By Brandon M. Gallagher Watson
If you have small kids, you no doubt have regularly scheduled doctor visits. In fact, the smaller the kid, the more frequent the visits. Babies get checked every few weeks, then every few months, until their toddler years where visits become an annual event. On the other end of the spectrum, if you care for your elderly parents, you also no doubt have regularly scheduled doctor visits, often getting more frequent near the end. During the middle years of life, you probably should see a doctor every once in awhile, but if you are anything like me, it’s really easy to blow off the annual exam and only seek medical assistance when sick or injured. In general, humans need more input from health experts when young and when old than they do during the middle ages of life. Trees have similar needs, and, as arborists, we are the doctors for the trees. So how does the care for young, middle, and old age trees differ?
Before we delve into the ways we approach trees of different ages, we have to first start at the beginning of any urban tree’s life: planting. I know arborists are not often consulted at the time a tree is being selected and stuck in the ground, but the importance of starting a tree off right cannot be overstated. As the first five years are critical to a tree’s long-term success, try to pass these three quick tips along to everyone you know. First, the tree needs room both above and below ground to reach its genetic potential. Second, the tree should be a species that will thrive in the soil type of the site. Third, the tree needs to be planted at the correct depth with the first order roots pretty much at the soil line.
Now that we have our tree in the ground with all the perfect conditions for success, how will our management strategies change as this tree grows? We could probably look at dozens of different factors from fertilizing to support cables, but let’s focus on the basics. How will watering, pruning, mulching, and pest management differ over time?
No external input is more important to a young tree’s future success than water. As a professional, you won’t be around to irrigate the tree on a regular basis, so teach clients to water correctly. A young tree should be watered once a week during the summer, getting 10-15 gallons each time. A good rule of thumb is to drag a hose out to the base of the tree, put it on trickle, then forget about it for an hour or two before turning it off.
Middle age trees
As a tree expands the reach of its root system, the need for supplemental irrigation decreases, but certainly does not go away. Watering, particularly during the drought-laden months at the end of summer, can be a lifesaver for middle age trees. Trees that suffer from drought are more likely to suffer from secondary pest problems, deplete their stored energy reserves, and not restock those reserves as their photosynthetic abilities are greatly reduced. How much water do these trees need? There are plenty of complicated formulas one can find, but any easy one to pass on to homeowners is five minutes per DBH”, three times per month from April through October. That means a 12” DBH tree should have the hose on for one hour, about every 10 days during the growing season.
See the section on middle age trees.
Young, middle age and old trees
We can lump all three age classes together here because the recommendation remains the same regardless of how many growth rings the tree has. The improved growth of roots under mulch, particularly when compared to root growth under turf grass, is well documented. As an organic mulch layer is closer to the surface of the forest floor than a well groomed lawn, this makes sense. Mulch, however, has a diminishing return on investment the thicker it is applied. We’ve all see ‘volcano mulching’ going half way up the trunk and know that is downright harmful to a tree, so keep it about 2” deep. While you can go too deep with mulch, you really can’t go too wide. How wide should a mulch ring around a tree be? I’ll defer to Dr. John Ball who is quoted as saying, “Show me the property line, and we’ll negotiate inwards.”
Besides getting a newly planted tree’s roots established, pruning young trees for proper development is probably the most important thing that can be done at this stage of life. Trees with poor structure are more likely to fail and become hazards later in life. To develop a strong structure, follow these simple guidelines from Dr. Larry Costello:
1. Remove dead, dying, damaged, diseased branches.
2. Select and establish the central leader.
3. Select the lowest permanent branch (LPB) based on tree location/purpose.
4. Select and establish scaffold branches.
5. Select temporary branches below the LPB and remove or head back others.
Middle age trees
While much of the structural development should have taken place during the tree’s formative years, there are still plenty of reasons to trim a middle-age tree, but any removal of live tissue should have a clearly defined objective. Reducing the risk of failure, improving structure, improving aesthetics, providing clearance, improving vista views, and even reducing the amount of shade the tree is providing are all legitimate reason to prune a tree.
The amount of live tissue that should be removed from a tree in a given growing season is not unlimited, so be judicious in how much trimming you do. A general rule for middle age trees in good health is not to remove more that 10 to 15 percent of live canopy per season; removing more than this slows root growth by shifting the root to shoot growth ratio. This adds significant stress to the tree. Heavy pruning also reduces carbohydrate reserves, making the tree less tolerant of insects, diseases and drought stress. If the tree is stressed or declining, pruning is not recommended until the tree recovers and resumes normal growth again.
The same rules apply when pruning an older tree that were followed when the tree was middle age. We still need to have a solid reason for removing any live tissue and not remove more than 10 to 15 percent of the canopy in a single season. Depending on the species, deadwood pruning may need to be considered as the tree ages. Dead branches in the canopy are not only unsightly, but pose a safety hazard as well.
In conjunction with pruning, or possibly as an alternative to it, tree growth regulators are another tool that can help maintain the size of mature trees. Palcobutrazol (Cambistat, Profile) is useful for larger, older trees and not only helps reduce the vegetative growth of the tree, but studies show the trees reallocate energy into root growth, carbohydrate storage, and drought resistance — all resources that benefit a mature landscape tree.
Insect and disease management
Monitoring for tree health problems such as insects and diseases is critical for a young tree’s health. Newly planted trees are focusing their energy on root production and getting established in the site. This can come at a reduction in energy placed on producing defense compounds, leaving the trees more vulnerable to attack from insects and fungi. As there are plenty of challenges as a new tree is settling in, be on the lookout for pests that will hinder the process. A pest that would just be a nuisance to a larger tree — for example a foliar-feeding pest like Japanese beetles — can cause significant stress and damage to a smaller tree. Not all pest problems require arborist intervention, so be aware of what pests are typical on this species in your area, and which are cause for concern.
Middle age trees
For trees of middle age, the adage of “a good defense is the best offense” holds true. There are invasive pest problems, such as emerald ash borer or hemlock woolly adelgid, that will attack and kill a tree regardless of its health, however, a vigorous healthy mature tree does a great job of fending for itself. Monitoring for insect and disease problems is still important for trees of this age. If you do find a pest that requires professional services, be on the lookout for predisposing stress conditions that lead a tree to being more susceptible to them. This is also the age where urban trees begin to really give back and provide all the benefits we love from them. Keeping them healthy and keeping an eye out for pest problems will be an investment that pays off.
Just as humans become more susceptible to pathogens as we age, so do trees. Monitoring for pest problems at this age is as critical as it was for young trees. Not only are these trees prone to opportunistic pests such as boring insects, decay inspections should become a regular part of these visits. Decay is a leading cause of failure in larger, older trees but it is often predictable and preventable with regular monitoring. Decay is nearly impossible to manage once it is established, so minimizing wounding, including pruning wounds, is important at this age. Even foliar diseases, such as anthracnose, can have a more serious health impact on an older tree than a tree in the growth phase of its life. As with every stage of life we have discussed, an ounce of monitoring and preventing issues is worth ten pounds of cure.
No tree lives forever, and we are well aware that trees in the urban environment live significantly shorter lives than trees in natural settings. There are, however, many ways by which an arborist can influence the health of the tree to get as many useful, productive, and safe years from that tree as possible. Educating the tree’s owners on the simple steps they can take is also an important part of a tree care professional’s job. After all, most tree owners don’t notice problems until it is too late, and the trees are unable to ask for help. This is why it’s good that tree doctors still make house calls.
Brandon M. Gallagher Watson is director of communications at Rainbow Treecare Scientific Advancements, and is an ISA Certified Arborist (#MN-4086A).