Tree Selection and Planting: Best Practices

Tree Selection
and Planting:
Best Practices
Recommended by the
Nebraska Forest Service
February 2012
Proper selection, planting and care is critical to the establishment of
healthy, thriving trees. The following selection and planting recommendations have been developed by the Nebraska Forest Service with input
from nursery and arboricultural professionals.
This publication online at http://arboretum.unl.edu.
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Species Selection, Siting and Design
1. Know the planting site, including soil type and conditions, and choose species
that will thrive in those growing conditions. Understanding soil pH and organic
matter content are especially important.
2. Broad species diversity is strongly recommended across a community or neighborhood. A general rule of thumb involves the 10/20/30 rule, which suggests
trying to achieve a limit of no more than 10% of any one species (red maple, hackberry, etc.); 20% of any one genera (oak, maple, pine, etc.); or 30% of any one family (beech, legume, elm, etc.). These are not hard rules but rather general targets.
Some genera, such as oak likely deserve a greater allowance. On an individual site
diversity should be balanced with the aesthetic benefits of repetition and massing.
Planting all of one thing or one of everything is rarely if ever advised.
3. Planting native species is good practice whenever it makes sense to do so. Native
species help provide a sense of place and they typically do a better job of enhancing biodiversity having evolved to provide food and shelter for a wide variety
of native birds, animals and insects. However, the appreciation of native species
should not mean the exclusion of non-natives. The reality is that many introduced species and cultivars have proven to be very beneficial for our community forests, helping to make them more diverse, beautiful and resilient. Caution
should be used, however, to avoid using any species that could become invasive in
surrounding natural areas. A list of such species is available on-line at: http://snr.
unl.edu/invasives/invasiveplants.htm.
4. Tree spacing recommendations can vary by species, purpose and design. However
if we examine how trees grow naturally (within forests) we realize planting trees
close together in mutually supportive groups increases survivability as the trees
help protect each other from strong winds and storms. Scattering trees too far
apart in the landscape is a larger problem than placing them too close together.
NFS generally recommends that shade trees be planted about 20-40’ apart, and
evergreens and understory trees about 10-20’ apart. However, there is nothing
wrong in planting large-maturing trees as close as a few feet apart. Keep in mind
that two or more trees planted tightly together will structurally rely upon each
other as they grow and should be viewed as a single unit rather than as individuals.
5. When possible, mass trees together with shrubs and other landscape plants in
larger beds or islands to help create separation from conflicting maintenance
zones, especially turfgrass. Such massing helps alleviate common problems such
as over-irrigation, mower/trimmer damage, herbicide damage, and soil degradation.
Nursery Selection & Purchasing
1. Buy from reputable nurseries licensed to do business in Nebraska. Be careful
buying from large box-retailers and other outlets that only offer plants seasonally.
The seed source and quality of such trees can be questionable.
2. Inspect trees thoroughly before purchasing. Reject any plants with significant
branch or trunk damage, poor branch structure and/or compromised root systems.
3. Although trunk and branch structure can vary by species, most nursery shade
trees should have these things in common: a straight trunk with a central, dominant leader; good trunk caliper and taper so that the tree can stand without
staking; branching properly distributed along the central leader and not clustered
at a few points; retention of low, lateral branches (see diagram); and a balanced
form. Allowances should be made for fruit trees, small ornamentals like redbud,
multi-stem types, and certain species that typically lack a main central leader such
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as coffeetree, yellowwood, zelkova, certain elms, etc.
4. An often overlooked, but vitally important part of any nursery tree is its root
system. Ideally, nursery trees should have fibrous, well-branched, laterally
spreading roots that possess many fine feeder roots. Regardless of the production
or digging methods used, the root system should be inspected for common problems such as stem girdling roots, excessively pot-bound roots, downward growing
main roots, stubbed-off and undercut roots, etc.
Nursery Production Methods
1. Bare-root trees have root systems that are completely visible and known
at planting time. Such root systems should be fibrous, well-branched, laterallyspreading and possessing fine feeder roots. Bare-root trees typically cost less than
most other growing methods and are easy to handle and transplant. A disadvantage is that some species, including most oaks, can have a difficult time breaking
dormancy if not “sweated” before planting.
