Reasons for pruning - PDF document

NA-FR-01-95 Figure 1. Reasons for pruning. USDA Forest Service Northeastern Area State and Private Forestry HOW to Prune TreesPeter J. Bedker, Joseph G. O’Brien, and Manfred M. MielkeIllustrations by Julie Martinez, Afton, MNIntroductionThe objective of pruning is to produce strong,healthy, attractive plants. By understandinghow, when and why to prune, and by followinga few simple principles, this objective can beachieved. Why Prune 2 crossing and rubbing branches. Pruning canbest be used to encourage trees to develop astrong structure and reduce the likelihood ofdamage during severe weather. Removingbroken or damaged limbs encourage woundclosure.Pruning for aesthetics (Fig. 1C) involvesenhancing the natural form and character oftrees or stimulating flower production. Pruningfor form can be especially important on open-grown trees that do very little self-pruning. All woody plants shed branches in response toshading and competition. Branches that do notproduce enough carbohydrates fromphotosynthesis to sustain themselves die andare eventually shed; the resulting wounds aresealed by woundwood (callus). Branches thatare poorly attached may be broken off by windand accumulation of snow and ice. Branchesremoved by such natural forces often result inlarge, ragged wounds that rarely seal. Pruningas a cultural practice can be used tosupplement or replace these natural processesand increase the strength and longevity ofplants.Trees have many forms, but the most commontypes are pyramidal (excurrent) or spherical(. Trees with pyramidal crowns,e.g., most conifers, have a strong central stemand lateral branches that are more or lesshorizontal and do not compete with the centralstem for dominance. Trees with sphericalcrowns, e.g., most hardwoods, have manylateral branches that may compete fordominance.To reduce the need for pruning it is best toconsider a tree's natural form. It is very difficultto impose an unnatural form on a tree without acommitment to constant maintenance.Pollarding and topiary are extreme examplesof pruning to create a desired, unnatura

l effect.Pollarding is the practice of pruning treesannually to remove all new growth. Thefollowing year, a profusion of new branches isproduced at the ends of the branches. Topiaryinvolves pruning trees and shrubs intogeometric or animal shapes. Both pollardingand topiary are specialized applications thatinvolve pruning to change the natural form oftrees. As topiary demonstrates, given enoughcare and attention plants can be pruned intonearly any form. Yet just as proper pruningcan enhance the form or character of plants,improper pruning can destroy it.Pruning ApproachesProducing strong structure should be theemphasis when pruning young trees. As treesmature, the aim of pruning will shift tomaintaining tree structure, form, health andappearance. Proper pruning cuts are made at a node, thepoint at which one branch or twig attaches toanother. In the spring of the year growthbegins at buds, and twigs grow until a newnode is formed. The length of a branchbetween nodes is called an internode. 3 2. Crown thinning - branches to be removed areshaded in blue; pruning cuts should be made at the redlines. No more than one-fourth of the living branchesshould be removed at one time. Figure 3. Types of branch unions. The most common types of pruning are:1. Crown Thinning (Fig. 2)Crown thinning, primarily for hardwoods, isthe selective removal of branches to increaselight penetration and air movement throughoutthe crown of a tree. The intent isto maintain or develop a tree's structure andform. To avoid unnecessary stress and preventexcessive production of epicormic sprouts, nomore than one-quarter of the living crownshould be removed at a time. If it is necessaryto remove more, it should be done oversuccessive years.Branches with strong U-shaped angles ofattachment should be retained (Fig 3A). Branches with narrow, V-shaped angles ofattachment often form included bark andshould be removed (Fig. 3B). Included barkforms when two branches grow at sharplyacute angles to one another, producing awedge of inward-rolled bark between them.Included bark prevents stro

