Planting Depth & the Root Flare

If we could ask every customer to remember one thing from this library, it would be this: the depth at which a tree is planted decides more about its long-term health than almost any other choice made on planting day. Fortunately, the tree tells you exactly where it wants to sit. You simply have to know where to look.

Finding the root flare

At the base of every well-grown trunk there is a gentle widening — the point where trunk tissue transitions into root tissue. Growers call it the root flare, or root collar. Above it, the tree is designed to live in air; below it, in soil. The flare is the boundary line, and the whole art of planting depth is keeping that boundary at ground level.

On container-grown stock the flare is occasionally hidden beneath a centimetre or two of potting media that has accumulated over successive potting. Brush the surface back gently until you can see the first structural roots leaving the trunk. Measure your hole depth from there, not from the top of the media.

The three outcomes

The diagram above shows the three ways a planting can finish, and only one of them is right.

Too high

The rootball stands proud of the surrounding grade with its shoulders exposed. Container media and the fine roots within it dry out quickly in sun and wind — in a Queensland summer, sometimes within a day. A tree planted high spends its establishment period under constant drought stress at the very moment it most needs steady moisture. If your soil demands a raised position for drainage, raise it only slightly and cover the shoulders of the rootball with soil and mulch, tapering away to grade.

Correct

The root flare sits exactly at ground level. Roots are fully in soil, trunk is fully in air, and water drains away from the collar rather than pooling against it. In heavy or slow-draining soils it is acceptable — often wise — to finish two to five centimetres proud, because a newly backfilled tree may settle slightly. A tree can tolerate sitting marginally high far better than it tolerates sitting low.

Too deep

The flare is buried and soil rests against trunk bark. Trunk bark is not root bark: kept permanently moist, it softens, admits fungal decay, and the tree develops collar rot — a slow decline that may take years to show and cannot be reversed once advanced. Buried flares also encourage roots to grow upwards and around the trunk, strangling it as both thicken. A tree planted ten centimetres too deep is not slightly compromised; it is quietly failing from the day it goes in.

Getting it right in practice

1. Measure the height of the rootball from its base to the root flare.

2. Dig the hole to exactly that depth — no deeper — onto firm, undisturbed soil. A tree placed on fluffed-up backfill will settle, and settling only ever moves the flare downwards.

3. Lay a straight edge — a stake or shovel handle — across the hole and check the flare sits level with it before backfilling.

4. After watering in, check again. If the tree has settled low, lift and re-set it now, while it is easy.

5. When you mulch, keep the mulch 5–10 cm clear of the trunk. Mulch piled against the collar buries the flare just as surely as soil does.

This discipline applies at every scale we grow, from a 45-litre container to a craned ex-ground specimen. On large plantings the consequences are simply larger: re-lifting a 500-litre tree that has settled is a crane job, so the straight-edge check before backfilling is worth thirty seconds of anyone's time.

From the growers — On sloping ground, set the flare at grade on the uphill side and build the downhill side up to meet it. Water arriving from above is what the collar most needs protecting from.

If you are unsure where the flare sits on a tree you have received from us, send a photograph and we will mark it for you — or ask us when you visit the estate by appointment.