Chapter 62 Wound Sealing and Filler Compounds

There is a never-ending argument in bonsai circles about whether or not to seal pruning wounds. Many artists say that pruning cuts never need to be sealed. Others say it is essential. If we dig deeper and apply some logic we find that NEITHER is true all of the time. There are situations where sealing is a good practice, and other times when it is inappropriate.

This chapter is longer than most others because the more I dug into this subject, and the more I experimented, the more I learned.

62.1 How Branches React to Pruning

To understand the use of sealants it helps to know what happens when a woody (bark-covered) branch or trunk is cut.

A recently pruned branch. This branch was cut at an angle away from the main trunk, leaving a small stub. This is standard for landscape trees, but not recommended for bonsai. Assuming this was a healthy branch, the thin layer of cambium that separates the bark and deeper wood will begin to expand and migrate out, covering the bare wood. The outermost layer of cambium will form bark to seal the wound. Link to original image..

Imagine a 1-inch diameter branch that has been cut off at a right angle to the axis of the branch. The cross section has an outer ring of bark then a thin layer of green cambium surrounding the older wood in the center of the branch. The center wood is dead, as is the outer bark. Only the thin cambium layer is alive.

Minutes after a branch is cut the ring of exposed green cambium begins to die back under the bark. How far back the cambium dies depends on moisture levels and the species in question. After 10-15 days, cambium cells adjacent to the dead zone reorganize to form a new layer of corky epidermis that protects the cambium beneath it from drying out. Now the cambium layer begins regrowing, pushing the corky epidermis back out towards the original cut face.

As the cambium begins regrowing, sub-groups of cells start to follow different paths.

  • Some cells will proliferate and form buds that can grow into new branches.
  • Some cells reconstruct the xylem an phloem tubes that carry water and nutrients.
  • The epidermal layer will continue to expand and thicken to form new bark.

Promptly covering exposed cambium with a barrier against moisture loss for 10 to 15 days can minimize initial die-back, shorten the time until regrowth starts, and hasten wound closure.

Image 1. A tree trunk with cambium rolling in to cover an old pruning cut. Image 2. A fully closed pruning scar. The natural junction points are ragged because some bark has been caught in the fusing cambium. Link to original image 1; image 2.

Once the cambium layer begins rolling (migrating out from under the cut edge of the old bark to cover recently exposed wood) the sealant material needs to lift away from the wood; if it does not lift easily, the rolling cambium will not be able to migrate in and close the wound.

When deciding whether or not to seal and how, we need to consider:

  • How sealing will affect aesthetics;
  • How the sealant affect disease resistance and rot;
  • How different species respond to pruning; and
  • How each type of sealant affects the healing process.

62.2 Aesthetic Considerations

Recommendations for pruning landscape and timber trees are driven by practical rather than aesthetic considerations. Standard practices are to make 45o angled cuts and to leave a branch stub long enough that the pruning cut does not break the branch collar.

Angled cuts reduce the potential for disease to enter the wound by letting water drain off rather than pool on the open woody end of the cut. Leaving a branch stub minimizes the odds of cambium die-back into the main stem of the tree (or the main branch if cutting side branches.) Die-back into the main trunk creates flaws in the wood that reduce the quality of the lumber produced from that tree. Die-back also creates weak spots where the tree will be more likely to break in wind or with a snow load. Taken together, the benefits of leaving a branch stub on a landscape tree outweigh the aesthetic concerns.

Pruning cuts for landscape trees vs. bonsai. When pruning full size yard trees, the recommendation is to make cuts 1 then 2 to remove most of the weight from the branch, before making a final cut at 45o that does not break the branch collar. For bonsai, branches are cut closer to the trunk. The ideal cut follows the green line from just outside the branch collar to a point below the branch. The cut should be angled away from the body of the trunk into the area marked B, and avoid cutting into the area marked A. Link to original image

In bonsai our first priority is to minimize visible scarring from pruning cuts. We usually do this by removing branch stubs completely; this is part of the rationale behind using concave branch cutters. The downside is that we must remove the branch collar, where the largest number of cambium stem cells are located. In this case, it makes sense to apply a short-term wound sealant and give what cambium remains its best chance to survive and repair the site.