ng attachment ofbranches, often causing a crack at the pointbelow where the branches meet. Codominantstems that are approximately the same size andarise from the same position often formincluded bark. Removing some of the lateralbranches from a codominant stem can reduceits growth enough to allow the other stem tobecome dominant. Lateral branches should be no more than one-half to three-quarters of the diameter of thestem at the point of attachment. Avoidproducing "lion’s tails," tufts of branches andfoliage at the ends of branches, caused byremoving all inner lateral branches and foliage. Lion’s tails can result in sunscalding, abundantepicormic sprouts, and weak branch structureand breakage. Branches that rub or cross 4 4. Crown raising - branches to be removed areshaded in blue; pruning cuts should be made whereindicated with red lines. The ratio of live crown tototal tree height should be at least two-thirds. another branch should be removed.Conifers that have branches in whorls and pyramidal crowns rarely need crown thinningexcept to restore a dominant leader. Occasionally, the leader of a tree may bedamaged and multiple branches may becomecodominant. Select the strongest leader andremove competing branches to prevent thedevelopment of codominant stems.2. Crown Raising (Fig. 4) Crown raising is the practice of removingbranches from the bottom of the crown of atree to provide clearance for pedestrians,vehicles, buildings, lines of site, or to develop aclear stem for timber production. Also,removing lower branches on white pines canprevent blister rust. For street trees theminimum clearance is often specified bymunicipal ordinance. After pruning, the ratio ofthe living crown to total tree height should be atleast two-thirds (e.g., a 12 m tree should haveliving branches on at least the upper 8 m).On young trees "temporary" branches may beretained along the stem to encourage taper andprotect trees from vandalism and sun scald. Less vigorous shoots should be selected astemporary branches and should be about 10 to15 cm apart along the stem.

They should bepruned annually to slow their growth andshould be removed eventually.3. Crown Reduction (Fig. 5)Crown reduction pruning is most often usedwhen a tree has grown too large for itspermitted space. This method, sometimescalled drop crotch pruning, is preferred totopping because it results in a more naturalappearance, increases the time before pruningis needed again, and minimizes stress (see dropcrotch cuts in the next section).Crown reduction pruning, a method of lastresort, often results in large pruning wounds to stems that may lead to decay. This methodshould never be used on a tree with apyramidal growth form. A better long termsolution is to remove the tree and replace it 5 5. Crown reduction - branches to beremoved are shaded in blue; pruning cuts should be where indicated with red lines. To preve dieback, cuts should be made at lateral that are at least one-third the diameter ofthe stem at their union. with a tree that will not grow beyond theavailable space.Pruning CutsPruning cuts should be made so that onlybranch tissue is removed and stem tissue is notdamaged. At the point where the branchattaches to the stem, branch and stem tissuesremain separate, but are contiguous. If onlybranch tissues are cut when pruning, the stemtissues of the tree will probably not becomedecayed, and the wound will seal moreeffectively.Pruning living branches (Fig. 6)To find the proper place to cut a branch, lookfor the branch collar that grows from the stemtissue at the underside of the base of the branch(Fig. 6A). On the upper surface, there isusually a branch bark ridge that runs (more orless) parallel to the branch angle, along the stemof the tree. A proper pruning cut does notdamage either the branch bark ridge or thebranch collar.A proper cut begins just outside the branchbark ridge and angles down away from thestem of the tree, avoiding injury to the branchcollar (Fig. 6B). Make the cut as close aspossible to the stem in the branch axil, butoutside the branch bark ridge, so that stemtissue is not injured and the wound can seal inthe shortest tim

e possible. If the cut is too farfrom the stem, leaving a branch stub, thebranch tissue usually dies and woundwoodforms from the stem tissue. Wound closure isdelayed because the woundwood must sealover the stub that was left.The quality of pruning cuts can be evaluated byexamining pruning wounds after one growingseason. A concentric ring of woundwood willform from proper pruning cuts (Fig. 6B). Flush cuts made inside the branch bark ridgeor branch collar, result in pronounceddevelopment of woundwood on the sides of thepruning wounds with very little woundwoodforming on the top or bottom (Fig. 7D). Asdescribed above, stub cuts result in the death ofthe remaining branch and woundwood formsaround the base from stem tissues.When pruning small branches with handpruners, make sure the tools are sharp enough 6 Figure 6. Pruning cuts to cut the branches cleanly without tearing. Branches large enough to require saws shouldbe supported with one hand while the cuts aremade. If the branch is too large to support,make a three-step pruning cut to prevent barkripping (Fig. 6C).1. The first cut is a shallow notch made onthe underside of the branch, outside thebranch collar. This cut will prevent afalling branch from tearing the stemtissue as it pulls away from the tree. 2. The second cut should be outside thefirst cut, all the way through the branch,leaving a short stub. 3. The stub is then cut just outside thebranch bark ridge/branch collar,completing the operation.2. Pruning dead branches (Fig. 6)Prune dead branches in much the same way aslive branches. Making the correct cut is usuallyeasy because the branch collar and the branchbark ridge, can be distinguished from the deadbranch, because they continue to grow (Fig.6A). Make the pruning cut just outside of thering of woundwood tissue that has formed,being careful not to cause unnecessary injury(Fig. 6C). Large dead branches should besupported with one hand or cut with the three-step method, just as live branches. Cutting largeliving branches with the three step method ismore critical because of the greater like

