Today's designers and contractors must know how to do more than simply design and install typical landscapes. They often must provide greenery on roof tops and interior spaces, typically with pottery and built-in planters. Contained plantings-whether in traditional pottery, modern planters or built-in cavities-require that we control the variables-air, water and chemical and physical soil factors within the planting medium-that affect plant performance and survival. Here, we'll discuss some of the many components, concepts and precautions necessary to make contained planting horticulturally possible and economically practical.
Waterproofing Building a planter into a structure subjects the surface of that structure to prolonged moisture. While this may not cause visible leakage, it may result in the deterioration of the structure and unattractive leaching of salts from the concrete components. To protect the structure, it is essential that you waterproof the planter area.
Waterproofing freestanding planters and pots is only necessary if they are composed of porous material. Either purchase the pots already waterproofed by the manufacturer, or field-apply waterproofing in strict accordance with the pottery manufacturer's recommendations. A specialty contractor will generally waterproof planters that are built into the structure of a building in advance of planting. However, you should seek written verification that the waterproofed planters have passed a flood test before you start your planting operations.
Drainage systems Each planter/root-medium/irrigation-system combination exhibits unique characteristics. In all cases, however, you'll need to eliminate surplus water from the root zone. In freestanding planters, this can be as simple as drainage holes in the bottom of the contrainer. Built-in planters require drain inlets designed into the planter as part of the building's plumbing. If this is not possible, it becomes necessary to capture the water in a sump, which will separate it from the plant roots until it is pumped out of the planter.
Surplus water must move along the bottom of the planter to the drain inlet. For this to occur, a drainage layer must span the floor of the planter. This layer generally consists of washed 3/8-inch pea gravel separated from the planting medium by a filter-fabric (geotextile) layer.
When weight is a concern, the design may substitute an engineered geocomposite drainage mat consisting of a geotextile fabric bonded to a semi-rigid, permeable underlayment. The underlayment holds the geotextile away from the interior surface of the planter, thereby creating a space where water can move laterally. Extending this product up the vertical sides of the planter enhances drainage on planter walls abutting occupied space.
I should note that the planting medium and geotextile fabric constitute an integrated system. The principal threat to this system is the migration of fine particles out of the planting medium and into the filter fabric. You can test component compatibility in advance by using the gradient ratio test (ASTM D5051 90). This test measures the relative decrease in filter-fabric permeability with repeated, controlled watering. From the graphed results of a representative sampling, you can predict whether a particular filter fabric will clog with "downwardly mobile" particles from a particular planting medium.
Planting media The most important part of a planter system is an appropriate planting medium. We use the term "medium" ("media," in plural) instead of soil because these materials are generally man-made and seldom include any naturally occurring soil. Planting media should allow for permeability and air exchange while compacting just enough to support the plant without excessive settling. Lastly, despite their high percolation rates, planting media must retain sufficient plant nutrients to sustain healthy plant growth.
Use a lightweight planting medium if the planter is on top of a structure (see related article, "Gardens in the sky," page 13). When the building design calls for extremely deep planters, the weight of the proposed medium volume may exceed the capacity of the structure. In such cases, you may need to displace some of the medium by using polystyrene-foam blocks. Arrange these blocks in layers, building them up from the bottom of the planter. Cover the blocks with the planting medium, leaving a surface layer of appropriate depth for the selected plant material. You also can place Styrofoam along the bottom and sides of the planter to provide protection for the waterproofing or thermal insulation of the root zone.
Most planting media begin with a blend of selected sands of various particle sizes. The sand particles must be hard to resist breakdown over time. Sharp, angular sand particles are preferable for their ability to compact tightly to support the weight of plants. A range of particle sizes, in specific percentages, is typically specified by the landscape architect or soils consultant as a means to finely tune the overall pore size and distribution throughout the blend.
