a partially- or fully-shaded space, there is benefit in establishing at least some organic soil layer because it provides weed-suppression and soil moisture retention.

That is why, in essence, we suggest that in changing from a turf lawn to a naturescape you’re not simply switching plants, but rather restoring the soil layers. And don’t worry now about how we do that – it is fun, easy and inexpensive and we’ll discuss it more below. First, however, let’s take out some lawn. … Yeah, let’s get out our tools, put on our boots, and go “kick some grass!” [Yeah, we know, cheezy humor, but we couldn’t resist]

There are primarily two ways to get rid of grass (1) dig it up, and (2) smother it. If you live in an arid climate there is a third, stop watering and wait. And if you are extremely patient there is a fourth, plant trees and they will naturally kill the grass by shading it out (may take 5-10 years). Here we will discuss the first two, and how to re-establish the organic soil layer.

Digging it up!
When it comes to digging up grass, you will likely need a rototill and rake or for smaller areas or folks who like to do things by hand, a shovel and/or pick and rake. For do-it-yourselfers, local rental shops often carry rototills. The larger self-propelled tillers (12-16 hp) tend to work well for average lawns. Rototills work best when the lawn and soil are somewhat dry. If too moist, you may create a mud bath.

Regardless of whether you use a rototill or do it by hand, thoroughly turn over the soil and rake away the grass. The goal is to remove as much of the grass plants as possible so that the grass does not grow back into the space creating a maintenance
issue. After tilling and thoroughly raking you should be left with smooth exposed dirt.

It is important to recognize that if you add water (rain, fog, etc.) to the exposed dirt you often get “insta-weeds” (not having "exposed dirt” is one of the benefits of the smothering technique discussed below). To help prevent or minimize this, we recommended that you be ready to plant your plants and otherwise cover the exposed dirt as soon as possible after it is cleared.

As discussed in more detail below, we recommend that you first plant your trees and shrubs in the exposed soil. Next we recommend that your cover the remaining exposed soil with a material that is both (1) an organic soil layer builder and (2) a weed suppressor. The number one material that we recommend for this is leaf mulch. Our second choice is a thick layer of leaves, though leaves tend to only be available in the fall and are more likely to blow away. Grass clippings and/or straw would be a distant third (distant because it is unnatural to have such a high concentration of cut grass, though the grass clippings could be mixed in with leaf mulch or leaves). Note that if you live in an area that has predominantly needle bearing trees, you can substitute the words "needle mulch" for "leaf mulch" in the present discussion.

Leaf Mulch – Organic Soil Layer & Weed Blocker
Leaf mulch is decomposing leaves and is about the closest commercially available substance to the organic soil layer that you are attempting to replace. Thus, it is the growing medium for a number of plants that you might want to plant. It is also an excellent weed suppressor. In addition, leaf mulch is relatively weed seed-free (leaves are not the reproductive parts of plants) and it breaks down relatively quickly, thereby releasing its nutrients to your desired plants (i.e., a natural fertilizer).

Caution on Yard Compost and No Bark Mulch!
In contrast, try to avoid commercial yard debris composts because they are often of unknown origin and contain weed seeds. The provider may claim that the weed seed are all dead, but in reality that is rarely the case. Also, avoid bark mulch because it is slow to break down, low in nutrient value, supports weed growth, and quickly becomes a maintenance problem. Bark mulch is only popular because it is aggressively marketed, and returns profits to its producers and the maintenance folks that are hired to spray chemicals on it and/or manually pull weeds from it. Large quantities of bark mulch covering ground is also an unnatural condition and thus there are no native plants that evolved to grow in it.

We will discuss below how you might obtain leaf mulch and how you might use it. First, however, let’s discuss the other method of getting rid of grass – smothering it – so that you can contrast the two methods and decide what is right for you and your situation.

Smothering It
While there may be a time and a place for removing grass by digging it up, that approach can be fairly labor-intensive relative to smothering it. In addition, and, perhaps of greater concern, digging up grass leaves bare soil which is a magnet for weeds. Thus, the smothering or obstruction method may have its advantages.

The smothering method is pretty straightforward though when practiced most effectively it requires some advanced planning and some patience. A typical approach would be as follows:

Cut the turf grass to the lowest setting on your mower and then cover it with newspaper at a thickness of 10-16 sheets. Be sure to overlap the newspaper sufficiently so that cracks don’t appear through which grass and weeds can grow. This starts the smothering process. Next, apply either several inches of leaf mulch (4-6 inches) or a thicker layer (6-8 inches) of leaves alone, or a combination of the two. If you use leaves, it is a good idea to wet them down so that they will stick together better. Note that while 6-8 inches of leaves might seem like a lot, a good rain may reduce that height in half.