Hiding all evidence of a pruned branch is not always the best strategy though. Sometimes the design of the tree actually looks better or more natural with either a stump remaining, a hard angle sculpted from the stub of a removed branch, or a cavity where a branch was removed. In these cases it is better to let the natural response of the cambium play out on its own without sealing it.

62.3 Preventing Disease and Rot

The second argument about sealing wounds centers on whether it keeps pathogens from entering a wound. A cut branch provides a potential path for disease-causing bacteria and fungi to enter and spread to an entire tree.

Those who recommend NOT sealing cuts argue that woody plants have several highly evolved mechanisms to block and wall off entering pathogens. That is largely correct; most pathogens need tears in the bark or roots, or similar large wounds to gain entry. Clean pruning cuts made with sharp tools are unlikely to allow in pathogens unless the pruning tools themselves are contaminated by pruning away damaged branches. Cross-contamination can be prevented by cleaning pruners with 70% alcohol between cuts.

What the preceding argument fails to recognize that pathological diseases are DIFFERENT from normal decay. Going back to the example of a freshly cut 1-inch branch, the outer cambium and bark make up less than 1/8th of the total diameter of the branch. The center of the branch is non-living wood. Residual sap deposits provide some protection, but ALL unsealed bare wood WILL start to rot at some point, even rot-resistant species like cypress and cedar. It is hard to predict how soon wood rot will begin or how fast it will progress, but generally rot progresses more slowly in drier or cooler conditions. Conversely, rot fungi and bacteria are most active in the warm, moist conditions that we have in the mid-Atlantic states from late April to mid-autumn.

If rot is inevitable, how can we create conditions that encourage the site to callus over before wood rot becomes established and damages the supporting mature wood underneath. The easiest way to do this is by applying wound sealant. If the goal is NOT to cover the area and instead allow natural wood to show, then wound sealant is NOT appropriate. Allow the wood to dry well, then apply a wood preservative that prevents wood rot.

62.4 Species Differences

The third consideration is what species is being pruned. For example, azalea are very prone to dieback if not sealed. For these, the right sealant will make the cambium more likely to roll in and cover scar sites. Sappy trees like maples are less prone to dieback because they contain considerably more liquid the cambium, which keeps it sufficiently moist so it undergoes little or no die-back.

62.5 How Different Sealants Behave

Wound sealants differ in how tightly they seal wounds and how readily the cambium can roll beneath them. I’ve outlined the traditional choices and alternatives that may be useful in certain situations.

62.5.1 Traditional Wound Sealants

These three options from Japan and China are widely available from suppliers, and most stylists use them currently or have in the past. The choice of a long- or short-acting product will depend on the goals for that location on the tree.

62.5.1.1 Kiyonal

A thick dull green paste that comes in a tube similar to toothpaste or oil paint. Kiyonal is composed of polyvinyl acetate, polybutene, and diocty phthalate. Polyvinyl acetate (plus polyvinyl alcohol) is the basic ingredient of Elmer’s Glue-All and Carpenter’s glue. It is degraded slowly by fungi, algae, and bacteria. Polybutene is used in lipstick, adhesives, and insect sticky traps to increase tackiness. Dioctyl phthalate is a widely used plasticizer that keeps the dried mixture flexible.

Kiyonal dries to a medium olive color, and remains flexible well below freezing. Initially Kiyonal forms a very durable barrier that is not pushed off as cambium grows over a wound. Treat it as a semi-permanent seal, not as protection until the cambium is repaired.

Intact layers will prevent rot but Kiyonal breaks down and falls away after 1-2 years. It can be replaced, or the wound re-covered with a more permanent wood preservative.

62.5.1.2 Cut Paste

A dark clay-like imported material that usually comes in a small wide-mouth jar. There are two colors, brown and gray. Unlike Kiyonal, this material dries out slowly and eventually peels off on its own. Rolling cambium WILL push this off as the wound edges close. It does not seal wood against rot.

62.5.1.3 Cut Clay

The least common traditional sealant. Not all suppliers carry it. This is an oil clay-based solid block sealant that has some grit added to increase bulk and improve tack. Like Cut Paste, this material is best handled with wet fingers. It does not harden, and peels away on its own within a few months. Use it for short-term protection while cambium layer recovers from pruning. It also does not seal wood against rot.

