'Improved Planting Standard' changes rules for trees
Telling someone how to plant a new tree or shrub is one of the most difficult tasks I face in my job. This is because a tree planting myth has been circulating for many years. The myth says: When planting a tree or shrub, thou shalt dig a hole twice as wide and twice as deep as the root ball and amend the backfill with organic matter. For those who enjoy myths and believe this is the best way to plant a tree or shrub, then I will probably have a difficult time convincing you otherwise. However, if you believe in using the scientific method and peer-reviewed research, I hope to change your mind.
The information that follows is the "Improved Planting Standard" and results from research conducted at universities across the nation. Begin by digging the planting hole only as deep as the root ball and loosen the soil in an area three to five times as wide as the root ball. The interface where the loosened soil meets the undisturbed soil should be rough and sloped (not glazed or vertical). When planting, place the tree or shrub in the planting hole such that the original soil line (root collar) of the tree will be at finished grade after planting. Backfill the tree with non-amended, native soil. Create a berm which becomes an irrigation basin that extends just beyond the root ball. Do not compact the soil by tamping it hard after backfilling the tree or shrub. After planting, place 3 inches of organic mulch on the soil surface but do not allow it to contact the trunk. Remove the original nursery stake. Stake young trees only if necessary. Irrigate the tree or shrub as necessary wetting the root ball and surrounding soil. Do not prune or fertilize the tree after the planting.
Think about how native trees and shrubs become established to further understand factors influencing landscape tree/shrub survival and growth. In a native setting, thousands of seeds are produced and these are disseminated by various methods to a multitude of sites. Only sites with an ideal microclimate and moisture regime allow seeds to germinate and survive beyond one or two seasons. In this time, the seedling puts down a taproot from which lateral roots develop. The taproot does not persist in most woody plant species. The lateral roots are rarely deeper than 2 or 3 feet (they need water, nutrients, and oxygen). On a mature native-grown tree, the diameter of the lateral root system can be three to five times the height of the tree.
There are two major problems associated with the old "twice as deep, amended backfill" planting method. As organic amendments in the backfill decompose, the volume of backfill shrinks, the root ball sinks lower into the soil, and the lower portion of the stem gets inundated by water and wet soil. The above-ground portion of the bark is permeable to air and will not tolerate the waterlogged conditions that will likely be present. This often results in fungal disease organisms getting established in the newly planted tree or shrub. In addition, lateral roots that grow into amended soil tend to stay in the amended soil. The planting hole can become much like a pot and roots will start to circle rather than utilize the surrounding native soil. These circling roots can girdle the plant as it matures.
Take care to select an appropriate tree species that suits the planting location. Avoid selecting trees that have circling roots in the nursery container - you should be able to look at the root system before purchasing by gently loosening the container and gently lifting the plant out of the pot for inspection. If it is too large to handle, ask someone at the nursery to assist you. You don't want to select the largest tree or the smallest tree - the tree should be "just right" for the container. If you do get a tree home and it has some circling roots, they can be pruned by making vertical cuts on the root ball with a clean, sharp utility knife on four quadrants.
The Improved Planting Standard is definitely a better practice than the old planting method. Nurseries and garden centers are in agreement with the size and shape of the planting hole, but still recommend organic amendments in the soil backfill. This has been shown to establish better root systems that will support and nourish the tree or shrub throughout its lifetime. For more information, see University of Arizona Cooperative Extension publication Planting Guidelines: Containerized Trees and Shrubs (https://extension.arizona.edu/sites/extension.arizona.edu/files/pubs/az1022.pdf). You may also pick up a copy of the publication at our office.
If you have a garden or landscape question, bring it to the University of Arizona Cooperative Extension Office. Our address is 840 Rodeo Drive, Suite C, Prescott, on the Prescott Rodeo Grounds. Hours are 9 a.m. to noon and 1 to 4 p.m. Monday through Friday. You may also call 928-445-6590 or email PrescottMG@gmail.com.
Jeff Schalau has been the Agriculture and Natural Resources Agent with the University of Arizona Cooperative Extension, Yavapai County since 1999.