The layer of leaf mulch or a greater thickness of leaves alone works in combination with the newspaper to block photosynthesis, which effectively kills the grass. The leaf mulch and/or leaves also provide the organic soil layer (the leaf mulch more rapidly than the leaves). The newspaper (a wood product) will breakdown and also contribute to the organic layer.

After application of newspaper and the appropriate leaf material, we suggest that you wait two months (and maybe more in the winter). This is where the advances planning and patience comes in. After two months, the grass should be sufficiently dead that you can pierce the leaf layer and newspaper at desired locations and plant your trees and shrubs. Note that when planting through the leaf and newspaper layer it is important that any dug up mineral soil not remain on the surface (it must be removed or covered) because it is a desired growing medium for many common weeds (whereas a leaf mulch or leaf layer is not). The herbaceous plants can be planted through or in the leaf layer as appropriate.

In the photo above, the trees where planted first and then surrounding ground was covered with newspaper and leaves. Two months later, shrubs where planted around the trees and herbaceous plants planted in and through the remaining leaves and newspaper. The same space now looks like …

Leaf Mulch – Getting it and using it
OK! Now we’ve sold you a little on the idea of leaf mulch. Now where, for Heavan’s sake, do you get it? If you don’t have your own leaf pile left over from last fall, this might require a little searching. Fortunately, however, more and more sources are popping up all over the place. The first place to check is your local municipality. If your city or other governmental entity collects leaves in the fall, then they must do something with those leaves. In contrast to “dumping ’em somewhere -> the old approach, many municipalities are turning their leaves into mulch which is used on public property or sold to individuals or landscape professionals for gardening and landscaping. The City of Portland, Oregon, for example, windrows their leaves, monitors the internal temperatures of the rows to “cook” any weed seeds and facilitate breakdown, and turns the rows when appropriate. The available leaf mulch is typically from leaves collected a year ago.

The City of Portland, Oregon, charges $16/cubic yard, and the lot is located at __________ (off NE 33rd by the airport). Call _________ for hours of operation. If you cannot make it during operating hours or do not have hauling capacity, you can call a private hauler often found in the service directory listing in the local newspaper – in this case the Oregonian. Note that if your municipality does not have a leaf mulch program, let them know that they should.

A second source of leaf mulch is private processors. Landscape products companies – the same folks from whom you might have previously purchased bark mulch – may sell leaf mulch. For example, in Eugene, Oregon, Lane Forest Products (www.laneforestproducts.com) sells leaf mulch. Note that smaller cities might contract out leaf collection. If this is done, find out who the leaf collection contractor is and when they provide leaf mulch. Good luck, and let us know if you find a good source in your community. We’ll post it.

With respect to using leaf mulch, we suggest that you spread the leaf mulch over the area you intend to naturescape to a desired depth. In areas that will have trees and shrubs, ie, the ground will be partially or fully shaded, we suggest approximately four inches. In areas that will have full sun, we suggest approximately two inches. Also, if you live in a more arid region you might use less. This is good for starters. Note that you can find out what works best in your situation by trial and error and that leaf mulch is an actively decomposing substance. The four inches you apply in the fall may be three inches in the spring.

Other Preparation Considerations

Removing Hard to Remove Invasives – Some of us are unlucky enough to have English Ivy or Himalayan Blackberry or Kudza or Vinca or some other invasive exotic plant that we are challenged to get rid of. We cut it and it growns back. We turn our back and it spreads to 10 new locations. We might even despair and ask ourselves, what’s the point, we’ll never get rid of it.

But the answer is acutally that we will get rid of it. The important thing to remember is that plants need to perform photosynthesis to survive. If you interrupt photosynthesis, you will kill the plant.

Applying this to the task at hand, these and similar species can be removed by cutting them low to the ground and then continually cutting them and/or smothering them. If you can pull or dig the plant out by the roots, good. If you can’t, cut them back to the ground. The plants will grow back from energy stored in the roots and will attempt to perform photosynthesis to recharge the roots. If you cut the new shoots before they have much time to perform photosynthesis, you reduce the rate at which the roots are recharged. The roots will again put out shoots, but if you cut these shoots quickly, the roots begin to die. You can repeat this process until the roots, and hence the plants, are completely dead. The key is to stay on top of it, and cut the regrowth quickly.

If the plant proves hard to kill, you can also try smother it – and with something more than newspaper and leaves. Cut it back to the ground as described above, and then place cardboard or wood or the like over it. If necessary, use stones or stakes to secure your barrier. For stubborn plants, you may have to combine smothering and frequent cutting, i.e., smother the new shoots and pick up the barrier from time-to-time to cut the new shoots.

If the plant still proves to be stubborn, you may consider a chemical pesticide. If you do, we recommend that you use a more mildly toxic one and that you apply it only when dry and not windy, so that it does not run off. Note that we do NOT advocate the regular use of pesticides, but feel that limited use may be a "necessary evil: to achieve a better, long-term end.