At a glance
Box is one of those plants that has become genuinely difficult to write about without starting with the same caveats everyone else starts with, and yet the caveats are true and necessary so there is no avoiding them. Box blight has been spreading through UK gardens since 1998. Box tree caterpillar has been doing the same since 2007. Between the two of them, a significant proportion of the box hedging in Britain is either dead, dying, or requires more intervention than most people want to put in. That has to be said at the outset, because a guide that does not mention it until the problems section would be misleading.
With that said: box is still grown in UK gardens, it is still the best plant for certain formal purposes, and understanding how to grow it well means you at least give it the best possible conditions if you decide to plant it. This guide covers how to do that, and there is also a section on alternatives for those who have lost plants to blight or caterpillar and are not sure what to replace them with.
What box is and why it has become complicated to grow well
Buxus sempervirens is a UK-native evergreen shrub. It occurs naturally on chalk downland in the south of England and has been in cultivated gardens since Roman times. The characteristics that made it so widely used are straightforward: small, glossy, dark green leaves; very dense growth; tolerates clipping repeatedly without weakening; grows in sun or shade including deep shade; and slow enough at ten to fifteen centimetres a year that a clipped hedge stays neat for several months after trimming rather than needing constant attention.
In April and May it produces small greenish-yellow flowers that are rich in pollen and attractive to bees. Left completely untrimmed, a box plant would eventually reach five metres, but most never get near that because they are clipped regularly for their entire lives.
The reason it has become complicated is box blight, a disease caused by two related fungi (Calonectria pseudonaviculata and Calonectria henricotiae), and box tree caterpillar (Cydalima perspectalis), an invasive moth whose larvae strip the foliage to bare twigs. Both arrived in the UK in the late 1990s and 2000s respectively and have spread steadily since. Millions of UK box plants have been lost or removed since 2008. There is no amateur fungicide that reliably suppresses box blight long-term. The caterpillar continues to spread north and west. In gardens where both are established, maintaining neat, healthy box is somewhere between very difficult and impossible.
None of this means box is ungrowable. In many gardens, particularly those in drier, more exposed areas where the humid conditions blight favours are less common, box performs well. Knowing what you are taking on before you plant is the reasonable starting position.
Varieties, site, and what to look for when buying
Common box (Buxus sempervirens) is the standard, most widely available, and the one with rich green foliage that most people picture when they think of a box hedge. For low hedging and formal parterres, ‘Suffruticosa’ is a dwarf, slower-growing, denser cultivar that has traditionally been used to edge knot garden beds; it is planted much closer than the standard at around 10 to 15 centimetres apart. ‘Blauer Heinz’ has a slight blue cast to its foliage, grows slowly, and is considered more frost-resistant, which makes it worth looking at if the site is cold. ‘Rocket’ is an upright, faster-growing variety suitable for taller hedging where you want height more quickly. Buxus microphylla, the small-leaved Asian species, is also planted at 10 to 15 centimetres for compact hedging and tolerates a wider range of conditions.
When buying plants for a hedge, bare-root plants available in winter from hedging specialists are considerably cheaper than pot-grown stock and establish just as well if planted promptly. Inspect any box you are buying carefully for blight symptoms: brown patches on leaves with darker leaf margins and white fungal growth at the leaf bases are the signs to look for. Do not buy infected plants regardless of price.
Planting in the ground and in containers
For hedging, preparation makes a significant difference to how quickly the plants establish and close up. Dig the trench to spade’s depth and up to 90 centimetres wide; this gives the roots room to spread outwards from the start. Mix in some organic matter, water the plants before planting and water them in after, and mulch along the line to conserve moisture and keep weed pressure down.
Planting too close together is a common mistake: the temptation to speed up coverage produces poor air circulation between plants, which is one of the conditions that accelerates box blight once it is present. Spacing at 30 to 40 centimetres produces a hedge that closes in three to five years rather than faster, but in better condition once it does. For individual specimens, the planting hole needs to be to spade’s depth and three times the width of the rootball; the wide hole encourages root spread outward rather than straight down. For containers, use at least a 45-centimetre diameter pot with peat-free John Innes No 3 compost, which has the weight and nutrient balance box needs in a confined space.
Pruning, and why August is the right month for mature hedges
The pruning timing for box matters more than for most hedging plants, and it matters primarily because of box blight. Trimming creates wounds in the stems, and fresh wounds made at the wrong time of year during warm, humid conditions allow the blight pathogen to enter. The conventional wisdom used to be to trim box in late spring, and some guides still say this. The current advice from the RHS is different: trim mature hedges in August.
Old or neglected hedges that have become overgrown or misshapen can be hard pruned in late spring, cut back to within 15 to 30 centimetres of the ground. Mulch and feed afterwards to support the recovery, and expect the regrowth to take two or three seasons to produce a presentable hedge again.
Box blight and box tree caterpillar: the current situation and what you can do
Box blight appears as brown patches on leaves with darker borders, rapid leaf drop, and dieback of stems. In wet, humid conditions, white fungal growth appears at leaf bases and on dead stems. The disease can kill a hedge in a single bad summer. It spreads on water splash, contaminated tools, clothing, and infected plant material. There is no amateur fungicide that the RHS currently describes as reliably suppressing it long-term.
For blight: trim in August rather than spring; avoid overhead watering; do not clip in wet weather; remove infected material promptly and bin it rather than composting it; disinfect tools after cutting infected material. For box tree caterpillar: check inside the hedge from April onwards; products containing Bacillus thuringiensis var. kurstaki sprayed directly at the caterpillars work well if caught early; nematodes (Steinernema feltiae) watered into the hedge are also effective. Pheromone traps tell you the moths are present so you know to check for larvae, but they do not control the population.
Do not use pesticide spray near an affected hedge. Most general insecticides kill beneficial insects as well as caterpillars, and none will touch blight. Use Bacillus thuringiensis (Bt) specifically for caterpillar; it is selective and does not harm other insects.
Alternatives when box is no longer viable, and what the honest trade-offs are
There is no perfect box alternative. Every substitute involves a trade-off of some kind. One thing that did not make it into the initial wave of enthusiasm for alternatives: Ilex crenata, the Japanese holly promoted as the obvious like-for-like replacement, has started showing problems in UK conditions too. The RHS advisory department has been receiving samples of poorly-growing Ilex crenata from UK gardens. It is a better option than box in terms of disease resistance, but it is not problem-free, and it performs poorly on heavy clay without excellent drainage.
The broader principle, if you are replacing a box hedge, is that mixing species spreads the risk. A hedge of mixed Ilex crenata and Lonicera nitida, or Taxus with Euonymus for the lower sections, is less likely to be entirely wiped out by a single future problem than a single-species replacement hedge. It is the same logic that made box monoculture a problem in the first place.
