Seedbed preparation

Before the mechanical equipment rolls in to prepare a seedbed for turfgrass plantings, the enlightened grounds-maintenance manager has already spent hours of time preparing for a successful project. In fact, before the first ripper teeth hit the soil surface, he or she has already worked out a well-orchestrated turfgrass-planting plan.

To help ensure you meet the definition of an "enlightened" grounds-maintenance manager, here is a simple checklist for getting a project going and making sure you use the appropriate mechanical equipment to do the job.

* Select the right turfgrass. Before you write the specifications for seedbed preparation, you must select the appropriate turf-seed mix. To do that, you must know the site's primary use. For example, will people use the site as a playing field, an open space or an entry area? Each of these specific uses demands the proper selection of the most suitable turfgrass.

* Test your soils. Test your site's soil - at the minimum - for pH, macro- and micronutrients and for soil texture (the relative proportions of sand, silt, clay and organic matter). Examine the soil's depth and determine its relative soil horizons: the 'A' (topsoil) horizons and 'B' (subsoil) horizons. You can achieve this with the use of a backhoe, a shovel or an auger.

This step is important because knowing the nutritional and physical status and depth of the soil in which you'll be working enables you to determine what soil amendments are necessary to produce the best turf possible.

* Evaluate other microclimatic considerations. Does the site of the turf installation experience prevailing winds? Are some areas sunny, while others are shady? Do different areas of the site have different soil types? Knowing your microclimatic differences can help you choose the most appropriate seed. For example, noting that some areas are sunny and others are shady - or knowing where two soil types interface - will help you better determine the correct turfgrass species, as well as irrigation systems, to use.

* Plan your irrigation systems well. Most large turf areas use rotor irrigation heads, which throw water great distances. Smaller areas typically use pop-up spray heads. Determine which is appropriate for your site using the advice of an irrigation consultant. In addition, consider sub-surface irrigation systems.

* Consider grading and drainage implications. Make sure your site has smooth flowing surfaces, slopes with a minimum 2-percent grade (but no greater than 25-percent grade - 1 foot of elevation change for every 4 feet of run) and gradual transitions between flat and steep portions. (Don't forget that mounds, berms and hills are forms of elevation changes.)

Also keep in mind maintenance aspects of the site after the turf has successfully grown in. For example, make sure slopes are not too steep for safe mowing. And check that irrigation laterals are installed horizontally to the slope - not vertically. In addition, make sure that top, middle and lower irrigation heads are individually circuited.

* Plan for equipment access and verify property lines. A commonly overlooked factor in the installation of a large turfgrass area is equipment access. Before starting a project, ascertain the travel needs of installation equipment, such as dump trucks, and determine their general dimensions. After all, even though a larger piece of equipment may be more useful, if access is difficult, you'll have to get by with a smaller piece of equipment - even if it is less efficient.

Evaluate property lines thoroughly before starting a project. If any access routes or property lines are in question, you may need a land surveyor to stake out those specific lines and boundaries.

Preparing the seedbed Seedbed preparation varies greatly from project to project, and the use of specific seedbed-preparation equipment depends on the scale and complexity of the job. The table on the opposite page illustrates the specific equipment best for each required task. After considering the types of equipment, refer to the items below for the step-by-step process you'll follow for completing a complex project or a simple one. In chronological order, these required tasks are:

1) Remove existing vegetation. If the site is heavily wooded, you'll need to remove trees, shrubs and other vegetation. Although shrubs and herbaceous materials are easy enough to remove, trees and their roots are another matter. Cut a tree as close to the ground surface as possible. Next, remove the stump. Depending on the tree's size, you may need a backhoe to do this. Then dig out large-diameter roots. How much of the tree's roots you need to dig out depends on your individual site. To determine if you've removed enough, look at what's left in terms of how close the remaining roots are to the surface; remove any large roots within a foot or two. Otherwise, subsequent decomposition of the roots will cause the soil surface to settle.

2) Remove large rocks, boulders and other impediments. If the site is rocky, remove large boulders and rocks (of roughly more than 1 foot in diameter) using mechanical equipment such as a backhoe or front-end loader. Once you've done so, you can remove smaller ones with a tow-behind rake during subsequent finish-grading operations.

3) Rough grading. If your site needs extensive grading, ask your landscape architect or engineer to prepare a grading plan. The plan should state the total quantity of soil you'll need to remove or replace. Your soil analysis should indicate the nature of the subsoil. If the soil is good, then - for extensive cuts - you can calculate what, if any, of the cut material you can stock pile on site for subsequent reuse. You then can specify the respective subsoil preparation (such as scarification or rough grading - that is establishing the contours on which you will place the topsoil). At this point, you also should rake the surface of the soil to a depth of from 6 inches to 1 foot to remove rocks, debris or other objects from the immediate soil surface.

4) Placing the topsoil. If your site needs extensive grading and requires outside fill soil, make sure you test the fill soil before placing it on the site. (Require your fill-soil provider to give you the results of the soil testing in writing.) If instead you have stockpiled site topsoil, now is the time to replace it. Don't place more than 1 foot of topsoil at a time. Make sure the project engineer is on hand to compact it. Then place the next 1 foot of soil and compact it, and continue doing so until you've reached the depth or thickness your site requires.

5) Dig trenches for the irrigation system. Two schools of thought exist on trenching for irrigation systems in relation to the success of the turfgrass. One method is to trench during rough grading. The other method is to trench after finish grading. The benefit of trenching during rough grading is that the subsoils are brought to the surface and are subsequently amended during incorporation of organic matter and amendments. The downside is that if you plumb irrigation laterals and stand pipes before finish grading, you're likely to accidentally break some during further rotary-tilling operations. However, if you do the trenching after finish grading, then you'll be bringing subsoil to the surface, which can be notably different than the amended soil. These subsoils can be problematic in terms of the uniform growth of turfgrass. Thus, you'll need to decide which of these techniques offers the "lesser" of two evils for your situation.

6) Amending soil and incorporating organic matter. Depending on your soil analysis, this is the time to amend the soil. This can include adding soil nutrients, chemical amendments or organic matter.

You can amend the soil in two ways. 1) You can spread the amendments over the soil surface and rotary till them in. 2) You can mix the amendments with the top soil before placing the soil back on the site. (You'll get more uniform mixing by using the second method.)

After making such additions, don't be surprised if your site suddenly looks like you have more soil than you need. Incorporating organic matter and soil amendments also incorporates air into the soil, and the soil may initially appear more voluminous than desired. After a few irrigations, however, the soil will gradually settle down to the planned grade.

7) Finish grading and rolling. Once you've amended the soil, the next step is to smooth the soil surface and slightly compact it. If necessary, rake the soil to remove smaller stones and roots. At this point, you have finally achieved the surface necessary to place the seed, plugs or stolons or perform hydraulic seeding or sodding - whichever your individual project specifies.

Steve McGuirk, ASLA, is a landscape architect at Madrone Landscape Group (Soquel, Calif.), and Dr. Ali Harivandi is with the Cooperative Extension Service at the University Of California (Hayward, Calif.).

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