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Tree Planting Tips & Myths
In many cities, the urban forest is in crisis – development, climate change, exotic disease and insects, apathy, scarce financial resources, and lack of replanting all play a part in the decline. In the City of Toronto, their street tree program is being redesigned to improve survival, while in many municipalities, tree planting programs are virtually non-existent. Arborists can play a vital role in the renewal of the urban forest by using their knowledge and expertise to plant high quality trees that will best complement site conditions and withstand the tests of time. Experience is showing that carefully selected planting stock installed by dedicated professionals has the highest chance of survival.
Arborists are in a unique position as we are one of the few groups that evaluate landscape design and installation techniques by the results associated with tree health and growth. We realize that poor stock and planting is often expressed as stressed and dying trees many years down the line. Working with individual homeowners, we can install the right tree in the right place, the right way, with instructions for the homeowner that will ensure the tree thrives. Local Enhancement and Appreciation of Forests (LEAF, www.yourleaf.org) has shown in the GTA that careful planting of trees can result in high success rates. If commercial arborists make a goal of including tree planting in their normal business operations, they can aid substantially in the long-term continuance of trees in the urban environment.
Rolling The Dice vs. Careful Planning
The planting of trees certainly appears straightforward. Visit www.treesaregood.com/treecare/tree_planting.aspx and/or grab a copy of The Principles and Practice of Planting Trees and Shrubs by Watson and Himelick (available through the ISAO Bookstore). As I’ve outlined above however, tree planting is often highly unsuccessful. In Toronto, survival of planted street trees is abysmal, but plantings are more successful in ravines. Trees planted in tree pits survive for short periods of time (http://spacingtoronto.ca/2008/10/15/the-urban-design-of-tree-pits/), seldom lasting 30 years. Modern approaches to tree planting in these situations have recognized that careful planning can lead to successful planting (see Up by Roots: Healthy Soils and Trees in the Built Environment by James Urban for a thorough discussion).
Planting trees on residential properties is far more successful, though planted trees still seldom reach the ages they would in natural environments. Since 1996, LEAF has planted over 16,000 trees with a survival rate of over 95% five years after planting. Michael Alkema, LEAF Field Operations Supervisor, states: “Planting, stock and maintenance all play a vital role in transplanting a tree. Each criterion is vital in the success of a tree as any one factor can have detrimental effect. Minimizing stress on the tree will allow it to establish quicker and more efficiently in the new habitat. One way to do this is to follow proper planting procedures such as correct height or minimizing girdling roots.”
We as arborists know how to plant trees, but many myths still exist amongst homeowners and uneducated contractors. Not surprisingly, these myths benefit neither the tree nor the urban forest and often result in poor performance. Let’s explore a couple of these urban legends.
Cut a Tree, Plant a Tree
When a tree is removed, it is often replaced with a small potted or balled and burlapped (b&b) tree. A new tree that has only a couple of hundred leaves cannot replace all the benefits lost with the removal of a mature specimen. Trees provide not only visual appeal, but also ecological functions such as cooling, air polishing, and habitat and oxygen production.
Older trees provide both a greater surface area and more microhabitats. Many of the reasons used to justify removal of old trees – dead limbs, hollows and fungi – are extremely important for biological diversity and cannot be replicated with the same surface area on hundreds of smaller trees.
Urban trees play a role in carbon dioxide sequestering and aid in the shading of structures, something not seen in most rural and forest trees. James Urban has stated that a transplanted tree will not begin to make a net contribution to carbon sequestering for 20 years due to high energy input into growing and moving the tree.
Clearly, one-for-one tree removal and replacement reduces the functioning of the urban forest over time. Multiple replacement trees are needed to help counteract the loss, and survival to a mature age is vital.
Big Box Buying & Marketing Powers
The proliferation of seasonal garden centres in big-box stores is training the public, through cheap prices and end-of-season sales, to view trees as a low priced commodity. Along with the low price mentality often comes reduced care of the tree. Countless times I have seen trees planted with little forethought and a doomed future.
Ed Gilman, a professor at the University of Florida, writes: “Planting and establishing trees is all about managing air and moisture in the soil. Manage these correctly and trees will grow quickly following planting. Four of the most common causes of poor plant establishment are 1) planting too deeply, 2) under-watering, 3) over-watering and 4) over-mulching” (see http://hort.ufl.edu/woody/planting-hole.shtml for more details).
