Pruning

What is pruning?

Most people are completely confused about pruning. A common misconception is that pruning is like hair cutting...you control the size and shape of a plant by pruning it. Because there is a small grain of truth to this misconception, people believe it "hook-line-and sinker." It is actually far more interesting than hair cutting!

When we cut a stem of a plant, for example to take a cutting, or to trim the plant, we alter the physiology of that plant. By removing a stem tip from the plant, we induce branching by releasing apical dominance. So pruning is not about what is removed, but what happens to what is left on the plant. Thus proper pruning is designed to provide proper branching on the plant...not to control its size or shape. Of course if branching is optimal, then the size and shape of the shrub will be improved and that is the grain of truth.

But again, I want to emphasize that pruning determines where branching will occur rather than controlling size or shape.

What is apical dominance?

A shoot consists of a stem with leaves. At the tip of the shoot is an apical bud. This actively grows, adds to the length of the stem, and produces more leaves along the extending stem. Where each leaf joins the stem of a shoot there is also a lateral bud. So a shoot with ten leaves has one apical bud but 10 lateral buds.

The apical bud produces the hormone, Indole Acetic Acid (IAA) which is also known as auxin. This chemical is produced in the rapidly growing apical bud and is transported down the shoot. Along the way, the IAA passes the lateral buds and the presence of the auxin keeps those lateral buds dormant. Dormancy means that the buds are "sleeping" or "inactive"; there is great potential for growth of the lateral buds but as long as the apical bud is intact, they remain dormant.

So apical dominance is a hormonal relationship in which an actively growing apical bud prevents lateral buds from growing.

When we prune a plant, removing the apical bud, the lateral buds in the axils of the leaves below the cut stump "awaken" and begin to actively grow. Just like the now-removed apical bud, these active lateral buds grow into shoots and their tips have apical buds that resume apical dominance over lateral buds in positions lower on the main stem.

So, where we prune, we get new branches locally. This is the concept that many semi-professional and most amateur pruners fail to understand. Pruning is about controlling branching.

Where you cut, you will next get new branches.

The mark of amateurish pruning is shown in the error of shearing. The pruner selects the wrong tool from the start. Rather than picking up a pruning shear, the person picks up hedge trimmers (manual or powered). At my university I have heard reports that the staff has been observed using a chain saw for pruning! These tools all do tremendous damage (lots of broken and crushed stem tips, torn bark, etc.) that leads to wound responses and bacterial and fungal invasions, and the cuts are all found at the surface of the shrub being sheared. Branches appear almost immediately near those cuts and the surface of the shrub becomes more densely branched. Within a week or so, the shrub will need pruning again, and repeated shearing will create a dense surface that shades out all of the leaves on the interior of the shrub and below the upper surface on sun loving species. So shrubs with dense upper surfaces and twiggy, hollow interiors and bases are the sign of shearing rather than pruning.

The amateur or uneducated staff person will think that powered hedge trimmers or chain saws reduce their pruning workload. But the rapid surface branching they induce multiplies their workload. Proper pruning with correct tools is a once-per-season job...shearing is job that must be repeated weekly or biweekly to keep shrubs "in shape." So if you like to do shearing, you set yourself up for biweekly work and lots of raking of small clippings from the landscape. You are also setting up your shrub for an ugly look and diseases. Excessive shearing also reduces the flowering of flowering shrub as the plant concentrates its efforts upon branch making rather than flower making. Are you sure this is what you want? Plus the shrubs begin to all take on the same sphere or cube shape...and are you sure you want all the various shrubs to look like poor attempts at the same sculptural shape rather than like naturally diverse shrubs?

I think the root of the pruning errors you see at just about every home in town, and all over our campus, is a pruning style called topiary.

Topiary

Topiary is an artform in which a shrub is the medium. The artist carefully selects the species of shrub to be sculpted...just as an artist would carefully select a specimen of marble prior to carving. The shrub is then pruned to achieve a desired sculptural shape. Of course an artist would not be satisfied with just a simple cube or sphere. Perhaps you have seen a shrub that looked like Donald Duck or Mickey Mouse at Disney World. Indeed amusement parks commission many topiary sculptures and these inspire us to try our hand at it at home. But we are not topiary artists and we make sophomoric mistakes.

The species a topiary artist chooses is generally boxwood. This shrub has small leaves that fill in surfaces smoothly, and the shrub is very shade tolerant. It will hold its leaves even if they are shaded by overhanging parts of the sculpture. The belly of Dumbo the elephant needs to be covered, not twiggy, so boxwood is the plant of choice.