Sand is essentially sterile and lacks the necessary properties to hold nutrients. Therefore, you must amend the sand with other components that hold nutrients. These amendments may be either organic (peat, bark, etc.) or inorganic (ceramics or volcanic products, for example) that, on a molecular level, have an affinity for the soluble nutrients you provide through fertilization.
This ability to hold nutrients until the root hairs pull them away is the cation exchange capacity (CEC). The primary considerations in selecting which of these products to use is their CEC relative to their resistance to decomposition. Current thinking suggests that when you use an organic component in the blend, it should constitute less than 25 percent by volume because any greater percentage will result in significant settling as the organic amendments decompose. Fortunately, you can succeed with a smaller-percentage organic component if you choose slower-decaying products with higher CECs, such as peat. These products are generally more expensive, but the initial higher cost is offset by the decreased likelihood that the planting medium will shrink and require replacement later. When planter mixes shrink and subside, the plants subside as well.
The cost of corrective action includes not only the cost to furnish and install added planting medium but the cost to raise plant material up to regain the proper relationship between the crown of the plant and the corrected finish grade.
Recent experiences during the construction of the Getty Center in Los Angeles are indicative of trends in on-structure planting and planting-media specification. Instead of using a homogenous blend throughout the full depth of the planter, we placed the media in layers that approximated natural soil horizons (see figure, below). The landscape-architecture firms of Olin Partnership Ltd. (Philadelphia) and Fong & Associates (Costa Mesa, Calif.) developed the following blends for the Getty planters:
* Upper 18 inches: 75 percent sand, 15 percent fir-bark fines and 10 percent composted sewage sludge * Next lower 18 inches: 90 percent sand and 10 percent AXIS (a diatomaceous-earth product) * All fill below 36 inches: 100 percent sand.
This combination supports the plant root ball, minimizes shrinkage, provides optimum pore space for drainage and includes sufficient organic and inorganic amendments to intercept and hold nutrients within the zone of maximum root development.
Irrigation systems The configuration and location of a planter dictates the type of irrigation system you use. If the planter is inside of the building, or represents a small part of the overall planting, it may be practical to hand water the planters with a hose or pressurized watering cart. If the number or size of the planters make it impractical to water by hand, an irrigation system should be part of the project's design.
When an irrigation system is necessary, it will most likely take the form of microspray or drip-emitter technology to limit overspray onto adjacent walls, windows or walks. If you use drip irrigation, the percolation pattern of the planting medium will dictate the spacing of emitter outlets to uniformly water each plant's root area.
Planting practices Selecting plant material for container planting involves aesthetic decisions beyond the scope of this article. Once you've selected the appropriate plant material, however, you can install it in conjunction with the placement of the planting media. With large trees, you may first need to brace them against the force of potential winds. Staking is impractical because sand-based media will not adequately support stakes and you could damage the planter's waterproofing. Instead, use guy sets held in place with buried deadmen instead of stakes or anchor bolts, which could breach the waterproofing.
After planting, you'll need to periodically check soil-moisture conditions with a soil probe to make sure that all of the systems are operating correctly. Of primary importance is the soil-moisture level throughout the planting medium. If moisture levels are too high, it may be the result of over-watering or restricted drainage. Check both of these systems before you adjust the maintenance routine.
Nutrient levels can be difficult to maintain in a sand-based soil medium. Monthly testing of soil-nutrient levels in the initial months will establish a baseline for fertilizing programs. After you establish this baseline, conduct testing every 6 months to a year.
Pottery and planters add an intimate dimension to our urban environment. However, because they are man-made systems from which we expect instant and lasting performance, it is important to plan in advance. Test the various component parts and integrated systems so that you can instantly achieve the equilibrium which, in nature, would require years to achieve.
Kelly Duke is director of pre-construction services for Valley Crest (Calabasas, Calif.). His most recent responsibility at Valley Crest was to serve as the project manager for the firm's work at the J. Paul Getty Center (Los Angeles).
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