lihoodof bark ripping.3. Drop Crotch Cuts (Fig. 6D)A proper cut begins just above the branch barkridge and extends through the stem parallel tothe branch bark ridge. Usually, the stem beingremoved is too large to be supported with onehand, so the three cut method should be used.1.With the first cut, make a notch on theside of the stem away from the branchto be retained, well above the branchcrotch. 7 Begin the second cut inside the branchcrotch, staying well above the branchbark ridge, and cut through the stemabove the notch.3.Cut the remaining stub just inside thebranch bark ridge through the stemparallel to the branch bark ridge.To prevent the abundant growth of epicormicsprouts on the stem below the cut, or diebackof the stem to a lower lateral branch, make thecut at a lateral branch that is at least one-thirdof the diameter of the stem at their union. Pruning Practices That HarmTrees and tipping (Fig. 7A, 7B) are pruningpractices that harm trees and should not beused. Crown reduction pruning is the preferredmethod to reduce the size or height of thecrown of a tree, but is rarely needed and shouldbe used infrequently.Topping, the pruning of large upright branchesbetween nodes, is sometimes done to reducethe height of a tree (Fig. 7A). Tipping is apractice of cutting lateral branches betweennodes (Fig. 7B) to reduce crown width. These practices invariably result in thedevelopment of epicormic sprouts, or in thedeath of the cut branch back to the next lateralbranch below. These epicormic sprouts areweakly attached to the stem and eventually willbe supported by a decaying branch.Improper pruning cuts cause unnecessary injuryand bark ripping (Fig. 7C). Flush cuts injurestem tissues and can result in decay (Fig. 7D).Stub cuts delay wound closure and canprovide entry to canker fungi that kill thecambium, delaying or preventing woundwoodformation (Fig. 7E). 8 When to Prune Conifers may be pruned any time of year, butpruning during the dormant season mayminimize sap and resin flow from cut branches.Hardwood trees and shrubs without showyflowers: prune in th

e dormant season to easilyvisualize the structure of the tree, to maximizewound closure in the growing season afterpruning, to reduce the chance of transmittingdisease, and to discourage excessive sap flowfrom wounds. Recent wounds and the chemicalscents they emit can actually attract insects thatspread tree disease. In particular, woundedelm wood is known to attract bark beetles thatharbor spores of the Dutch elm disease fungus,and open wounds on oaks are known to attractbeetles that spread the oak wilt fungus. Takecare to prune these trees during the correcttime of year to prevent spread of these fataldiseases. Contact your local tree diseasespecialist to find out when to prune these treespecies in your area. Usually, the best time isduring the late fall and winter.Flowering trees and shrubs: these should alsobe pruned during the dormant season for thesame reasons stated above; however, topreserve the current year's flower crop, pruneaccording to the following schedule:?Trees and shrubs that flower in earlyspring (redbud, dogwood, etc.) shouldbe pruned immediately after flowering(flower buds arise the year before theyflush, and will form on the new growth).?Many flowering trees are susceptible tofireblight, a bacterial disease that can bespread by pruning. These trees,including many varieties of crabapple,hawthorn, pear, mountain ash,flowering quince and pyracantha,should be pruned during the dormantseason. Check with your countyextension agent or a horticulturist foradditional information.?Trees and shrubs that flower in thesummer or fall always should be prunedduring the dormant season (flower budswill form on new twigs during the nextgrowing season, and the flowers willflush normally).Dead branches: can be removed any time ofthe year.Pruning Tools Proper tools are essential for satisfactorypruning (Fig.6). The choice of which tool touse depends largely on the size of branches tobe pruned and the amount of pruning to bedone. If possible, test a tool before you buy itto ensure it suits your specific needs. As withmost things, higher quality often

