Common Mistakes When Planting Larch on Wet UK Sites

Larch has a reputation for toughness. It grows quickly, shrugs off exposure and will establish on sites where other conifers sulk. That reputation encourages people to plant it on wet ground — and wet ground is where larch plantings go wrong.

The failures tend to look alike. Yellowing leaders in the second summer, dead tops by the fourth, a ragged stand that never closes, and canker on the survivors. Almost every one of those decisions was made before the first tree went in the ground.

Check the soil before you order the trees

Take a spade. Not an auger by the gate, and not last decade's drainage map — a real pit, at least 60cm deep, in three or four places across the block you intend to plant. Dig them in winter or early spring, when the water table is at its highest.

What you are looking for in the pit:

• Mottling. Grey-blue subsoil flecked with rusty orange means the ground sits waterlogged for part of the year. Larch roots will spend that period starved of oxygen.
• A pan. A hard, dense layer that a spade or root cannot push through. Water backs up above it and the tree never gets deep roots.
• Rooting depth. If there is only 20cm of workable soil over clay or rock, you are planting a shallow-rooted tree on a wet site. Expect windthrow as well as poor growth.
• Standing water. If the pit fills and stays full, larch is the wrong tree for that spot. Not "needs a bit of drainage" — wrong tree.

Opening new drains is rarely the answer, and it is not always allowed. Anything touching a watercourse, a peatland or a protected site comes with its own consents, so check before a machine arrives. Far more often the better move is to leave the wet ground as wet ground and plant the drier parts properly.

Fit the tree to the microsite, not the map

Wet UK sites are seldom uniformly wet. There is usually a dry shoulder, a flush through the middle and a boggy bottom corner. Walk that variation before you mark out rows, because a larch on a mound will outgrow one sitting in the hollow beside it by a wide margin. The difference is simply oxygen at the roots.

On imperfectly drained ground, plant on a mound or an inverted turf, and make sure the root collar sits at the top of it, not buried in the loose soil at the base. Do not let the planting bag dictate the layout: if a run of rows crosses a flush, shift them or leave a gap.

Remember that wet hollows are also frost hollows. Larch flushes early, and one late frost on a damp flush can strip a whole planting. Ground that is both wet and frosty is not larch ground.

Choose the right larch — or decide against larch

European, Japanese and hybrid

European larch (Larix decidua) suits drier, more inland sites with reasonable depth of soil. It is the least susceptible of the three to Phytophthora ramorum, but it cankers badly on mild, humid, frosty ground — which describes plenty of western Britain.

Japanese larch (Larix kaempferi) is faster, more wind-firm and more tolerant of exposure and heavier soils. That is why it is the larch people reach for on wet upland sites. It is also the most susceptible to P. ramorum. On mild, wet western ground, that combination is a genuine problem rather than a theoretical one.

Hybrid larch (Larix × marschlinsii, Dunkeld larch) combines vigour with good form, but it is still a sporulating host for the same pathogen. It is not a way round the disease question.

Provenance matters too. Buy British-grown stock from a known seed source rather than whatever is cheapest that season. Late-flushing provenances are more frost-prone, and form varies more than most people expect.

And be willing to say no. On genuinely wet ground, alder, downy birch and willow will do a job that larch cannot. Leaving the wettest third of a site unplanted is a decision you will not regret.

Spacing too tight, blocks too big

Larch is light-demanding. Cramped spacing on a wet site keeps the canopy humid and the air still — exactly the conditions that favour canker and the dispersal of P. ramorum spores.

Typical restocking densities sit somewhere between 2 and 2.5 metres apart. On wet or high-risk ground, work at the wider end, or wider still, and accept a little more branching on the lower stem. Air movement is worth more than an extra few hundred stems.

The shape of the block matters as much as the spacing between trees. Large pure blocks of larch are the worst arrangement on a damp site: no airflow through the middle, and if disease arrives, the whole block counts as one unit. Break the planting with rides, open ground and other species. A larch component in a mixed stand is a far safer bet than a larch monoculture.

Disease you can design out, and disease you cannot

Larch canker shows as sunken lesions, dead twigs and resin on the stem. It hits trees already stressed by poor drainage, frost or the wrong soil, so much of the risk disappears once the site is matched properly.

Heterobasidion root and butt rot is a real consideration where larch follows conifers on old woodland sites. Stump treatment at thinning helps, and so does not planting a susceptible species straight back onto the stumps.

Phytophthora ramorum is the one you cannot design out entirely. Look for crown thinning, dark bleeding lesions on the stem and rapid decline, sometimes within a single season. If you suspect it, report it through the appropriate plant health service and do not move timber or brash off the site. Keep larch well away from rhododendron and other known hosts, which can carry the pathogen without showing much themselves. In the wettest, mildest western areas, plenty of managers now avoid planting larch at all — that is a sensible position, not a defeat.

A pre-planting checklist

1. Dig at least three soil pits per block, to 60cm, in winter.
2. Return after heavy rain and check whether they fill.
3. Map the wet and dry patches, and the frost pockets.
4. Match species to microsite, not to the whole field.
5. Plant mounds or turves wherever drainage is imperfect.
6. Space at 2.5 metres or wider on wet, high-risk ground.
7. Avoid pure blocks; mix species and leave open ground.
8. Buy known UK provenance from a reputable nursery.
9. Check consents and tree health rules before ordering.

Getting it right from the start

Larch failures on wet ground are decided in the weeks before planting, not the years after. If you are planting more than a small shelterbelt, walk the site with a forester who knows the local soils, the local frost pattern and the current disease position in your area. It

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