The other day I was searching for peer-reviewed research articles on the planting depth of peach trees. For years I have promoted the proper planting depth of ornamental trees and shrubs based on such research and wondered what the research showed on the planting of peach and other fruit trees.
Research on ornamental trees shows planting depth directly influences plant health and has provided the recommendation that the uppermost structural roots be placed no deeper than 2-3 inches below the soil surface depending on the species of tree. The deeper the roots are placed in the soil, the more stress the trees suffer due to a soil oxygen deficiency and the more susceptible the tree is to insect and disease problems. Improper planting depth even results in the decline of the tree and death.
A number of publications on the planting of peaches made no mention of the proper planting depth for peaches. Some literature recommended setting the graft scar (where the bud was grafted onto the rootstock) 4 inches above the soil line; another recommended placing the graft scar 1-2 inches above the soil line, while other articles recommended planting peach trees at the same level they were planted in the nursery. I can think of a number of problems with these recommendations.
Planting based on the location of the graft scar assumes the peach was grafted onto the rootstock at a uniform height above the soil line. When you look at the grafting procedure on YouTube, you find the height of the graft can be from 4 inches to over 12 inches above the ground depending on the ability of the person doing the grafting to bend over.
Another of my concerns is whether the trees used for rootstock were all planted at the proper depth to begin with. One publication even recommended peach trees be planted with the graft union 1 inch below the soil line. Can this get any more confusing? No wonder we have peach trees in the Grand Valley with problems.
Placing the roots of peach trees too deep has been shown to result in trees improperly anchored and unable to withstand the push of the wind. Planting too deep also puts these trees under stress due to inadequate soil oxygen levels which means more chances of infection by Cytospora canker and other pathogenic diseases, physiological problems such as chlorosis, as well as insect pests.
According to Clemson University, "planting too deep can stunt tree growth, lead to soil compaction and a lack of oxygen for feeder roots, and even adversely impact tree stability." Planting too deep can result in defoliation, reduced leaf size and shoot growth, and crown dieback. I've always wondered if the problem local growers have with "Yellow Peach Syndrome" is due to improper planting depth and the resulting oxygen-deficient soils.
The deeper the roots are in the soil, especially in clay soils, the less oxygen is available for the metabolic activity of the roots. Cellular metabolism is necessary not only for nutrient and water uptake by the roots but for the production of the biochemicals the tree needs to protect itself from disease pathogens. Instead of protective substances, stressed trees are more attractive to attack by insects and are less able to respond defensively.
Most backyard orchards are started with trees grown in containers or in a field and provided as balled-and-burlap trees. Even these trees can be planted too deep especially if you don't check to determine the location of the roots in the container or burlap-wrapped root balls.
Commercial orchards are established using bare root trees. The trees are planted into holes dug with a posthole digger usually mounted on the back of a tractor, or into a trench dug the length of the orchard.
I don't like the posthole technique when the soil contains clay as these holes have glazed compacted sides from which the roots have a hard time escaping. Such trees are put under stress immediately upon planting. Cutting a trench also can be a problem if the sides of the trench are glazed by the implement used to cut the trench. Breaking up the glazed sides with a shovel might help correct this problem but might not. Digging the planting holes or trenches with a backhoe that rips and tears the sides is probably the best way to prepare the area for planting.
Based on research with other trees, peach trees should be set in the ground with the roots no deeper than 2-4 inches below the soil line. Following this guidance avoids the confusion of how high above the ground you should set the graft scar. Care needs to be taken to ensure each newly planted tree does not slip deeper than this recommended depth. When the planting hole is filled with soil, hold onto the new bare-root tree to ensure it does not slip deeper. Fluff out the roots as soil is placed under and around the young tree. Root density decreases with depth, especially in clay soils that limit soil oxygen levels to below that required for continued root growth. The critical area for root development is in the top foot of the soil. A shallow planting depth increase root growth in this area.
Planting depth, soil compaction, soil amendments (i.e. organic matter), soil moisture content and subsequent watering, soil phosphorus deficiencies and excesses, the competition of grasses and other weeds, mulching, hard pans resulting in water logging, and other factors affect the success of establishment of newly planted peach and other trees. All of these must be taken into account to ensure the backyard and commercial orchard is a long-term success. Neglecting any of these factors can result in increased Cytospora canker, root rot, and insect attack. A visit to your orchard can help identify these problems and develop techniques necessary to reduce or avoid these problems and increase plant health and the quality and quantity of next year's peach crop. Whether you have a small backyard orchard or are a commercial peach grower, you can reach me at 970-778-7866 and schedule a visit.
Winter is a great time for such a visit.
Dr. Curtis E. Swift is a retired horticulture agent with the CSU Extension. Reach him at Curtis.Swift@alumni.colostate.edu or check out his blog at http://SwiftsGardeningBlog.blogspot.com. He owns Swift Horticultural Consulting and High Altitude Lavender.