Dig It - Tree Planting for Long Life
by Sage & Snow Garden Club
June 6, 2010
Planting trees too deep has become an epidemic leading to the decline and death of 57% of landscape trees due to the development of trunk-girdling roots. Trunk girdling roots develop when a tree is planted too deep in the root ball and/or the root ball is planted too deep in the planting hole. Trunk girdling roots may be below ground and cause tree decline and death 12 - 20 years after planting. Here are ways to minimize or avoid this problem when planting a tree.
Step 1: Determine depth of planting hole - The depth of the planting hole should be 1-2 inches less than the height of the root ball. For small (one-inch diameter) trees, the top of the root ball should be about one inch above surrounding ground level. For larger (2-4 inch diameter) trees, the top of the root ball should be about two inches above surrounding ground level. Generally, at least two structural roots should be within the top 1-3 inches of the root ball, measured 3-4 inches from the trunk. Do not assume that it was planted correctly at the nursery. The presence of the root flare is an indication of good planting depth. A good way to evaluate planting depth in the root ball is with a slender implement like a slender screwdriver, knitting needle, or barbeque skewer. Probe around the root ball 3-4 inches out from the trunk to locate structural roots and determine depth. If the tree is planted too deep in the root ball, excess soil should be removed from the top during the backfill step of the planting process. Adjust the depth of the planting hole to compensate.
Step 2. Dig planting hole - A saucer-shaped planting hole three times the root ball diameter with sloping sides allows the root system to grow rapidly to 400% of the root ball volume before being slowed by the lower oxygen levels in the surrounding soil. When a root ball sits on un-dug soil it stabilizes the tree and prevents sinking and tilting. The wide saucer-shaped planting hole gives the tree more tolerance to over-watering problems and allows for root ball wrappings to be removed after the tree is situated in the planting hole.
Step 3. Set tree in place - If the tree has a slight curve in the trunk just above the graft the inside of the curve must go to the north to avoid winter bark injury. Container grown nursery stock have limited root spread and the advantage of being planted spring, summer or fall. Plant a container tree by laying it on its side in the planting hole and cutting off the container. Shave off the outer 1-1½ inches of the root ball. Tilt the tree into place. Ensure proper depth of root ball in planting hole. Align vertically. For stability, firm a shallow ring of soil around the bottom of the root ball. Cut away and remove fabric or other material. Field grown, balled and burlaped (B&B) type trees are dug from the growing field with the root ball soil intact. In the harvest process, only 5- 20% of the feeder roots are retained in the root ball. B&B nursery stock is best transplanted in the cooler spring or fall season. In addition to above the steps described above for planting of a container tree, additional instructions for planting B&B stock including removing all the wrapping (burlap, fabric, twine, wire basket, etc.) on the upper 12 inches or upper 2/3 of the root ball (which ever is greater) and if roots are circling the root ball, shave off the outer 1-1½" of the root ball.
Step 4. Backfill
Backfill with soil removed when digging the hole to cover the top curve of the root ball, gradually tapering down, being careful not to over-pack the soil (especially in clay soils) and reduce soil oxygen levels. A good method is to simply return soil and allow it to settle when irrigated. Dirt clods up to the size of a small fist are acceptable.
Step 5. Water & adjust
Water the tree thoroughly. The use of a wide planting hole may result in backfill soil settling after watering. Final grading may be needed after watering.
Step 6. Mulch
Do not place mulch directly over the root ball on newly planted trees. As a rule of thumb, 3-4 inches of wood/bark chips gives better weed control and prevents soil compaction from foot traffic when place over the backfill area and beyond. Do not place wood/bark chips up against the trunk or make mulch volcanoes. Too much mulch will reduce soil oxygen and hold excessive moisture on wet soils.
Step 7: Stabilization (optional)
Unless planting in windy areas, when properly planted with the tree set on un-dug soil, most trees do not require staking or underground stabilization. Install staking before watering to avoid packing down of wet soil.
Summarized Tree Planting Checklist
Top of root ball rises 1-2" above surrounding ground level
Tree sits on undisturbed soil
Saucer-shaped planting hole 3 times root ball diameter
No backfill soil covers top of root ball
Backfill soil covers the top curve of the root ball, tapering down to surrounding ground
At least two structural roots should be within the top 1-3" of the soil surface, measured 3-4" from the trunk
The Sage and Snow Garden Club typically meets on the 3rd Tuesday of each month at the Sublette County Cooperative Extension Service Office, 621 South Pine St., Pinedale. Contact us at PO Box 2280, Pinedale, WY, 82941, by email at email@example.com or call 307-859-8606 to confirm meeting time and location. To find out more about the Garden Club, go to www.pindealeonline.com and click on the link under "clubs".