62.5.2 Alternative Sealants

These are options if you run out of wound paste or cut compound, or want to experiment with some non-traditional and less expensive products. Items listed are non-toxic according to US federal standards, and I have tested them on live trees and seen no harmful effects.

62.5.2.1 Grafting Tape/Parafilm

Grafting tape is available from arboriculture suppliers, while Parafilm is available from scientific suppliers. Both are plastic films coated with low-temperature melting wax. When the film is stretched it warms, softening the wax. When wrapped around a branch, the wax layers bond together and form a nearly waterproof seal. Grafting tape is engineered to decompose in a few months; Parafilm can last 6-12 months outdoors. Both materials provide a SHORT TERM moisture barrier that allows cambium to grow and cover open areas. Neither protects against rot.

62.5.2.2 Elmer’s White, Carpenter’s Glues

Most Elmer’s glue formulations are based on polyvinyl alcohol (PVA), which is non-toxic. They dry to a translucent film (milky for white glue, pale yellow-tan for carpenter’s glue). White glue will slowly erode away after drying, but carpenter’s glue is more or less permanent. Both can be tinted by adding a little matte exterior grade acrylic paint to the liquid glue, or painted over after drying.

Brushing several layers of wood glue onto bare wood create an impermeable seal that blocks rot. Once dried, wood glue can be painted with exterior-grade matte acrylics to blend into other structures. Be aware that this glue layer will not stretch or flex with the wood, and eventually will separate and allow water to seep underneath.

Avoid more specialized wood glue formulas, as many have additional components that are more toxic.

62.5.2.3 Doc Farwell’s Grafting Sealant

Proprietary aqueous emulsion of polyvinyl ester and kaolin clay. Thinner than Kiyonal, but works almost identically. Purchase from nursery supply companies.

62.5.2.4 Children’s non-drying modeling clay; Plasticine clay

Oil-based clays will not dry or shrivel over time, and remain flexible below freezing. As cambium rolls into a wound, it can push the clay up and away, making removal easier. The drawback is they can be difficult to clean out of cracks or crevices. It is hard to find more than a few colors, but the basic primary colors can be mixed to match most bark colors.

Like children’s modeling clay, plasticine clay is non-toxic and does not dry or harden. Look for it in artists’ supply stores.

DO NOT USE PLAY-DOH! It is extremely salty, and will kill surrounding tissues.

62.5.3 Home Made Sealant Mixes

62.5.3.1 Glue-Based Cut Paste

A mixture of PVA wood glue, a non-toxic thickener, and a coloring agent is a reasonable substitute for Kiyonal paste. Like Kiyonal, PVA pastes adhere too tightly for the rolling-in cambium to dislodge. Do not use them if you want the cambium to be able to lift the wound cover off. Use non-hardening cut pastes if lift-off is the goal.

Starting Mix:

  • 10 parts (by weight) Elmer’s carpenter’s glue
  • 0.5-2 parts (by weight) thickener / bulking agent
  • Colorant as desired (hint: final color will be slightly darker after drying)

There are several options for the thickener / bulking agent. I have tried all of these.

  • Powdered mica is a non-toxic filler used to reduce greasiness of oil-containing mixtures. It has anti-caking properties (prevents clumping) that make mixing easier. The mica-based mix is smoother and adheres very tightly. In a 2-year field test, it only came off when the wood beneath broke off with it. Treat as a permanent seal.
  • Powdered kaolin is a non-toxic clay. It is very absorbent but does not swell as it absorbs water. Add half the amount you would expect to use to the glue base, as it will draw up water for 1-2 days and make a thin paste much thicker. Kaolin (and mica) containing pastes form a smooth, easily packed mix. They dry into a strong wound cover that lasts at least 2 years. Neither absorbs water or shows any signs of degradation. The kaolin-based mix adheres more loosely, and is slightly rougher. It will separate from the wood underneath when pried away, so MAY be pushed off by infilling cambium.
  • Zinc oxide is used as a filler for cosmetics and soap, and in sunblock to block UV rays (usually in a mix with titanium dioxide). It also has some antimicrobial activity. Zinc oxide-containing paste is stiffer and breaks up when exposed to freezing temperatures, so would be best suited for short-term wound coverage.
  • Talc is most often seen in talcum powder, but also is used as a lubricant and bulk additive.
  • Bentonite clay is formed by weathering of volcanic ash. It expands when wet, absorbing several times its dry weight in water. It is the main active ingredient in fuller’s earth, absorbent cat litter, and oil-absorbing powder.