From the artist's point of view, sculpting in boxwood is lucrative. The initial commission is for the artist's work in shaping the shrub. Within a few weeks of installation, the sculpture will need some trimming. The owner's staff will try to keep the shape true to the artist's design. However, within some months or certainly within a year or two, the owner will be calling the artist back to visit the sculpture to "fix" it up because it will have lost some of its original form. This is a repeat commi$$ion!

So our local gardeners try to do some simple topiary on their home shrubs. They do the surface shearing to make a sphere or cube thinking that this should be easier than the Little Mermaid. But then the gardener is not an artist and chooses the existing shrub rather than installing boxwood! Forsythia or hemlock or rhododendrons just do not make a good topiary medium. Even a talented artist could not turn these shrubs into nice sculpture.

The Forsythia cube starts out looking promising, but this aggressive shrub puts out lateral branches very quickly. The surfaces become dense. The top of the cube shades the interior and sides of the cube from the sun. The leaves fall from the twigs on the interior. The sides lose their leaves too. You end up with a green table held up by twigs. IF it flowers, the plant will have flowers only on the table...rather a disaster!

A Forsythia sphere agains starts out looking promising, but again the branching is rapid and becomes dense on the surfaces. To top of the dome quickly fills in. The interior loses leaves and becomes hollow. The sides below the equator of the sphere lose the leaves. You end up with a mushroom-like dome held up by twigs. Not a very successful sphere. And again, the flowering will only be on the dome...if it flowers at all.

So the advice for your spheres and cubes is to not use a sun-loving shrub like Forsythia. Stay with shade-loving species. Then, reconsider the shapes. If the sides can remain in the sun they have some chance of retaining the leaves. So the shape needs to be widest at the bottom and should narrow toward the top of the shrub. So rather than a cube, consider a pyramid. Rather than a sphere, consider instead a hemisphere. These shapes have at least some chance for success.

It is also critical in home topiary to remember to thin surfaces so that some light penetrates the surface of the sculpture and and help the interior retain leaves to avoid hollow shapes.

OK, so what do I do with my Forsythia?

Rule of Thumb Pruning

Natural pruning for shrubs is not done with hedge trimmers but with pruning shears. This small hand tool makes individual stem cuts. The pruner also does not make his cuts at the surface of the shrub, again because pruning leads to branching. So the cuts are made closer to the ground to keep the branching dense from the base up rather than from the top down. This also thins the canopy of the shrub so that light penetrates and the shrub remains well covered with leaves.

For Forsythia, the Rule of Thumb pruning method provides a low-maintenance solution for keeping this aggressive shrub under control and flowering beautifully throughout the shrub each year.

Right after flowering, the pruner makes the annual pruning visit to the shrub. That is right! It is once-per-year! The pruner looks past the surface of the shrub, but looks at where the stems come up from the soil. Any stem that has achieved a diameter of the pruner's thumb is cut at the ground surface. That stem and all of its branches are removed. That is all there is. Every shoot greater than thumb diameter is taken out each year.

The Forsythia responds by sending up new shoots from the soil each year. These young canes bear flowers most profusely. The shrub looks like a small fountain going off, with young canes covered with yellow flowers each year. The shrub is green inside and out, and the shrub stays relatively small. The gardener only has to prune once per year...and can just enjoy the Forsythia for the rest of the year! And the Forsythia has a shape that is uniquely its own. It does not look like a lilac, a rhododendron or a spiraea.

Bonsai

Perhaps the most extreme form of topiary is Bonsai. Again, this is an artform in which an artist picks a species and prunes it over many years to create a sculpture. But the goal is not an animal or cartoon figure, rather it is to create what looks like a very large old tree in miniature. This tree should show signs of age, an accumulation of accident and environmental stress. It will be asymmetrical and windswept.

Bonsai is achieved by extreme pruning of shoot and root. As needed to achieve the form of the canopy...often showing "cloud pruning" form...the shoot is carefully pruned. Again the cuts are made individually and with due consideration of where branches are desired. Very fine pruning shears are the tool used. At least once per year, the tree is tipped out of its pot or pan and the lower third of the soil and root mass is cut away. This extreme pruning causes the dwarfing we observe as a result.

A closely related artform is Penjing. While Bonsai is Japanese, Penjing is Chinese. In Penjing, the older of the two artforms, the goal is focused more on producing a miniature landscape featuring usually more than one plant, and includes architectural details (pagodas, footbridges, etc.) and often includes the use of moss to create "lawn" areas in the miniature landscape.

