Pruning Wound Sealant: What the Research Says, and How It Is Used in Japan
After a large branch is removed, many people ask the same question: should I paint the cut? In Japan, wound pastes are widely used on garden trees. In the English-speaking world, many arborists have advised against them since the 1970s.
We are a small tree care and landscape business in Mie Prefecture, central Japan. This page sets out what the research says about wound dressings, the three products commonly used in Japan, and how we choose between them on site.
Trees do not heal, they seal
A tree cannot repair wood that has been cut. What it does is grow new tissue, called woundwood or callus, in from the edges of the wound until the bark closes over it. Inside, the tree walls off the damaged wood so that decay does not spread through the rest of the trunk. This process is called compartmentalization, and it was described by the American forest pathologist Alex Shigo and his colleagues from the 1970s.
Shigo concluded that the dressings in use at the time did not help this process and could even work against it. According to a later review of the research, his work led most conscientious tree care professionals to stop using wound dressings.
What the research says
Linda Chalker-Scott of Washington State University sums up the evidence in a fact sheet called The Myth of Wound Dressings. In her view, dressings do not prevent decay organisms from entering, they can hold in moisture and slow the tree's own response, and they eventually crack. Using one for appearance alone is never justified. She makes one exception: if a disease-prone tree has to be pruned in the warmer months, when insects and fungi are active, a light coating of a fungicide or insecticide may be warranted.
A field study by Hudler and Jensen-Tracy, published in 2002 in the journal Arboriculture and Urban Forestry, tested Lac Balsam, one of the products we use, on Norway maple, honeylocust, eastern white pine and eastern hemlock. After 16 to 24 months, the dressings had little or no consistent effect on wound closure or on how well the tree walled off the damage. The one exception was honeylocust wounded in June, where the column of discoloured wood was about 25 percent shorter. The authors say the practical meaning of that difference is unclear.
Two other results are worth knowing. On eastern white pine, none of 36 wounds coated with Lac Balsam were colonized by a resin-feeding moth larva, while 5 of 27 untreated wounds were. And the authors found no sign that the dressings harmed the trees. Where a scar is considered unsightly, they had no reservations about using Lac Balsam to soften its look.
So the two sources agree that a dressing does not speed healing, but differ on whether appearance alone is a good enough reason to use one.
Three products used in Japan
Topjin M Paste is a fungicide paste registered as an agricultural chemical in Japan (registration No. 13411). Its active ingredient is thiophanate-methyl at 3.0 percent, and it is orange-yellow. The maker describes it as a protective coating for pruning cuts that kills fungi and helps callus form.
Callus Mate is a vinyl acetate resin paste with no pesticide registration. It contains no fungicide. It forms a film that keeps out rain, prevents drying and blocks physical entry. Because it is not a pesticide, it can be used anywhere, including schools and hospitals, and it is good value.
Lac Balsam is made in Germany. The study above describes it as an artificial resin dispersion emulsion. We use it where the finished look of the tree matters.
How we choose on site
We carry all three products and choose for each cut, looking at the state of the wound and the conditions of the site.
Where the risk of decay is high, such as a large-diameter cut, a hardwood like zelkova or Japanese hackberry, or a wound that has already begun to discolour, we use Topjin M Paste.
At shrines, temples, parks and gardens where appearance matters, we use Lac Balsam.
Where pesticide use is restricted, on ordinary garden trees, or where cost needs to be kept down, we use Callus Mate.
One thing we do not do is mix charcoal into Topjin M Paste to darken its colour, a practice some people have heard of. It dilutes the active ingredient. If the colour matters, we start with Lac Balsam instead.
Common questions
Does wound sealant help a tree heal faster? The research does not show that it consistently does. The tree closes the wound itself by growing woundwood in from the edges.
Is it harmful to seal a pruning cut? In the Arboriculture and Urban Forestry study, the authors found no sign that the dressings they tested harmed the trees.
When might a fungicide paste make sense? Chalker-Scott notes that a light coating may be warranted when a disease-prone tree has to be pruned in the warmer months. In Japan, Topjin M Paste is a registered fungicide made for this purpose.
How long does a coating last? In the same study, Lac Balsam stayed intact until the tree had added about 1 cm of growth, then lifted, cracked and peeled.