Options for coloring agents:

  • Acrylic paint will blend into the mix and not leach out.
  • Powdered pigments will leach out over several months, leaving the off-white base. The base still can be painted to match the rest of the bark.

62.5.3.2 Wax-Based Cut Paste

This paste is designed to hold in moisture, and will not remain in place for very long or after the temperature drops much below freezing.

  1. Mix equal amounts of candle wax and vegetable oil together.
  2. Melt them in an old saucepan inside a container sitting on boiling water.
  3. Remove the saucepan when mixtures is melted.
  4. Let it to cool until it is thick, slippery gloop when you stir it.
  5. Add some ground cinnamon as a fungicide and coloring.
  6. Whisk the mixture for a few minutes until it is fluffy like chocolate mousse and not warm when you touch it.
  7. Store in a jar for use. Dip your fingers in water before you use it because it is a bit sticky.

62.5.3.3 Basic Plasticine-Style Clay

This serves as a temporary wound cover. The exact composition of commercial Plasticine is a trade secret, but the basic mixture is calcium carbonate, petroleum jelly, and long-chain aliphatic acids (principally stearic acid). It is non-toxic, sterile, soft, malleable, and does not dry on exposure to air.

Version 1

This blend’s handling properties depend on what dry solids are used.

  • 1 part (wt.) petroleum jelly
  • 2 parts (wt.) paraffin wax (beeswax will make a softer alternative)
  • 2 parts (by weight) mineral oil
  • 4-10 parts (wt.) dry solids (thickener and bulking agent)

Melt the wax and petroleum jelly over low heat, add oil and mix well, then add solids. The final consistency will be thicker when cooled, so do not add too much dry solid.

The original formulation was made with powdered calcium carbonate (oyster shells, basically). Other options for the solid portion include powdered talc, clay, or mica.

Version 2

This is more difficult to make because it is harder to find the raw materials. They need to be ordered online.

  • Calcium carbonate
  • Polybutene (used in adhesives to improve tackiness)
  • Oleic acid
  • Linoleic acid
  • Ferrosoferric oxide

Calcium carbonate is the active ingredient in agricultural lime and limestone, and oyster shells.

Ferrosoferric oxide is a black pigment known as iron (II, III) oxide. Also called ferric ferrous oxide; tri-iron tetraoxide; black iron oxide; magnetic iron oxide. Used in paints, linoleum, ceramic glazes, and colored glass.

Polybutenes are polymerized butylenes that are viscous, non-drying liquid polymers. They are non-toxic, and impart tackiness or adhesion.

62.5.4 Wound Fillers

Wood fillers are rarely used in traditional bonsai and penjing. However filling in a deep gouge or crack takes more than just cut paste. There are some modern commercial products that are useful for solving very specific problems. I cannot recommend them for routine use though because none of them allow out trapped moisture, so any decay that has started will continue.

62.5.4.1 Two-Part Epoxy Putty Sticks

Epoxy putty is the most permanent repair option. It is waterproof and will not shrink or crack. Raw epoxy is an irritant, but cured epoxy is non-toxic.

The two components are sold as a stick. Cut off a piece, knead to mix, and pack into wound or open space. Working time is about 10 minutes, so have the space to be filled prepared before mixing, and work quickly.

  • JB Weld KwikWood (tan color only) is available at most home improvement stores.
  • Putty for furniture repair (marketed as “touch up epoxy putty sticks”) comes in several colors. Look for it online from woodworking suppliers.
  • Aquarium epoxy putty is available from aquarium suppliers in limited colors.

62.5.4.2 Exterior Wood Filler

Water-based wood fillers can be used to fill areas that will not be in direct contact with live cambium. Do not apply wood filler over rotted wood; moisture and trapped fungi will continue destroying wood beneath the filling. Scrape down to sound wood before applying.