Planting a tree isn’t rocket science, which is good. If it were that complex, I’d wager we’d have a lot fewer trees around. It may not take a genius to plant a tree correctly, but a lot of money is wasted each year to buy and install trees which may as well be rented, because they will only live a fraction of their potential lifespan.

When trees decline and die after 20 or 25 years, the last thing we probably suspect is shoddy planting. Although landscape trees like mountain-ash and gray birch are short-lived, sugar maple and red oak should easily last a century or two. Yet all too often, a long-lived species will expire at twenty because it was planted “fast and dirty.” You can find examples of trees declining as an age-class in housing developments, and especially along major routes where contractors replaced older trees that were cut for road improvements. We may as well consider such trees rentals.

Deep planting sets the stage for a sickly tree, one often headed for an untimely end. Every tree comes with a handy “depth gauge” called the trunk flare, which should be just visible above the original soil grade. Planting too deep leads to serious future health problems. For the tree, primarily. Here’s an arborist joke: What do you call a metre-deep planting hole for a tree? Its grave.

Given their druthers, tree roots extend more than twice the branch length, or drip line, but 90% of them are found in the top 25 centimetres of soil. To reflect this fact, a planting hole should be saucer-shaped and 2 to 3 times the diameter of the root system, but no deeper – ever – or the Planting Police will ticket you. Okay, that’s fiction, but if an arborist happens to come along, she or he may scowl ominously.

When a tree is dug in the nursery, most of its roots are cut off by the tree spade used to dig it. The term transplant shock refers to this catastrophic loss of roots. Obviously, trees can survive transplanting, but they need to have the right conditions for re-growing roots. It’s essential a transplant’s roots be able to penetrate the surrounding soil, as any slight barrier can induce them to turn aside in search of an opening.

Even burlap around the root ball has been shown to cause roots to circle inside the fabric. Wire cages surrounding the burlap can last decades, and often lead to further problems as roots enlarge. Once a tree is at the right depth in the hole, remove all burlap as well as the wire cage from ball-and-burlap trees. Roots of container-grown trees need to be teased out straight. It is often necessary to cut them to do this. Over time, circling roots increase in diameter and constrict one another. Some eventually become girdling roots which strangle the trunk, either partially or wholly, below the soil line, and stress symptoms like early fall color and twig dieback appear, followed by decline and death.

Selection is important. Like kids, trees look cute when you bring them home from the nursery, but they tend to grow fast and need more room than you expect. If a site is under wires or has restricted space for branches, pick a species and variety that can grow full-size without causing conflicts. Your local municipality may have rules about planting near roadways, and it’s always wise to have underground utilities marked before digging.

Choose a tree hardy to the region, as not all stores carry trees well-suited to our climate. And not all trees have sunny dispositions. Maples can handle deep shade, but a shaded crabapple will get crabby. Large-caliper trees fare worse than smaller ones. Research says 2” is the largest caliper size that should be planted.

Adding gobs of organic matter to the backfill likely dates back to ancient times, when people would sometimes grab an arborist, if one was handy, and throw them in the planting hole. Possibly in response to this, most arborists today recommend little or no additional organic matter in native soils with reasonably good fertility.

However, in cases where soils are very poor such as in subdivisions and urban sites, up to 30% by volume of organic matter can be mixed with the soil from the planting hole before backfilling. A higher percentage can cause a “teacup effect,” where water fails to drain from the planting hole. This leads to root suffocation. Fertilizer is stressful on new trees, so wait at least a year on that.

Water as you backfill, prodding the soil with a stick or shovel handle to eliminate large air pockets. Unless a site is very windy it’s best not to stake trees. They need movement for strong trunks to develop. Five to eight centimetres of mulch over the planting area (not touching the trunk) will help conserve moisture and suppress weeds.

It’s good to embrace as many trees as possible, but in our changing climate, certain non-regional trees should be more widely embraced:

    • Ginkgo (Ginkgo biloba) Handles severe drought, high soil pH, salt, air pollution. Very low-maintenance.

    • Japanese Tree Lilac (Syringa reticulata) Produce lush blooms, perfect for under utility wires, and have few pests and diseases.

    • Norway Spruce (Picea abies) Very resistant to needlecast diseases that decimate Colorado and white spruces, good on marginally wet sites.

    • Crabapple (Malus spp.) Use only disease-resistant varieties, on full-sun sites. Can live 100+ years.

    • Kentucky Coffeetree (Gymnocladus dioecious) Very drought-resistant; survives pollution, high pH.

    • Honeylocust (Gleditsia triacanthos) Salt, drought, and compaction tolerant. 

    • Hackberry (Celtis occidentalis) Good in severe drought, OK in seasonally wet conditions.

   • Bur Oak (Quercus macrocarpa) lives 800+ years. Super drought-tolerant, OK with seasonal flooding. Tricky to transplant, so use only small stock.

    • River Birch (Betula nigra) Resistant to bronze birch borers. Great for seasonally wet sites.

With the same amount of cost and effort, we can plant a tree so that it lasts for generations, or we can plant it “fast and dirty” and have it fizzle out before the kids are grown. It’s just a matter of attending to a few details; no rocket science required. Tree Canada offers a trove of great resources on this topic at https://treecanada.ca/resources/canadian-urban-forest-compendium/8-species-selection-and-planting/

Paul Hetzler has been an ISA-Certified Arborist since 1996. He gets nervous around planting holes.