Potted is Better
Traditionally, b&b trees have been considered stressed because approximately 90-95% of their roots are severed when dug. Trees grown in pots have been viewed as a superior option as no roots are severed and the reduced weight allows for a broader range of people to plant trees. Unfortunately, potted trees exhibit their own evils. Girdling and contorted roots, desiccation-prone potting material, and root-bound plants can all result in poor survival (see Up by Roots for extensive discussion). The increasing prominence of potted material in the marketplace indicates that we should all be vigilant about inspecting roots.
Once a root is formed, it does not straighten itself. The Canadian Standards for Nursery Stock, 8th ed. (available through the Canadian Nursery Landscape Association) says: “All normal quality nursery stock must have an adequate fibrous root system that has been developed by proper cultivating practices, particularly transplantings or root pruning.” Florida’s Grades and Standards for Nursery Stock are stricter; they require that many circling roots be removed or the plant be culled (see http://hort.ufl.edu/woody/powerpoints/Floridagrades.ppt). It is of utmost importance to remove or straighten girdling and contorted roots at the time of planting. The root-crown and trunk-flair intersection must be exposed and planted at or above the soil level. The tree may be planted two to three inches high as addition of soil and excessive mulch by homeowners and property maintenance companies can effectively raise the soil level.
Remember: Plant it high and it will thrive; plant it low and it won’t grow.
With b&b trees, it has been commonplace to prune the crown to “balance” with the severed root system. This appears to be a myth but a more immediate need may be to begin formative pruning while the arborist is on site and to reduce the size of the sail. Trees are often topped in the nursery to form a dense head. This clump of sprouts needs to be thinned and a dominant leader formed from day one.
Planting instructions often include adding soil modifiers such as manure, compost, triple mix, peat moss, fertilizers or mycorrhizal fungi to the planting hole. Amending soil with organic matter may reduce bulk soil density but can also lead to the bathtub effect in dense soils. Also, as the organic matter is decomposed, the soil will subside. It is better to add organic matter in the form of woodchips (mulch) to the soil surface.
Again, simple advice can greatly benefit establishment. Alkema (LEAF) says, “The benefits of mulch are profound as it helps mimic a natural environment, maintain soil moisture levels, soil temperature, restore organic matter, inhibit weed growth, and foster beneficial fungus, bacterial and animals.”
Unless a specific soil test indicates fertilizer deficiency, omit high nitrogen fertilizers. Fish-based fertilizers are known to have a positive effect on soil microbial numbers and the soil food web without being high in salt. Recently the addition of biopacks, which contain microorganisms such as bacteria, agrobacterium and mycorrhizal fungi, have been advocated. Research indicates that in limited circumstances, these may be beneficial (particularly in “sterile” soils) but often natural processes are fine (discussion by Gerrit J. Keizer is available on various threads on http://arbtalk.co.uk/).
Rather than worrying about modifying the backfill soil, it appears far better to dig a wide, shallow hole and allow the new roots to find their way through the loosened soil.
Though rigidly staking trees was once very common, recently there has been a movement to not stake and rather allow the tree to flex and sway. It is believed that trees build taper and root structure in response to movement (thigmotropism) and staking counters this. Unfortunately, trees act like sails and the root ball of a transplanted tree may not have the mass to counter the sail. Shifting positions produce leaning trees and may tear roots. Small amounts of movement allowed by flexible staking seem adequate to allow trunk taper and anchoring to occur while preventing tipping of the root ball.
Tree Care Ends With Mulching
As the mulch is applied, the most important time for the tree begins. Ben Kobes of Kobes Nurseries in Hampton says that potted material at his nursery are watered daily as the potting soil is 60% pine bark and just one day will desiccate this material. Michael Alkema notes, “Care and maintenance are important, the number one reason (~95%) why newly planted trees die is lack of water. Proper care can help the tree overcome issues from planting or stock but not the other way around. The greatest tree with the best planting will still die without sufficient water. Mulch and regular watering (2-3x a week for 10-15 minutes from a hose on a slow trickle a couple feet from the base) is best.”
Instructing your client in proper watering technique may be the most important thing you do for the tree. Watering during initial root formation and during droughts will ensure successful establishment and survival of newly planted trees.
In The End
Tree planting is simple given a little knowledge and may be one of the most profound things an arborist can do. Chain sawing a tree is quick and easy; planting a tree begins a relationship with the customer and nature that can last for decades. An arborist who carefully selects the right tree for the right place, chooses high quality nursery stock, and meticulously plants the tree will leave a legacy of their professionalism that can be seen for decades to come.