Bulk landscape soil may contain bark, aged tree debris, rice hulls, wood ash and other materials. (Photo by Allen Owings) Proper landscape bed preparation along with soil pH management is important in long-term landscape success. (Photo by Allen Owings) News Release Distributed 11/07/14
By Allen Owings
LSU AgCenter horticulturist
HAMMOND, La. – Whether renovating an existing landscape or starting from scratch, we need to remember that soil pH and proper bed preparation will be essential in assuring top performance of our ornamental plants. A Louisiana landscape planned for long-term success includes these important considerations.
Optimum soil pH is critical. Louisiana soils are somewhat variable in pH ranges. Ideally, a perfect soil pH for most ornamental plants in Louisiana is 5.5-6.5. Soil pH is a measurement of its acidity or alkalinity. A pH value of 7 is neutral while a pH value less than 7 is acidic and a pH value greater than 7 is alkaline or basic. Soil pH is raised by using lime – normally dolomitic lime in landscape situations – and is lowered by using sulfur. Always adjust pH based on the results of a soil test.
The LSU AgCenter Soil Testing and Plant Analysis Lab can conduct a soil test for you. It will provide a report with information on soil pH as well as the levels of many essential nutrients present in your soil. A routine test is $10. You can learn about the soil testing Lab by clicking here.
Some plants in Louisiana landscapes and home gardens are classified as acid-loving. These plants do best with a soil pH slightly lower than other plants we commonly grow. A soil pH in the 5.0-5.5 range is preferable for plants that require more acidic growing conditions. Common examples are blueberries, camellias, sansanquas, dogwoods, azaleas, periwinkle, petunias and pansies. In turfgrass, centipede grass prefers acid soil, while St. Augustine grass prefers neutral to slightly alkaline soil.
Once you know your soil pH, you can move on to bed preparation. Several factors need to be carefully considered when you are developing beds for ornamental plants. Improving internal drainage should be the first priority. This can be accomplished by amending some of our existing soils, but more intensive work may be needed in more poorly drained soil types.
French drains can remove water from poorly drained areas by providing subsurface drainage. You can construct a French drain by first selecting an area lower than the landscape site. Dig a trench, fill it partially with gravel and lay pipes to carry water away from the planting site. Sometimes lawn areas benefit from French drains, and landscape beds may need French drains, depending on the situation.
Raised beds are almost always essential for successful landscape plant establishment if French drains or “pitcher’s mounds” are not used. A raised bed at least 6-8 inches deep can be enclosed with decorative bricks, concrete edging, landscape timbers, railroad ties or 4x4s. Chemically treated wood is safe for use around ornamental plants. A raised bed does not necessarily have to have a physical border on the edge. If properly prepared and well mulched when completed, the soil should hold in the bed and not wash away in heavy rainfall.
We recommend a “pitcher’s mound” or berm when planting an individual tree or shrub. This accomplishes the same thing as a raised bed, but it’s done for an individual plant. The berm should be 1 foot tall and come out from the center gradually, sloping down to the level of the surrounding soil.
If you’re planting directly into a heavy clay soil, incorporate a 3-inch layer of new soil to form a transition layer between the existing soil and any soil that is added. A sudden change in soil texture disrupts the flow of water and causes a stagnant area beneath the new soil. It’s highly likely that roots of a newly planted tree or shrub will not move out of the planting hole if you don’t follow proper planting procedures.
Faculty at the LSU AgCenter also advocate the “kill, dump, rake and plant” method of bed preparation for annual bedding plants, herbaceous perennials and small shrubs. Kill – with a spray of glyphosate herbicide – the area where you want the new landscape bed to be, then spray again in 10-14 days to remove last existing vegetation. Obtain good landscape bed soil from a reputable dealer and dump this soil over the area. Then rake the new landscape soil and plant into it. It is best to make the bed a minimum of 6 inches high. Also, wait for one rain to settle the soil prior to planting. We recommend laying mulch prior to planting and planting through it instead of mulching afterwards.
Soil preparation, drainage and pH are very important in landscape gardening success. Don’t overlook these very important factors.
You can see more about work being done in landscape horticulture by visiting the LSU AgCenter Hammond Research Station website. Also, like us on Facebook. You can find an abundance of landscape information for both home gardeners and industry professionals at both sites.
Rick Bogren