One particularly lovely Penjing that I have seen was in the collection of the Matthaei Botanical Garden at the University of Michigan in Ann Arbor. The artist had used maple as the specimen trees. The grove of miniature trees with black bark had an emerald green lawn of moss. It was autumn and the work had been properly kept outdoors in natural photoperiod and temperature swings. The leaves on the trees were suitably dwarfed by the pruning, but perfectly shaped maple palmate forms. At the time of my viewing the leaves were a mixture of green, orange, yellow, and red colors. It was just beautiful.

Pruning for fruit trees

Fruit or nut trees are often part of a home landscape. Pruning these can be a bit confusing too. As is the case for the "rule of thumb" pruning, this is a once-per-year job. The form is not miniature forests or individual trees, but the form is critical.

In winter, while the tree is dormant, the pruner visits the orchard and prunes to form. The Central Leader form is used for nut trees. In this the main trunk is not cut back so that the apical bud continues to grow upward and the tree will achieve full height. The side branches are pruned to provide a wider base and narrower top for good sun coverage, and the canopy is thinned to allow good sun exposure and air movement for good flower and fruit development.

For fruit trees, the format depends on the varieties found in the orchard. Ungrafted standard fruit trees are usually pruned into the Open form. The main stem is pruned just above the pruner's knee. Branches at nice wide angles are allowed or forced to grow outwards and then upwards so that the canopy has a bowl-like shape. It is hollow in the center. Sunlight hits all sides of the branches and all sides of the fruit for full skin color development on the apples or cherries. The reduced humidity and increased airflow in the canopy reduce fungal attack on the trees. Some dwarfing of the tree induced by this pruning helps with safety in spraying, picking, and pruning processes in the orchard.

In the last few decades, many orchard fruit trees are grafted to dwarf the tree. As we shall see, a tree scion is grafted to shrub root stock so that the tree grows slowly and short...limited by its shrub roots. These grafted dwarfs do not need the Open system as they do not get large enough to have shading or light or airflow penetration problems. So pruning is more limited. The modified central leader allows the apical bud to achieve a main trunk of perhaps human height or so and then is decapitated to make nice side branches. Again getting nice wide branch angles is a goal and limb spreaders might help to that. So the Modified Central Leader form is somewhere between the Central Leader and the Open forms.

Central Leader

Open System

Modified
Central Leader

Espalier

If Topiary, Bonsai, and Penjing are sculptural (3-D) artforms, Espalier is a 2-D artform...more akin to painting or drawing than to sculpture. In this artform, the artist prunes a tree into a planar form just in front of a wall or a fence.

The exact form of the espalier will vary and can be formal or informal. Informal forms might have the planar tree showing a more "natural" branching system against the wall. Formal forms might show various levels of U shaped branching patterns, with strong geometry and either with or without symmetry.

Simple U

Multiple U

The species chosen for espalier will depend upon the sun exposure for the wall or the fence. But often the species are flowering and/or fruiting plants. Apples for example provide flowers in the spring and fruits in the fall and nice leafy form for most of the summer. The planar pruning shows off all of the flowers and fruits nicely...so fruit trees are a common subject.

But not all espalier is done for aesthetics...it can be practical and functional. For example in the former Soviet Union, labor for orchards was quite limited and espalier provided the solution to growing tree fruits for the population. The orchard is planted with the trees in the plane of a cyclone (chain-link) fence, and the fences are spaced apart just slightly wider than the width of a pickup truck. The pruning is accomplished in winter by three people. A truck is driven by one worker, two others have powered hedge trimmers and stand in the truck bed. As the driver slowly drives the truck between the fences and the pruners cut back all branches that stick out from the fence. Only those branches in the plane of the fence remain (it is espalier!). For spraying, the same three workers can do the spraying very efficiently. They have one tank and two pumps in the bed of the truck and two sprayers direct the pesticide onto the fences as the driver passes between all the fences. At picking time, the same three people can do the orchard in a very short time. Again as the driver drives, the two pickers work each side of the bed to pick the fruit into boxes in the truck bed.

Belgian Fence

Another beautiful 2-D artform is the Belgian Fence. Here, rather than planting the trees into the plane of a wall or fence, the trees become the fence! Trees are planted in close spacing and allowed to grow up. The pruner controls branching with judicious cuts with pruning shears. Lateral branches are bent into crossing forms to make a lattice pattern in what will be the plane of the fence. Where the branches cross, the pruner will make approach grafts. The bark on both crossing branches is sliced from the branch on the side where the branches will touch. The wounds are pressed together and wired with a twist-tie. The branches heal together forming a solid joint at each crossing.

The Belgian Fence becomes a living fence and thus needs little to NO maintenance! The annual pruning is just to keep the lattice form and to keep the trees limited to growing in the intended plane. You do not have pickets to replace or to paint! And if you picked fruit trees, you will have lovely flowers in spring and lucious fruits in the fall. And, each year the wood gets thicker with annual growth rings, making a fence that gets stronger with age rather than weaker with age!

