By Brandon Gallagher Watson
Thieves sneaking into the orchards of Leonardo da Vinci to steal apples may have been successful once — but not likely twice. Legend has it that da Vinci was so fed up with these impious pome pinchers he devised a way to inject lead arsenic into the trunks of the trees in hopes of poisoning the fruit to, in turn, poison the thieves. If this is true, it not only makes da Vinci a spiteful neighbor who’s stingy with his apples, but also makes him one of the first pioneers in the field of tree injection.
There are even earlier accounts, from the 12th Century, of Arabian horticulturalists attempting to inject various compounds into trees in an attempt to alter the fragrance or color of flowers. The success of these trials has been lost to history, but it does show that people have long been interested in the idea that various treatments could be injected into the vascular system and transported throughout the whole tree.
Modern tree injection began in the mid 20th Century and looked to utilize this technology for the benefit of tree health rather than simply aesthetic or apple-orchard-security-system purposes. Just as the advent of the syringe and needle revolutionized the way doctors approach human medicine, the advent of reliable equipment, the formulation of injection-specific chemistries, and a growing knowledge of the science behind tree injection has revolutionized how arborists are able to care for trees.
Tree injection has many advantages over other application methods. It can be used in situations where spraying the whole tree is not feasible or where soil applications are not an ideal option. There are also certain tree health treatments, especially for the management of vascular wilt diseases, where tree injection is the only treatment that has been shown to be effective. Additionally, in the era of increasing public sensitivity to chemicals in the environment, tree injection also has the positive perception that the treatment is contained entirely inside of the tree, minimizing environmental exposure.
Despite these advantages, tree injection remains a mystery to some, and even controversial to others. There are some disadvantages, of course — with tree wounding and application time at the top of that list — and injection is certainly not a “silver bullet” for every tree problem. Adding to the confusion, a bevy of devices are available to you these days from manufacturers that all claim to have benefits over the others. So how does one know which devices to buy, and in which situations to deploy them?
Tree injection systems and methods
Tree injection can be loosely defined as any method by which a solution is introduced directly into the vascular system of the tree, but there are many ways this can be accomplished. There are passive injection systems, by which a bag of solution is hung on a tree and a series of tubing and tees delivers it by gravity into holes drilled in the base of the tree.
As far as the science of tree injection is concerned, these passive methods work just fine for getting treatments into a tree, but the business of tree injection also requires treatments to be applied quickly as labor costs are often the largest expense of this method. This is one of the reasons that the vast majority of tree injection devices utilize pressure to move treatments into the tree. Depending on the device, this pressure can range from 8 psi upward to 60-70 psi.
Pressure is not the only factor that influences uptake time for an injection treatment. The species of tree has a significant impact, with some trees — such as elm and oak — taking the treatments fairly quickly, while others — such as red maple or white birch — are known to be notoriously slow. The time of year, recent weather, and any tree stress prior to treatment will all influence the time it takes to get a full dose into the tree. An easy fact to remember is that our tree injection treatments are going into the current year’s growth ring, so anything that would cause smaller growth rings — such as drought or stress from an insect attack — will have an influence on a treatment’s uptake.
The vast majority of today’s tree injection techniques are lumped into either macro- or micro-injection. The difference between the two simply comes down to the volume of solution that will be introduced in the tree. Although these terms are not official by any means, in general, “micro” refers to volumes of milliliters up to a few liters of solution while “macro” implies the application of a few to several dozen gallons of solution. (As an aside, the term “infusion” versus “injection” can be confusing, but the terms can be used interchangeably. Technically, none of the treatments we apply to trees are “infusions;” they are all “injections.” I believe “infusion” sounds softer so it is frequently used by marketers, but don’t let it trip you up).
Micro-injectors come in many shapes and sizes. There are pre-filled capsules, as well as refillable capsules. Some applications simply require drilling a hole, inserting a capsule, and waiting for the capsule to empty. Pre-filled capsules have a historical complaint of unpredictable uptake time — some may empty in a matter of minutes, others may be several hours — causing scheduling nightmares for commercial arborists. Another downside is disposing of empty capsules; but in terms of fast set-up and ease-of-use, the pre-filled capsules are hard to beat.
More elaborate micro-injection systems may use a small bottle as a reservoir and then employ a series of tubes and needles to move the treatment into the tree. These systems often require a plug inserted into the hole to prevent the treatment from coming back out of the hole. This type of system has gained popularity in the past few years as it has proven to be a fairly rapid way to inject insecticides into ash, and other widely recognized slow-as-molasses-uptake-time species of trees, including pines and spruce. These are refillable systems that can be utilized for a number of different treatment protocols, such as insecticides, nutrients and certain fungicides.
Macro-infusion systems are also available in a few different versions. Dutch elm disease, which has been treated for more than three decades by macro-infusion, requires a large volume of fungicide solution applied to the tree every three years. For example, a 30-inch-diameter elm tree would require more than 36 gallons of solution to be injected (hence, macro). To accomplish this, the reservoir is often a 55-gallon drum and then an electric pump moves the solution through a series of tubing and tees into the base of the tree. Other macro-infusion setups will replace the electric pump and 55-gallon reservoir with a 3-gallon manual pump tank when such high volumes are not required.
As a general rule, research has shown micro-injection is most effective for insecticides, miticides and antibiotics, while macro-infusion has been shown most effective for fungicides and micronutrient supplements such as iron and manganese.
You may notice that every injection device manufacturer seems to carry a full suite of injection treatments — insecticides, fungicides, nutrients, and so on. As stated earlier, there are no “silver bullets” in tree injection, so don’t think about getting into the tree injection business by thinking “what device do I need that can do it all?” None of them can do it all. If you have any doubts about the results of a treatment recommendation, do not hesitate to request research data backing the claim.
Our tree care company in Minnesota treats roughly 4,500 trees each year via injection, and we are nearly evenly split between micro- and macro-infusion. It’s not about “what device makes most sense,” it is about “what does the research show is the best option for treating this tree?” Sometimes it is micro-injection, sometimes it is macro-injection, sometimes it is soil applications, sometimes it is spray treatments, and sometimes the tree needs to be removed.
Tree injection is a valuable tool in the toolbox, but it is just that — another tool. Drinking the Kool-Aid served by injection device makers may be just as dangerous as drinking Leonardo da Vinci’s cider.
Brandon Gallagher Watson is director of communications at Rainbow Treecare Scientific Advancements, and is an ISA Certified Arborist (#MN-4086A).