Southern Magnolia: Size It Before You Plant It

Magnolia grandiflora is one of the great flowering trees — glossy evergreen leaves with rust-brown undersides, and creamy flowers up to a foot across that smell of lemon and vanilla.

It is also 60 to 80 feet tall at maturity, and it drops thick leathery leaves twelve months a year. Both facts are routinely discovered after planting.

Size, and the cultivars that solve it

‘Little Gem’ is the one most gardens want. It stays around 20 feet, flowers from a young age, and flowers over a much longer season than the species — sporadically from late spring into autumn rather than one concentrated flush.

A nursery plant of the straight species and a ‘Little Gem’ look nearly identical at six feet tall. The label is the only thing distinguishing a 20-foot tree from an 80-foot one.

Conditions

• Sun: full sun to light shade. More sun means more flowers and denser growth.
• Soil: moist, rich, free-draining, slightly acidic at roughly pH 5.5 to 6.5. In alkaline soil the leaves yellow between the veins — see lowering soil pH.
• Shelter: cold, dry winter wind scorches the foliage. A sheltered spot or a wall makes a real difference at the cold end of its range.
• Space: the roots are shallow and spread wide, well beyond the canopy. Do not plant close to paving, drains or foundations.

Plant in spring where winters are cold, autumn where they are mild, at the same depth it sat in the pot. Mulch generously and keep the mulch off the trunk.

Water and feed

Water regularly for the first two or three years — this is the establishment period and it matters more than anything you do later. Deep soaks rather than frequent sprinkling.

Once established it is moderately drought-tolerant, though it looks better with consistent moisture.

Feed lightly in spring with a balanced or acid-loving fertiliser. Excess nitrogen gives vigorous leafy growth at the expense of flowers, which is one of the standard reasons a healthy-looking magnolia does not bloom.

Flowering, and why it might not

Age is the usual answer. A seedling M. grandiflora can take ten to fifteen years to flower. A grafted named cultivar flowers in two or three. If flowering matters to you, buy grafted — and that is most of what you are paying for.

The other causes, in order of frequency:

• Too much shade.
• Too much nitrogen — often from lawn feed applied nearby.
• Pruning at the wrong time. Buds form during the previous growing season, so a winter tidy-up removes the flowers.
• Recent transplanting. Magnolias sulk for a year or two after moving.

Pruning

As little as possible, immediately after flowering.

Magnolias compartmentalise wounds slowly and large cuts heal badly, so restrict yourself to:

• Dead, damaged or crossing branches.
• Light shaping while the tree is young.
• Removing lower limbs if you want a clear trunk — decide this early.

Do not attempt to reduce the height of a mature magnolia. It responds poorly, and the shape rarely recovers.

Problems worth knowing

Yellowing between green veins — iron chlorosis from alkaline soil. Treat with chelated iron short-term, sulfur long-term.

Scale insects — brown or white bumps on stems with sticky honeydew and sooty mould below. Horticultural oil in the dormant season.

Leaf spot — cosmetic in most cases; improve airflow and clear fallen leaves if it is severe.

Nothing growing underneath — not a disease. The combination of dense evergreen shade, shallow greedy roots and a thick leathery leaf litter makes it genuinely difficult to plant beneath a mature magnolia. Plan for mulch rather than underplanting.