equates tohigher cost.Generally speaking, the smaller a branch iswhen pruned, the sooner the wound createdwill seal. Hand pruners are used to prune smallbranches (under 2.5 cm diameter) and manydifferent kinds are available. Hand pruners canbe grouped into by-pass or anvil styles basedon the blade configuration. Anvil style prunershave a straight blade that cuts the branchagainst a small anvil or block as the handles aresqueezed. By-pass pruners use a curved cuttingblade that slides past a broader lower blade,much like a scissors. To prevent unnecessarytearing or crushing of tissues, it is best to use a 9 by-pass style pruner. Left- or right-handedtypes can be purchased.Slightly larger branches that cannot be cut witha hand pruner may be cut with small pruningsaws (up to 10 cm) or lopping shears (up to 7cm diameter) with larger cutting surfaces andgreater leverage. Lopping shears are alsoavailable in by-pass and anvil styles.For branches too large to be cut with a handpruner or lopping shears, pruning saws must beused. Pruning saws differ greatly in handlestyles, the length and shape of the blade, andthe layout and type of teeth. Most havetempered metal blades that retain theirsharpness for many pruning cuts. Unlike mostother saws, pruning saws are often designed tocut on the "pull-stroke."Chain saws are preferred when pruningbranches larger than about 10 cm. Chainsawsshould be used only by qualified individuals. Toavoid the need to cut branches greater than 10cm diameter, prune when branches are small.Pole pruners must be used to cut branchesbeyond reach. Generally, pruning heads can cutbranches up to 4.4 cm diameter and areavailable in the by-pass and anvil styles. Onceagain, the by-pass type is preferred. Forcutting larger branches, saw blades can befastened directly to the pruning head, or aseparate saw head can be purchased. Becauseof the danger of electrocution, pole prunersshould not be used near utility lines except byqualified utility line clearance personnel.To ensure that satisfactory cuts are made andto reduce fatigue, keep your prun

ing tools sharpand in good working condition. Hand pruners,lopping shears, and pole pruners should beperiodically sharpened with a sharpening stone.Replacement blades are available for manystyles. Pruning saws should be professionallysharpened or periodically replaced. To reducecost, many styles have replaceable blades.Tools should be clean and sanitized as well assharp. Although sanitizing tools may beinconvenient and seldom practiced, doing somay prevent the spread of disease frominfected to healthy trees on contaminated tools. Tools become contaminated when they comeinto contact with fungi, bacteria, viruses andother microorganisms that cause disease intrees. Most pathogens need some way ofentering the tree to cause disease, and freshwounds are perfect places for infections tobegin. Microorganisms on tool surfaces areeasily introduced into susceptible trees whensubsequent cuts are made. The need forsanitizing tools can be greatly reduced bypruning during the dormant season.If sanitizing is necessary it should be practicedas follows: Before each branch is cut, sanitizepruning tools with either 70% denaturedalcohol, or with liquid household bleach diluted1 to 9 with water (1 part bleach, 9 partswater). Tools should be immersed in thesolution, preferably for 1-2 minutes, and woodparticles should be wiped from all cuttingsurfaces. Bleach is corrosive to metal surfaces,so tools should be thoroughly cleaned withsoap and water after each use. 10 Treating woundsTree sap, gums, and resins are the naturalmeans by which trees combat invasion bypathogens. Although unsightly, sap flow frompruning wounds is not generally harmful; however, excessive "bleeding" can weakentrees. When oaks or elms are wounded during acritical time of year (usually spring for oaks, orthroughout the growing season for elms) --either from storms, other unforeseenmechanical wounds, or from necessary branchremovals -- some type of wound dressingshould be applied to the wound. Do thisimmediately after the wound is created. In mostother instances, wound dressings areunnecessary, and may

even be detrimental. Wound dressings will not stop decay or cureinfectious diseases. They may actually interferewith the protective benefits of tree gums andresins, and prevent wound surfaces fromclosing as quickly as they might under naturalconditions. The only benefit of wounddressings is to prevent introduction ofpathogens in the specific cases of Dutch elmdisease and oak wilt. Pruning GuidelinesTo encourage the development of a strong,healthy tree, consider the following guidelineswhen pruning.GeneralPrune first for safety, next for health,and finally for aesthetics.?Never prune trees that are touching ornear utility lines; instead consult yourlocal utility company.?Avoid pruning trees when you mightincrease susceptibility to importantpests (e.g. in areas where oak wiltexists, avoid pruning oaks in the springand early summer; prune treessusceptible to fireblight only during thedormant season).?Use the following decision guide forsize of branches to be removed: 1)under 5 cm diameter - go ahead, 2)between 5 and 10 cm diameter - thinktwice, and 3) greater than 10 cmdiameter - have a good reason.Crown Thinning?Assess how a tree will be pruned fromthe top down.?Favor branches with strong, U-shapedangles of attachment. Remove brancheswith weak, V-shaped angles ofattachment and/or included bark.?Ideally, lateral branches should beevenly spaced on the main stem ofyoung trees.?Remove any branches that rub or crossanother branch.?Make sure that lateral branches are nomore than one-half to three-quarters ofthe diameter of the stem to discouragethe development of co-dominant stems. 11 Do not remove more than one-quarterof the living crown of a tree at one time.If it is necessary to remove more, do itover successive years. Crown Raising?Always maintain live branches on atleast two-thirds of a tree's total height.Removing too many lower brancheswill hinder the development of a strongstem.Remove basal sprouts and vigorousepicormic sprouts.Crown Reduction?Use crown reduction pruning only whenabsolutely necessary. Make thepruning cut at a lateral branch that is atl

