Japanese Maple: Choosing One, Siting It, and Leaf Scorch

Almost every problem people have with a Japanese maple is the same problem: crisp brown leaf edges, appearing in midsummer, on a tree that looked perfect in spring.

That is leaf scorch, and it is a siting and watering issue rather than a disease. Getting the position right at planting is most of the work of growing one.

Choosing a form first, a colour second

The single most useful distinction is habit, because it decides how much space the tree needs and how tough it is.

Bloodgood is the reference upright: deep purple-red through summer, 15 to 20 feet, and one of the more sun-tolerant cultivars. Crimson Queen and Tamukeyama are the reference dissectums — cascading mounds of lace-like foliage, superb in a courtyard or over a rock, and noticeably fussier about wind and afternoon sun.

Sango-kaku (coral bark) is worth naming separately: green leaves, gold in autumn, and coral-red young stems that carry the winter. Osakazuki is grown for what is arguably the best red autumn colour of any of them, from plain green summer leaves.

Siting: this is the whole game

• Morning sun, afternoon shade. Enough light for good colour, without the heat that scorches.
• Shelter from wind. Wind is at least as damaging as sun — those thin leaves lose water fast, and a windy site scorches even in shade.
• Free-draining but moisture-retentive soil. They dislike drought and they dislike waterlogging equally.
• Neutral to slightly acidic, roughly pH 5.5 to 6.8. Very alkaline soil causes yellowing between the veins — see lowering soil pH.
• Not against a hot wall or over a paved area, both of which radiate heat back at the canopy.

Plant in autumn or early spring, at the same depth it sat in the pot — never deeper — and mulch with compost or bark, keeping the mulch clear of the trunk.

Water and mulch

Japanese maples are shallow-rooted. That is why they respond so quickly to dry surface soil, and why mulch does so much for them.

Water deeply and regularly through the first two or three years, and in any dry spell after that. A 2 to 3 inch mulch of compost or bark keeps the root zone cool and damp and is worth more than any feeding.

Feeding is barely needed. A light balanced feed in spring is plenty; heavy nitrogen produces soft, sappy growth that scorches and breaks.

Leaf scorch, diagnosed

Brown, crisp margins with the leaf centre still green. It appears from midsummer, worst on the windward and sunniest side.

Scorch is disfiguring rather than fatal. The tree usually comes back fine the following spring — but it will do it again every year until the cause is removed.

Pruning: less than you think

The reason to own a Japanese maple is its natural shape. Hard pruning destroys it and provokes congested, upright regrowth that never looks right.

Limit yourself to:

• Dead, damaged, diseased wood, at any time.
• Crossing and rubbing branches, in late autumn or winter when dormant.
• Light thinning to open the canopy and show the branch structure.

Never take more than about a fifth of the canopy in one year. Avoid pruning in late winter and early spring, when the sap is rising and cuts bleed heavily.

In a container

Japanese maples are among the best trees for a large pot, and dwarf and dissected forms are made for it.

Use a large, heavy container with good drainage and a loam-based or ericaceous mix — see potting soil. Then the two rules that matter:

• Never let it dry out. Drought kills more potted maples than cold does.
• Shelter it. A pot in wind and afternoon sun is the worst-case combination for scorch.

Repot or root-prune every two to three years in late winter, and use rainwater if your tap water is hard.