Proper Planting will Prevent Deep Roots

Even when trees meet the requirements of American Standard for Nursery Stock (2004, www.anla.org) which states that “soil above the root flare shall not be included in the root ball depth measurement”, there can still be excess soil over the structural roots. You may have to adjust planting techniques so that the structural roots will be at the correct depth. (illistration by: S. Wegener)

For additional tree planting information:

• Watson, G. W. and E.B. Himelick. 1997. Principles and Practice of Transplanting Trees and Shurbs. International Society of Arboriculture, Savoy, Illinois. 200pp.
• Watson, G.W. and E.B. Himelick. 2005. Best Management Practices: Tree Planting. International Society of Arboriculture, Champaign, Illinois.
• Tree planting details and specifications on the web— www.isa-arbor.com/publications/cadDetails.aspx

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Even when trees meet the American Standard for Nursery Stock (2004, www.anla.org) which states that “soil above the root flare shall not be included in the root ball depth measurement”, there can still be excess soil over the structural roots. You may have to adjust planting techniques so that the structural roots will be at the correct depth.

When root balls arrive on site, the depth of the structural roots should be checked before placing the tree in the planting hole. When burlap over the root ball cannot be removed to locate the roots before placing the root ball in the planting hole, use a chaining pin to probe for roots through the burlap to estimate root depth.

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Many tree cultivars in the northern half of the US are bud grafted; this is less common in the southern half of the US. Evidence of the “dogleg” in the stem and bark texture change should be approximately 3-6 cm (1-2 in) above the soil surface for a young tree.

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The first priority is to have the structural roots at the correct depth when the planting job is finished. If the structural roots have been located within three inches of the surface, the root ball should be planted with the surface no lower than the same level as existing grade. Slightly higher is usually preferable to allow for settling and ‘flattening’ of the root ball. Do not plant so high that the cut ends of the structural roots at the edge of the root ball are above the surrounding grade.

When planting, the roots should also not be too close to the soil surface. The roots must be covered with soil.  If the roots are too shallow on a small tree, especially on one with horizontal roots, there may not be enough soil covering them to prevent exposure by erosion or frost heaving.  As roots increase in size over time, the amount of soil over them will be reduced.

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Though the roots may be covered with a couple of inches of soil when planted, as a tree grows, roots thicken faster on the top side. Each year the amount of soil covering them is less. Indirect evidence of this process was provided by a study of nursery trees where the roots of 2 year older trees were a half-inch shallower (See Rathjens [pdf]). If planted too shallow, both thickening and frost heaving can eventually result in excessive surface roots.

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It does appear to be possible to ameliorate prior planting depth problems from poor production practices by adjusting planting depths in the landscape. (Bryan, LBG [pdf]) If the structural roots are found deeper than three inches below the root ball surface, the root ball and the planting depth will have to be adjusted. Elevate the root ball so that the structural roots are at the correct depth relative to surrounding grade.

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Of course, there will be times when the roots are just too deep and the tree must be rejected. (photo by: V. Phillips)

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If there are a few small roots several inches above the main group of roots with a gap of more than 8 or 10 cm (3 or 4 in) in between, planting deep enough to cover the small roots with soil would force the main group of roots deeper than 10 cm (4 in). If the high roots are small, cut them off and raise the main group of roots to the correct depth. If the high roots constitute a substantial portion of the root system and can’t be removed, the tree may have to be rejected. (photo by: V. Phillips)

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Removing excess soil over the roots could be problematic for a variety of reasons.

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If this soil is included in the root ball depth measurement, but has no roots, the root system will be undersized. The functional root ball volume shown in the photo (red line) is only 25% of normal because the roots were too deep in the root ball. This tree did not survive and should have been rejected before planting.

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In container plants, the extra substrate over the structural roots could also be filled with fibrous roots. If these roots are removed at planting to position the structural roots at the proper depth, the remaining root ball will be undersized and extra stress is likely to result. (photo by: D. Fare)

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When the extra soil is suddenly removed from the base of the trunk (this could actually be a portion of the original primary root in some cases), there have been reports of cold damage, sunscald and borer problems in the newly exposed tissue. Late fall plantings may be the most susceptible to cold damage. Earlier plantings may have time to harden off normally. Thin-barked trees may be most susceptible to sun injury.

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The only research data available showed that in November, green ash root shank tissue from just below the soil surface was as cold hardy as stem tissue, while root tissue from several inches deeper in the soil was less cold hardy than stem tissue (Watson, unpublished data). There was not difference earlier or later in the season. Based on this data, root shank tissue recently exposed by planting high, but not yet hardened off when a rapid drop in temperature occurs, could be less cold hardy.

Until this situation is more fully understood, exercise caution when removing soil and exposing tissue that is accustom to being below grade.

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• Leaving the extra soil in place above grade to be removed slowly or erode away, as described above, may help to protect sensitive trunk tissues. In one study, there were no adverse effects of the exposed rootballs reported. (See Bryan [pdf])
• Mulch can be used to hide the protruding root ball. If the soil is removed, replacing it with organic mulch may help to protect from sun and cold damage initially, exposing the trunk tissues gradually. The mulch should not be maintained at this depth indefinitely. Trunk protection may help to prevent sunscald.

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One of the most controversial topics centers around methods used to remove the excess soil from B&B and container root balls. Some have advocated removal of nearly all the soil, to the point of making them bare root plants.

This does allow thorough examination of the root structure, and makes it easy to see where the roots are when planting.

An ongoing bare rooting study of field and container-grown 8 cm (3 in) caliper dormant or actively growing red maple and willow oak trees includes root ball soil removal by soaking, pressure washing, air excavation and dropping the root ball from nearly 4 m (12 ft). No bare rooting method or season reduced survival or growth of red maples. Survival of willow oaks bare rooted while actively growing was lower, with smaller differences resulting from production method and bare root method. (Appleton, LBG [pdf]).

In other studies of whether bare-rooting containerized materials would have a deleterious effect on establishment of smaller container-grown plants, the results have been mixed. Survival and growth of pines were reduced, but the response was species dependent (Hummel, LBG [pdf]). No deleterious effects resulted from bare-rooting Myrica californica and Thuja occidentalis. (Chalker-Scott, LBG [pdf])

There must also be certain drawbacks. The cost of buying a more expensive tree, plus the cost of removing the soil may not be practical. (photos by: B. Appleton)

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