2. Container-grown (plastic containers). Trees grown in smooth-sided plastic
containers are now the most common production method used for retail sales. Unfortunately they often have root systems that are “pot-bound” and circling in the
container, and often with hooked and downward plunging main roots. Such root
systems can be very difficult to plant properly. Pot-bound trees planted without
efforts to cut deformed roots or to spread out circling roots often end up with
stem-girdling roots and missing anchor roots, making them more likely to fail in
storm events later in life. NFS discourages the use of any such trees with severely
compromised root systems. Recent research is showing that the shearing of circling roots and matted roots at the container’s edge might help in salvaging some
pot-bound trees at planting time. However, this shearing of roots limits the planting window to early spring and fall. Some trees are now being grown in plastic
containers designed to prevent root circling. These containers, generally known
as “root-production containers” have slits or openings that force air-pruning of
roots and graduated or layered sides which help reduce the amount of root diving within the container. Although some of these systems look promising, greater
evaluation needs to be done before recommendations can be made.
3. Fabric Containers (grow bags): The growing of trees in fabric containers, commonly known as “grow bags”, is a growing trend in the nursery industry. Grow
bags are designed to allow some root penetration into or through the bag and the
consequent “trapping” or “pinching” of the roots at that point, greatly reducing
their circling or girdling within the bag. Potential benefits of fabric containers
are a fibrous, non-circling root system and a longer season of transplant. NFS is
working with Nebraska nurseries and growers to expand the availability of trees
grown in fabric containers. We are also awaiting research and trial results being
conducted in other states that will compare some of the more prominent fabric
container systems.
NOTE: For any container growing system, timing is very important. Plants
should not languish in the container for any length of time and should to be
bumped up frequently, with proper root pruning, to prevent major root deformities.
4. Balled and burlapped (B&B) trees are hand or spade dug with a soil ball typically
wrapped in burlap and supported by a wire basket. The advantages are longer
transplanting season and the benefit of root-soil contact at planting time. The
primary disadvantage is that much of the root system, including most of the fine
feeder roots, is left behind at digging time. Other disadvantages includes a heavy
soil ball and often a root system buried deep within the ball, which leads to trees
being planted too deep in the landscape. NFS recommends a minimum ball size of
18” diameter for each 1” trunk caliper measured at 4” above soil line.
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5. Spade-dug trees are moved directly with a tree spade from the growing field or
the landscape to their new location. The benefits of tree spading include the ability to move larger trees, no need to directly handle the root system, and no need
to hand dig or auger the planting hole. The primary disadvantage is that much of
the root system, including many of the fine feeder roots are left behind, so timing
and size of tree spade are critical. Typically, the larger the spade, the better. NFS
recommends a minimum spade width of 24” for up to a 1” caliper trunk, with 12”
additional spade width for each ½” increase in trunk caliper (i.e. 48” spade for 2”
caliper trunk). Timing of digging should be late winter or early spring before bud
break and then again in the fall after leaf drop.
Timing:
In general, the best time to plant trees is early spring (March through early May)
or in the fall when temperatures are cooler and trees are dormant or nearly dormant. Trees can be planted in the winter if the ground is workable. It is best to
avoid the hottest part of the summer (late June through August) for all trees.
Timing can vary depending on the growing and planting methods used. Keep in
mind that significant cutting or shearing of roots, such as with container grown
stock, should not occur when the tree is most actively growing (May-July). This
is one reason why fall can be a good time for the planting of such trees. Ideally
the digging of trees for transplant should occur in late winter or early spring
before bud break and then again in the fall and early winter after leaf drop. Most
evergreens are best dug in early spring before candling (bud break), and gain in
late summer to early fall when there is still some growing season remaining after
digging.
Site Preparation:
In general, trees grow best in native, healthy top soil. Healthy soils are highly
organic and are alive with a myriad of insects, fungi and microorganisms. Avoid
planting in sterile fill soils and soils that drain poorly. On sites with poor soils, it
is especially important that species selection be carefully considered. Compacted,
sterile and degraded soils can be improved by working in organic matter (compost, mulch, etc.) and planting deeply-rooted or nitrogen-fixing grasses and perennials. Another strategy is to plant soil building cover crops a year or two before
planting trees.
Digging the Hole
1. Dig a saucer shaped hole wider than the root system so that there is room for
loose soil to be backfilled around the root ball (see attached diagram). The hole
should not be dug deeper than the root mass. Time should be taken before digging to figure out how deep the root mass will be by locating the first main lateral
roots and measuring to the bottom of the root mass (see discussion below on root
depth). If the hole has been dug too deeply and needs to be raised, the bottom
should be tamped firm enough to prevent the tree from settling deeper after planting. This is especially important when digging the hole with a mechanical auger
– deeper is not better!
2. The soil should be friable and workable at planting time, not excessively dry,
hard, wet or gooey. A small spade-fork comes in handy to help break up soil clods.
For excessively hard or chunky soils, a small rototiller can be used to till up the
planting site before or during the planting process. Care should be taken to not
destroy the surrounding soil tilth with excessive tilling.