east one-third the diameter of the stemto be removed.?If it is necessary to remove more thanhalf of the foliage from a branch,remove the entire branch.GlossaryBranch Axil: the angle formed where a branchjoins another branch or stem of a woody plant.Branch Bark Ridge: a ridge of bark thatforms in a branch crotch and partially aroundthe stem resulting from the growth of the stemand branch tissues against one another.Branch Collar: a "shoulder" or bulge formedat the base of a branch by the annualproduction of overlapping layers of branch andstem tissues.Crown Raising: a method of pruning toprovide clearance for pedestrians, vehicles,buildings, lines of sight, and vistas by removinglower branches.Crown Reduction Pruning: a method ofpruning used to reduce the height of a tree.Branches are cut back to laterals that are atleast one-third the diameter of the limb beingremoved.Crown Thinning: a method of pruning toincrease light penetration and air movementthrough the crown of a tree by selectiveremoval of branches.Callus: see woundwood.Decurrent: a major tree form resulting fromweak apical control. Trees with this form haveseveral to many lateral branches that competewith the central stem for dominance resulting ina spherical or globose crown. Most hardwoodtrees have decurrent forms.Epicormic Sprout: a shoot that arises fromlatent or adventitious buds; also know as watersprouts that occur for on stems and branchesand suckers that are produced from the base oftrees. In older wood, epicormic shoots oftenresult from severe defoliation or radical pruning.Excurrent: a major tree form resulting fromstrong apical control. Trees with this form havea strong central stem and pyramidal shape.Lateral branches rarely compete fordominance. Most conifers and a fewhardwoods, such as sweetgum and tuliptree,have excurrent forms.Flush Cuts: pruning cuts that originate insidethe branch bark ridge or the branch collar,causing unnecessary injury to stem tissues.Included Bark: bark enclosed between 12 “How to Prune Trees” was written to helppeople properly prune the tr

ees they careabout. If you doubt your ability to safelyprune large trees, please hire a professionalarborist. Information in this publication canbe used to interview and hire a competentarborist. branches with narrow angles of attachment,forming a wedge between the branches.Pollarding: the annual removal of all of theprevious year's growth, resulting in a flush ofslender shoots and branches each spring.Stub Cuts: pruning cuts made too far outsidethe branch bark ridge or branch collar, thatleave branch tissue attached to the stem.Tipping: a poor maintenance practice used tocontrol the size of tree crowns; involves thecutting of branches at right angles leaving longstubs. a poor maintenance practice oftenused to control the size of trees; involves theindiscriminate cutting of branches and stems atright angles leaving long stubs. Synonymsinclude rounding-over, heading-back,dehorning, capping and hat-racking. Topping isoften improperly referred to as pollarding.Topiary: the pruning and training of a plantinto a desired geometric or animal shape.Woundwood: lignified, differentiated tissuesproduced on woody plants as a response towounding (also known as callus tissue).ReferencesANSI Z133.1. 1994. Safety standards.American national standard for tree careoperators. Washington, DC: AmericanNational Standards Institute.ANSI A300. 1995. Standard practices fortree, shrub, and other woody plantmaintenance. Washington, DC: AmericanNational Standards Institute.Fazio, J. R. ed. 1992. Don't top trees. TreeCity USA Bulletin No. 8. Nebraska City, NE:The National Arbor Day Foundation.Harris, R.W. 1994. Clarifying certain pruningterminology: thinning, heading, pollarding.Journal of Arboriculture 20:50-54.ISA Performance Guidelines Committee. 1994.Tree-pruning guidelines. Savoy, IL:International Society of Arboriculture.Ryan, H.D.P. III. 1994. Arboricultural pruningmethodologies. Arborist News Volume3(4):33-38.Shigo, A. 1991. Modern arboriculture.Durham, NH: Shigo & Trees, Associates.Shigo, A. 1989. Tree pruning: a worldwidephoto guide. Durham, NH: Shigo & Trees,Associate