Make sure you do it correctly from the beginning to avoid problems later
By Dr. Garry V. McDonald
I live in a town that professes to be a tree city. Citizens and businesses are encouraged to plant trees and tree ordinances abound. While I am all for tree planting and a self-confessed tree-hugger, it frustrates me to see hundreds of trees planted that will be dead within two years. Especially maddening are trees planted along streets that are either the wrong species for the site, incorrectly planted, or receive no care after planting. The aesthetic and ecological effect has been ruined by gaps left by dead trees, or trees that you wish were dead. Trees are a long-term investment and a gift to future generations and tree researchers have added a great deal to our knowledge of best planting and care practices.
When it comes to tree planting, there are a couple of terms that need to be defined: transplanting and establishment. Transplanting is the stage of physically moving or planting trees to a new site, either bare-root or container-raised, and of any size: from a seedling to a full-sized tree. The goal of transplanting is to provide a soil environment or condition that encourages root regeneration as quickly as possible while reducing transplant shock. Establishment, on the other hand, is long term, the length of time a tree takes as it adapts to a particular site or environment and metaphorically settles into its new home. Since this article is more about transplanting than long-term establishment (a separate article in itself), the following subjects will be covered: tree size, planting depth, soil amendments, mulching, and staking.
One commonly asked question is “What size tree should I plant?” A fair question, which often depends on the tree species. Some trees, such as elms (Ulmus spp.) or maples (Acer spp.), transplant easily and have a rapid growth rate, while many oaks (Quercus spp.) and nut-bearing trees can be agonizingly slow. The temptation is to buy as large a tree as financially feasible in an effort to have a large tree as quickly as possible or to create an instant landscape. However, recent research suggests that a smaller tree that is properly planted and cared for will catch up and even grow into a much larger tree in five years. How small is small and how large is large? Personally, I like to plant trees in the 7-10 gallon range, as they are more affordable, transplant easily, and quickly regenerate roots. Trees can be purchased up to several inches in trunk caliper but will take years to fully establish. A general rule of thumb is about 2 years establishment time per inch trunk diameter – meaning a 4-inch caliper tree will take at least eight years to get on its feet and see appreciable trunk growth. In eight years, I can have a 1-inch-diameter tree grow that large, if not larger. Full-sized trees, or nearly so, can be transplanted using a mechanical tree spade, but this is an expensive endeavor and even using a professional can be dicey. And as mentioned, transplant success is very much dependent on the species. Transplanting fully grown trees is usually reserved for legacy trees and best left to the experts. Purchasing bare-root trees, which is common for fruiting species, is not usually a problem since the trunk diameters tend to be smaller and in proportion to the root-system when grown in the nursery.
There is an old plant nursery adage that says a 50-cent plant needs a 1-dollar hole. Factoring for inflation, the adage still holds. What is increasingly becoming common knowledge is how critically planting depth affects transplant success.
Our lab group conducted several studies on this very topic. While some tree species such as sycamore (Platanus spp.), a bottomland tree used to flooding, were tolerant of various planting depths, others, such as ash (Fraxnius spp.) and crapemyrtle (Lagerstroemia spp.) were extremely sensitive to planting too deep. Planting 3 inches below grade, the level the plant was growing in the nursery container, significantly (in the statistical sense) adversely affected transplant success and subsequent growth. It was better to plant 3 inches too high than 3 inches too low. But how tempting it is to plant a tree a bit deeper when it is a windy day because we don’t want the tree to “rock” or sway. The same thing can happen if the soil is very loose or sandy and the plant settles after being watered and so extra soil gets piled on top of the newly planted tree. I observed several trees die one summer during a planting project on campus. I assumed they weren’t being watered. When I finally checked them out, they had received plenty of water, but because they were planted on a windy day, the landscape crew had planted them a good 6 inches too deep so they would stand upright. They were all dead shortly after planting. Research from the same group showed that planting too deep starting at the seedling stage and continuing through subsequent up-canning to larger containers affects the long-term establishment of the tree.
My recommendation is to dig a hole the same depth as the root ball but twice as wide. Unless the soil is so rocky that there is no soil to backfill, the current thinking is to backfill with the native soil and not add extensive amendments, such as peat moss or potting media. The rationale is that tree roots are fat and happy in the amended soil and therefore have no need to expand out to the surrounding environment and so the roots grow in a circle. The result: The trees topple over in heavy rain or wind. While exceptions abound, I have found this to be generally true. Cut or prune any circling roots and gently loosen the root ball before backfilling. Avoid trees that are extremely root-bound, as the tree may recover. Avoid cramming roots in a hole that’s too small. Once backfilled, water thoroughly to remove any air pockets. Water regularly the first two growing seasons for establishment. After that, the tree should be on its own unless there is prolonged drought.
Around 20 years ago, I began noticing a new phenomenon in landscapes: the rise of the mulch volcano. I think it was a reaction to string-trimmer “blight,” which became pandemic when gas-powered trimmers became readily available. The solution was to mulch around the base of a tree to prevent grass from growing, thus lessening the chance of string-trimmer injury. Over time, the mulch got higher and higher, resulting in a mulch volcano resembling Mt. Fuji. To cut to the chase, additional research done by myself and others demonstrated that 2-4 inches of mulch is adequate. More than 4 inches either kept a clay soil too wet or did not allow water to penetrate into sandy soil – where it created a hydrophobic, or water-repelling, layer. Additionally, if you live in fire-ant country, ants simply love the warm, moist environment of a mulch volcano. The same environment also encourages crown rots. For years, the recommendation has been 2-4 inches of any organic mulch. If it ain’t broke don’t fix it.
A final question that inevitably pops up is “Do I stake my trees?” I avoid staking trees unless absolutely necessary. More often than not it is improperly done or, worse yet, not removed in a timely manner. The guy wires or straps end up girdling the trunk and damaging the tree and are a pain to mow around. I only recommend staking trees in very windy locations. If a tree is so tall that it needs staking, the root to shoot ratio is probably off and will slow the establishment phase