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Planting The Tree
1. Planting elevation. Plant so that the base of the trunk is at the original
ground level or slightly higher. The trunk flare or root collar, the transitional
area between the stem and roots, should be visible above grade. In heavy, claydominated soils and soils that stay wet longer, the tree should be planted slightly
above grade (2”-4”) with the soil sloped gradually back to original ground level.
2. Root depth is critical! Regardless of planting elevation, the first lateral roots
should end up just under the soil surface (1” to 2” deep) and the trunk for most
species should flare visibly where it meets the ground. Always locate the first main
lateral roots and remove any excess soil above them before setting the plant in the
hole. The first main roots are often several inches below the top of the container
or root ball. All graft unions should be visible above the soil line.
3. Remove all plastic containers, fabric containers and grow bags before planting!
4. Remember that the primary goal of planting is to insure a laterally spreading root system! For container-grown trees, carefully remove any loose growing medium from around the roots by shaking, soaking or washing with
TYPICAL TREE
PLANTING DETAIL
If staking is necessary, use
two opposing belt-like straps.
Place top of root mass at or
slightly above existing grade
(2” max) first lateral roots 1-2”
below soil surface.
Use plastic guard to protect
from rabbit or mower damage.
Mulch 2-4” deep to dripline or
beyond. Keep mulch off trunk.
Remove containers,
wire, string, rope and
tags. See note on
burlap & wire basket,
page 2.
Root flare should be visible at
base of trunk.
Set root mass
on undisturbed
or firm soil.
Dig hole 2-3x diameter of root mass
(No Scale)
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a hose. Exposing the root system helps reveal circling, hooking and girdling
problems and will also allow for better root-soil contact when the tree is planted.
Circling roots should be loosened and spread laterally if possible before planting. Cutting of larger roots that can’t be straightened may be necessary to
prevent girdling and to better insure a laterally spreading root system. This includes any hooked roots that are growing downwards. Recent research is showing
that the shearing of the 1-2” outer layer of tightly circling and matted roots at the
container’s edge may be necessary at planting time. The shearing can be done with
a sharp spade after the tree is in the hole and properly backfilled. Keep in mind
that significant cutting or shearing of roots will shock actively growing tree,
making the timing of such cutting important (see prior discussion on timing).
5. For balled and burlap (B&B) trees in which root integrity can’t be guaranteed
before planting, NFS recommends removal of the wire basket and burlap at
planting time. The primary purpose for this is to help expose the root system so
that any root problems can be corrected before planting. It’s our experience that
much of the soil ball can be allowed to fall apart or even be carefully removed
before planting without harming the integrity of the tree (see discussion on timing). The simplest method is to remove the wire basket and burlap before carefully
placing the tree in the hole. If maintaining the integrity of the soil ball is important or is required by the nursery, then the tree can be placed in the hole first
before the top 12-18” of the wire basket and burlap are cut away. Excess soil over
the root ball should be removed at this time and the roots should be examined for
any problems such as hooking or girdling. Remember to check for proper planting
depth by locating the first main lateral roots before planting. A surveyor’s pin is a
good tool for this purpose. Finally, cut away any twine or tying materials that may
be wrapped around the base of the trunk. Many trees are girdled and die for lack
of this simple effort.
6. Once the tree is set in the hole, it should be backfilled with the original soil dug
from the hole. Large clods and soil chunks should be broken up beforebackfilling.
Composted organic matter or good top soil can be added to the backfill if desired,
especially if planting in poor soils. The tree should be watered during backfilling to help remove air pockets and moisten the roots.
Mulching
1. Adding wood mulch around new trees has several benefits including helping to
preserve soil moisture, cooling the soil in the summer, improving soil organic content, reducing weed competition, reducing mower/trimmer damage, and improving aesthetics. However, problems can occur when mulch is misapplied, especially when piled too deeply or directly against the trunk. Recent research shows
that mulch placed deeply and directly up to the trunk can lead to the development
of stem girdling roots.
2. Although there is some debate over best mulching practices, NFS recommends
mulching individual trees with a 2-4” layer of wood mulch extending from
near the trunk (but not touching the trunk) to at least the drip line of the
tree. The wider the better (typically a 5 to 10 foot diameter circle) as long as the
mulch ring can be properly maintained over time. An even better solution is to
mulch trees in mass with other trees, shrubs and/or companion plantings. Such
massing helps create a wider organic rooting zone that more closely mimics forest
conditions.
3. Mulch should not be piled against the trunk or mounded deeply like a volcano.
Mulch should also be allowed to degrade significantly before new mulch is reapplied. Mulch should not build up deeply up over time.