Grape Vines

Grape vines are almost espalier too. A vine needs support, so there are fenceposts along the rows of a vineyard. Heavy wires are strung between the posts. The grapes are planted between the posts.

After a year of growth the apical bud is removed and the vine will be allowed to be no taller than the top wire. The lateral branches that form will be trained along the wires. The wires will support the weight of the developing fruit.

In winter, each year the pruner will remove all branches except one two-year branch along each wire in each direction, and one renewal branch (for next year). In other words more than 90% of all the branches on this vine will be removed! This leaves all of the materials stored in the roots from photosynthesis of those branches to support the fruits of just the few remaining branches. Hence you get nice large bunches of larger, higher-quality grapes!

We have learned several pruning styles for various parts of our landscape, and hopefully you are now better prepared to know what to do when you walk into your yard to do some pruning. However, if you still feel unsure, here is a list of some items you can prune out with very little uncertainty.

What Do I Remove When I Prune?

  1. Dead wood
  2. Diseased wood
  3. Crossing branches
  4. Vertical branches on horizontal limbs
  5. Weak crotches (those less than 90°) but leave strong ones (those more than 90°)
  6. Anything not fitting the pruning form
  7. Excess flowers or fruits (avoid competition = plums)

Obviously just as in universities, dead wood is not productive and can be a pathway for infection or disease pest. So removing it is good.

If you can tell a branch has a disease, removing it and taking the prunings away from the plant is important. You do not want to keep reservoirs of disease attached or nearby your plants.

Limbs that cross, unlike those grafted in Belgian fences, move in the wind and keep each other wounded all the time. The exposed wounds provide pathways for disease and pests, so eliminating these is important.

A vertical branch on a horizontal limb is often called a water sprout. These verticals behave like the main trunk and grow rapidly in height and diameter. They get very heavy while the horizontal to which they are attached grows more slowly in the canopy shade. Obviously the water sprout becomes a big weight on the end of a wimpy lever, with the tree trunk as a fulcrum. A nice load of fruits or ice during an ice storm will tear this tree apart. The water sprout needs to be removed before this happens.

Branches that appear at narrow angles from the main trunk again grow rapidly and as they increase in diameter the trunk and the narrow-angle branch push each other apart. Again with ice or fruit loads, this tree splits right down the middle! The battle zone between a main trunk and a limb at 90° is quite limited by comparison and the crotch is far stronger!

Of course you want to continue to observe the pruning technique that is appropriate for the shrub, tree, or vine in question. Branches out of form just have to go.

The removal of excess flowers and fruits is important to get high-quality fruit from fruit trees. Again because all fruits are made of genetically identical carpel walls, all are equally competitive for the tree's water, sugar, and mineral supplies. If there are too many developing fruits on a tree, they all turn out to be compromised. This is quite obvious in plum trees. The produce vast quantities of flowers which are pretty but when they all start forming fruits you will quickly have a "loser" crop. And when plums are compromised in competition, the results are exactly what you would expect. The seed in the pit is NOT compromised, the pit wall is NOT compromised (it is the protection for the embryo in the seed), so the tasty fleshy layer of the plum is what gets reduced. So you get tiny plums with full-size pits and seeds inside!

Excess developing fruits on olive trees are traditionally removed by beating the branches of the the trees with poles. The vibrations knock off the weakly attached individuals, leaving behind what will likely be a fewer nicer fruits. A grower gets more money for jumbo olives than for micro-olives.

The modern way for removing excess fruits is to spray the tree with an ethylene releasing agent. This causes the abscission zone to form on some of the fruit pedicels. A machine is brought to the tree a few days later and shakes the trunk. The weakened fruits fall to the ground...well-attached ones remain to grow into jumbo fruits.

There is one thing to avoid removing in pruning fruit trees. Most of the flowers (and therefore fruits) are produced on short shoots sometimes called spur shoots along the main branches and twigs of apple trees. If you cut those off, you will not have flowers for several years! It would be a disastrous mistake for an orchardist!

When do I prune?

Our final consideration is when to prune. You might have noticed some mention of this in the discussions of each technique above. So here we will just see a list:

I will explain the last item, though. There is a good reason NOT to prune in the fall. As you have learned here, the response of a plant to pruning is to form new branches. If you prune in fall, you have nice tender young branches and these will be hit by frost in mid-October at our latitude! Now you have dead branches and a pathway for disease and pest attack. So you stop pruning in August and resist the urge to prune anything until the dormant pruning that we do for fruit trees in Feburary.

 

This page © Ross E. Koning 1994.

 

 

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