Installing mature trees into new landscapes has become a major trend across the country. Every year, we sell hundreds of mature trees that are planted on the island. Over the years, it has become clear that improperly planted trees don’t stand a chance on the Grey Lady. Gardeners that don’t understand the science of tree planting are at a distinct disadvantage.
Goal: Promote rapid root growth to reduce water stress. Trees dug from the field are left with less than 20% of the fine water absorbing root hairs they need to support the canopy.
Large trees are grown in the field at production tree farms on the mainland. When trees reach a salable size they are dug out of the ground with a mechanical tree spade. The wide root system that the tree once had to support the canopy is mostly left behind. Once the tree and root ball are lifted, the root ball is wrapped in burlap, twine, and often a wire cage. These coverings keep the root ball intact, so that it can be transported and planted later on. Trees are packed on flat bed trailers or box trucks with their bare branches tied tightly together. Many of the trees we sell at the nursery travel thousands of miles on the highway before taking the ferry to Nantucket!
Keeping in mind the production method, it’s clear that the major goal in planting large trees is rapid regeneration of water-absorbing fine roots. Scientific studies have proven that many common planting practices are harmful, and possibly fatal to trees. Here is the short list of major DON’T s:
• Don’t dig a narrow hole with steep sides.
• Don’t plant trees too deep.
• Don’t put ANYTHING on top of a newly planted tree’s root ball – NO MULCH, NO SOIL.
• Don’t plant a tree unless follow-up watering and care is available for the first TWO years after planting.
• Don’t waste money on excessive amounts of backfill amendments.
• Don’t aggressively tamp backfill soil around the root ball.
If at all possible, plant trees in spring or early fall. Trees planted while just waking from dormancy have far less water needs than a tree in full leaf.
Follow these steps to plant trees properly according to the most recent science:
1. Determine the depth of the planting hole:
• First, measure the height of the root ball. Keep in mind: most trees are actually low in the root ball, with too much soil on top of the roots. Use a screwdriver or other thin implement to gently poke into the top of the root ball to find the depth of the first two to three structural roots. These roots should lie no more than two inches below the top of the ball. If they are deeper, subtract that from the original depth, making the hole shallower.
• The top of the root ball will sit one or two inches above grade, subtract one or two more inches to account for that, as well.
2. Dig the hole:
• The ideal planting hole will be saucer shaped (sloping sides), three times the width of the root ball, and at the proper depth, with undisturbed soil on the bottom.
• The shape of the hole is the MOST important factor in aiding rapid root growth. A hole with straight sides encourages roots to circle in the hole, and eventually girdle the trunk, causing death of the tree. The width of the hole is the second factor. The wider the hole, the better, as roots can quickly grow in lighter soil, with adequate pore space.
3. Put the tree in the hole:
• Large trees have extremely heavy root balls. A ball cart can be used for smaller root balls, or a skids steer for larger ones. Once the tree is next to the hole it can either be rolled in (for smaller root balls) or eased in with the forks of the machine (for larger trees). Enormous trees are planted with the help of a crane. However the tree is delivered to the hole, be sure it is placed straight, with the appropriate face in the appropriate direction. If the hole appears too deep once the tree is in place, remove it and add soil to the bottom of the hole, tamping it firmly so that no further settling will occur.
4. Remove any covering on the top 2/3 of the root ball:
• It has been proven time and again that burlap, twine and metal cages DO hamper root growth, especially when trees are planted improperly. Why risk it? Cut the top of the metal cage off with bolt cutters and the burlap with a utility knife. Leave the lower portion of the cage and burlap in place, the vast majority of tree roots grow out from the root ball, not down.
5. Stabilize the root ball:
• Shovel a small ring of backfill soil around the root ball.
• Tamp the soil in place firmly.
6. Backfill the tree:
• Considerable debate surrounds whether or not to amend backfill soil for trees. The prevailing wisdom is that excessive amendments do more harm than good. However, in lean soils (like on the majority of Nantucket) adding 5% or so of organic matter will help with soil texture, and water and nutrient retention.
• Shovel the back fill into the hole, without any further compacting.
• Do not place any soil on top of the root ball. Allow it to sit above grade with the top of the root ball showing.
7. Stake the tree, if it is deemed necessary:
• Staking is a whole other can of worms. It is preferable to leave trees un-staked. If staking is necessary, be sure the lines used to keep the tree stable are loose enough to allow for some flexion of the trunk.
8. Water the tree to settle the soil, and hydrate the roots.
9. Mulch the new root zone:
• Another Hot topic! Recent studies have shown that mulch is best used to cover the backfill area and anticipated root zone, not the root ball, and certainly not ever touching the trunk.
This article summarizes practices outlined in the Colorado State University CMG GardenNotes #633. Check it out for more in depth information on best tree planting practices.