4. It is preferable to use locally generated mulch from tree trimmings or other
wood waste. Bagged cypress mulch should not be used as it may have come from
environmentally-sensitive swamp forests. It also has the tendency to mat tightly
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together as it decomposes forming an impermeable layer.
5. Do not use rock mulch, rubber mulch, rubber tree rings, or plastic film under the mulch.
Staking, Bracing & Trunk Guards
1. The primary purpose of staking is to prevent the root ball from dislodging
in the wind and tearing newly formed roots. Staking also helps prevent mower
and trimmer damage on young trees. Most new trees likely need staking or bracing to prevent dislodging in the wind. There is more than one method for bracing, including the use of two or three posts opposite the tree and fastened to the
trunk (see diagram); one post driven diagonally and tied directly to the trunk; or
root ball braces. Whatever method is used, some sway should be allowed in the
tree trunk after staking and anything attached directly to the tree trunk should
not cause rubbing injuries. Broad, belt-like materials should be used to attach any
bracing to the trunk. Do not brace with wire, rope or wire through hose as they
can all cause trunk injury.
2. Staking should be monitored closely and all attachments to the tree removed
within one year (for most trees). Trees in especially windy areas may require
a longer staking period. Typically, if a tree is not stable one year after planting,
then it likely has significant root problems that staking will not correct. Posts can
remain longer to help discourage mower damage and reduce potential vandalism.
3. Trunk guards (typically 12-18” tall) can be placed around the base of the trunk to
help prevent mower, trimmer and rabbit damage. Rabbits especially love to chew on
the trunks of young oak trees. Trunk guards might also help alleviate frost
cracking and sunscald of young trees. Several products are available for this purpose,
including plastic mesh guards that allow for better air flow around the trunk.
Watering
1. Trees should be watered thoroughly at planting time. Thereafter the root zone
should be kept moist but not waterlogged. Insuring good soil to root contact at
planting time will greatly reduce the amount of supplemental watering needed.
Watering needs will vary greatly depending on soil type, weather conditions and
the type of nursery stock used. A general rule-of-thumb is newly planted trees
should receive the equivalent of about 1” of moisture per week, including rainwater, during the first two growing seasons. The root zone should be checked frequently for moistness. Many trees are actually lost to over-watering, especially if
planted within irrigated turfgrass.
Fertilizing
1. New trees properly selected for a site and planted into native top soils with good
organic matter will generally NOT need fertilizer. A soil test is recommended
before planting on any site where soils have been altered or are not well understood. Soil problems should be addressed before planting. Adding organic matter such as compost, compost tea, ground leaves, grass clippings or aged manure
to a planting site before planting can often improve soil fertility. If a soil test
determines that a fertilizer is necessary, it is generally recommended that a slowrelease, low-nitrogen product be used and used only until the health of the soil is
improved organically.
2. Many soils in Nebraska have high pH levels making them unsuitable for acidloving species such as pin oak. Adding sulfur or chelated iron to a planting site
can temporarily reduce soil pH and improve the health of certain species. However, over time it becomes nearly impossible to adequately alter the soil of a tree’s
far-reaching root system. It is much better to choose species that will thrive at the
native pH level.
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Pruning
1. At planting time, pruning may be necessary to remove damaged branches and
to correct structural defects such as codominant leaders and week branch
unions. Pruning should be kept to a minimum and the crown should not be generally thinned or cut back to try and “balance” the top with the roots. Since leaves
produce the food necessary to help overcome transplant shock, the more branching
and leaf material retained at planting time the better.
2. Research has shown that the best nursery trees retain small horizontal branches
lower on the trunk. If possible, these low branches should be left on the tree for
several years after planting as they will help the tree develop good trunk taper
and might also help prevent sun scald and frost cracking of the lower trunk.
As long as a tree’s structure is sound and there is no compelling reason to remove
lower branches such as for visibility, sidewalk clearance, safety, etc., a tree’s lower
limbs can be retained throughout much of its young life. In fact some species such
as linden, bald cypress, beech and most evergreens look better and function better
when allowed to retain their lower branches for as long as possible.
SHARE YOUR THOUGHTS: If you have a comment or if you don’t agree with
any part of this, let us know. Contact Justin Evertson at (402) 472-5045 or jevertson1@unl.edu. This publication can be found online at http://arboretum.unl.edu.
PARTS OF A TYPICAL TREE
Central Leader
(one main branch that
extends straight up from
the trunk)
Branches
Branch Bark Ridge
Crown
(branches
and leaves)
Trunk Collar
Dripline
(line on the ground
under the outermost edge of
a tree’s crown)
Trunk
Graft Union
Root Collar
Roots
Note:
Flowers and fruit vary by
tree type in shape and time
of appearance
Ground Surface