The proper installation of plants in the landscape involves much more than just digging holes and setting plants in them. The existing soil is often compacted and poorly drained. Tilling or digging to improve aeration and drainage is essential for satisfactory plant growth. In some cases deep tilling or digging is necessary to eliminate a hard pan that was formed below the soil surface during construction.
The current trend is to plant trees and shrubs in large beds. Preparation of an entire bed is preferred over preparing individual holes since the roots will have a larger area to grow before they encounter native soil that might be compacted and poorly aerated. To achieve a 25 percent increase in organic matter, incorporate 3 inches of organic matter, such as pine bark mulch or compost, into the top 12 inches of soil. The organic matter used should be composted or aged. Incorporating uncomposted organic matter can create nutrient deficiency problems. Adding organic matter when preparing individual holes is not recommended.
Plants can be purchased as bare-root, ball and burlapped (B&B), or as container-grown plants. Planting procedure varies somewhat with each type.
The planting depth should be such that the plant is exactly the same depth after transplanting as it was in the container (Figure 1). Ideally, the hole should not be dug any deeper than the root ball. The loosened soil below the root ball can settle resulting in the plant being planted too deep. If you dig too deep, firm the bottom of the hole to reduce settling. Since most new roots will grow horizontally from the side of the root ball, soil firmed at the bottom of the hole will not substantially affect root growth. In most compacted urban soils, root growth from the bottom of the root ball will be limited by inadequate aeration and possibly excessive moisture. In some cases the roots in the lower portion of the root ball die after transplanting. Efforts to improve the soil should be directed near the soil surface by preparing the entire bed or digging wide individual holes.
Widening the planting hole is an ideal way to enhance plant growth. A planting hole that is three times the width of the root ball with the sides of the hole sloping towards the bottom is ideal in most situations. When digging in heavy soil the sides can become slick especially if the soil is somewhat wet. Slick sides can act as a physical barrier to root growth and moisture movement. Use a shovel to make the sides of the hole rough and irregular.
Always water plants thoroughly before transplanting. Remove the plant from the container by turning the plant upside down and giving the top edge of the container a sharp rap. Catch the root ball in your hands as it slips from the container. If plants have become overgrown in the container and the root mass is growing in a tight, compact circle the roots should be loosened before planting. If the roots are only slightly encircled you can loosen and spread them out by hand. Many gardeners cut the outer roots with a sharp knife by making vertical cuts approximately 2 inches into the root ball on two to four sides of the root ball (Figure 2) Some gardeners split the lower half of the root ball and spread the roots horizontally (Figure 3). This practice raises the lower roots closer to the soil surface. Plants such as azaleas, rhododendrons, and daphne that are especially prove to developing root rot in poorly drained sites could benefit from this butterflying practice. After the roots have been loosened or cut, carefully place the plant in the hole. Always pick the plant up by the root ball never by the trunk or stem (can cause damage to fine root hairs).
Soil that was removed from the hole should be used to refill the hole. Traditionally, the recommendation was to incorporate organic matter into the backfill (soil used to fill a planting hole). Some gardeners took the practice further and completely replaced the removed soil with purchased topsoil. Research has shown neither practice helps plants grow and in some cases can be detrimental. When water enters soil with one type of texture and later comes in contact with soil that has a very different texture water movement (drainage) is impeded. Excessive water can accumulate in the bottom of the hole which can lead to root suffocation or root rot development. Some researchers report that amended backfill can cause roots to remain in the planting hole instead of growing into the surrounding soil (roots will grow in the area of least resistance and greatest soil aeration). The primary reason for digging a wide hole is to improve soil aeration and to reduce compaction.
Lime should be mixed with the backfill, if needed, based on a soil test. Fertilizer should not be added at planting since it can burn the roots. An exception would be the application of phosphorus which moves very slowly, or not at all, in the soil and plays a key role in root formation. It will not burn the roots. Liquid fertilizers are some times applied after planting, but their benefit has not been proven.
Fill in around the plant with soil until the hole is one-third full. Firm the soil around the root ball, however, be sure not to use excessive force since soil compaction should be avoided. Loosen and break up any clods of soil before backfilling. Clods can create air pockets around the root ball. Before finishing the filling process, make certain the plant is straight and at the proper planting depth. It is important when planting (particularly container-grown material) to avoid covering the top of the root ball with more than 1/2 to 1 inch of fine soil. Ideally it should be level with the soil surface. Otherwise, water can be diverted sideways through the native soil and not soak down into the root ball where it is needed.
After a tree or shrub has been planted, construct a ring of soil 2 to 3 inches high to form a water basin at the outside edge of the hole (plants in beds probably will not require a water basin). This permits water to go into the root zone rather than running off the surface. Water the plant to eliminate air pockets around roots. The water basin does not need to be a permanent fixture and can be removed after the plants become established. Apply a 3-inch layer of mulch over the planting hole. The mulch will help maintain moisture and reduce fluctuations in soil temperature. The mulched area should be expanded as the plant grows.
in poorly drained sites
Prepared by: Erv Evans, Consumer Horticulturist, NC State University
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