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The single most common mistake with kobus magnolia is planting it as though it were one of the compact garden magnolias. It is not. Left alone in reasonable soil it builds a broad, open-crowned tree of 30 to 50 feet (9 to 15 m), occasionally more, with a spread not far short of its height. A young plant in a nursery pot gives no hint of that, because it spends its first several years as a thin, upright, unbranched whip.
You cannot correct this later by cutting. Magnolias take reduction pruning badly, the cuts seal slowly, and a tree that has been headed back never regains a natural outline. So the planting distance is a permanent decision. Allow at least 20 feet (6 m) from a building wall, 15 feet (4.5 m) from a boundary, and keep it clear of overhead lines and of anything you would mind being shaded in the afternoon.
The reward for giving it room is a tree that carries hundreds of white, fragrant, six-tepalled flowers on completely bare wood before any leaf appears, at a size where you look up into the display rather than down at it. Crowded against a fence, the same tree flowers only on the side that gets light and looks permanently lopsided.
| Distance from | Minimum clearance | Why |
|---|---|---|
| House wall or eaves | 20 ft (6 m) | Crown spread, and roots that resent later trenching |
| Property boundary | 15 ft (4.5 m) | Half the mature spread will cross the line otherwise |
| Drive or paved path | 12 ft (3.6 m) | Shallow roots lift slabs and are damaged by them |
| Another large tree | 25 ft (7.5 m) | It flowers poorly in side shade and leans away |
| Drains or a soakaway | 15 ft (4.5 m) | Digging near them later means cutting roots |
Aspect decides whether you keep the flowers
Kobus magnolia opens its flowers on bare wood while frost is still a real possibility, and a frosted flower is finished. What decides the damage is not simply how cold the night was but how fast the flowers thaw in the morning. Petals that warm in minutes under direct early sun rupture; the same flowers thawing slowly in shade often come through untouched.
So an east-facing open position, which sounds attractive, is the worst one you can pick. A west-facing or north-facing site, or anywhere the tree is shaded until mid-morning by a building or a hedge, buys you flowers in years when the exposed tree loses them. Shelter from cold wind matters for the same reason, and also because the tepals are thin and shred in a gale.
The tree itself is extremely cold hardy and survives winters down to around -25F (-32C) without trouble. It is only the open flowers that are fragile. Do not confuse hardiness of the plant with hardiness of the display; they are different problems with different answers.
Soil, the hole, and how to handle the roots
It wants deep, moisture-retentive soil that still drains, and a pH between about 5.5 and 6.8. It tolerates neutral ground but goes chlorotic and thin on shallow chalk. Heavy clay is workable if the site does not sit wet in winter; standing water over the roots for weeks will kill it.
Dig a hole two to three times the width of the rootball but no deeper than the rootball is tall. Depth is the part people get wrong. Magnolia roots are fleshy, close to the surface, and intolerant of being buried; the top of the rootball should finish level with the surrounding soil or a half inch (1 cm) proud, never below. Break up the sides of the hole so roots are not turning in a glazed pit.
Handle the roots as little as you can. Do not tease them out aggressively, do not wash the soil off, and do not cut circling roots hard unless they are genuinely strangling. These roots are brittle and thick rather than fibrous, and every one you snap is one the tree has to replace before it can grow. Backfill with the soil you took out, firm gently with your hands rather than your heel, and water in thoroughly to settle it.
Do not put fertilizer in the planting hole. A magnolia planted into fed soil pushes soft top growth before the roots can support it. Mulch instead, and leave feeding until the second season at the earliest.
The first two seasons decide the next fifty
Water is the whole job. For the first two full growing seasons, give a young tree a deep soak of 4 to 5 gallons (15 to 20 liters) once a week in dry weather, delivered slowly so it goes down rather than running off. Frequent light watering trains roots to stay in the top inch of soil, which is exactly the wrong habit for a tree that has to survive future droughts on its own.
Mulch a circle at least 3 feet (90 cm) across with 2 to 3 inches (5 to 8 cm) of bark or leaf mold, kept a hand's width clear of the trunk. That mulch is doing three jobs at once: holding moisture, keeping the shallow roots cool, and stopping anyone from planting or digging in the one place they must not.
Stake only if the site is windy, and then low. A short stake reaching about a third of the way up the stem, with a wide flexible tie, lets the top move and thicken. Check the tie every autumn and every spring, and remove the whole thing after two years. More young trees are ruined by forgotten ties cutting into the bark than by wind.
Write down what you planted and where
Note the planting position with a measurement from two fixed points, such as a corner of the house and a fence post. In ten years the tree will be large and you will have forgotten where the rootball sits, but you will need to know before you ever dig a trench, lay a path, or plant anything nearby.
Record the height and stem thickness at planting, the aspect you chose, and the soil pH if you tested it. Then each year add the date of first open flower and the overnight lows around it. A kobus magnolia is a fifty-year relationship measured in single events per season, and memory is not good enough to compare one spring against another.