Box blight: identification, symptoms & prevention

Shaped and topiarised box (Buxus sempervirens) is the best plant for adding winter structure and few great gardens could manage without it. But even in modest plots, a carefully placed box ball or two weaves its own magic.

Growing your own box plants is a good skill to acquire at the moment because commercial stocks and private gardens have been hit by a disfiguring fungal disease called Box blight (Cylindrocladium buxicola).

Box blight strips the leaves away to leave dead twiggy growth leaving an unsightly bare patch. I first noticed bare box hedges over twenty years ago in several large gardens I visited. The gardeners thought it was old age, but the disease spread along whole hedges and when new box was replanted it also succumbed.

Box blight - a very distressing disease

Box blight has driven some gardeners to tears because they’ve had to remove all their box plants. There are other 'box' substitutes and they include the floppier wall germander (Teucrium chamaedrys), the small-leaved Japanese holly (Ilex crenata) and Phillyrea. But none are quite the same. If you’re affected you cannot replant box, because the spores linger in the soil and re-infect the new plants, so you have to find an alternative.

What is box blight?

Box blight was only identified in Britain in 1994, but it’s thought that the disease was introduced from Central America because it’s endemic in Mexico and the Caribbean. However frost doesn’t touch this fungal disease - despite its exotic, warm heartland. Under favourable mild and damp conditions box blight spreads quickly.

The symptoms of box blight

The disease starts with spots on the leaves and these develop into larger brown areas. The fungus also causes black streaks on the stems and branches. Patches of greyish fungus appear on the lower leaf surface, however most gardeners only notice the disease once the leaves begin to disappear and reveal the twiggy growth.

How does the box blight affect the plant?

The fungal disease clogs the plant's arteries slowing down the sap until the box eventually dies. Water droplets carried on the wind spread the sticky spores and when the box is cut in early June. The shears or clippers also spread it along. Washing the shears with detergent will help, but your clothing can also transfer spores.

It’s worth remembering that fungal diseases tend to thrive in damp, muggy conditions so certain areas of the country seem more prone than others.

Propagate your own box to avoid blight

Firstly you need a healthy box plant to start with. The best source would be a garden-grown box plant in a friend’s garden, or a small healthy plant from a reputable nursery.

The end of summer/beginning of autumn is a good time to acquire your stock plant and plant it in the ground.

Grow your box plant on and then cut away the new growth in late-summer. Look for five-inch long, semi-ripe cuttings from this year’s growth. Trim below the leaf joint and remove some lower leaves before plunging into compost.

You can plunge your cuttings into an outdoor sand bed in a makeshift cold frame, or place them in the ground in a shaded position.

The two-pot technique for propagating box

There is another technique using two pots of different sizes. The larger one has a layer of gritty compost in the bottom and the box cuttings are placed round the edge.

The smaller pot sits on top and keeps the cuttings damp and in place. The beauty of this technique is you can lift the top pot up to see if the cuttings have rooted or not.

Plant the cuttings into pots in the following spring and then put the young plants out into the ground in early autumn - planting them apart to make a hedge.

If you want to make a box ball quickly, use several plants close together.

Feed your box plants

If you’re growing young box plants you can encourage growth by using seaweed extract. Feed every two weeks between June and late August.

Trimming box plants

Early June is the recognised time to trim box and generally once a year is enough, although you might want to smarten up topiary in late August if needed.

Box was traditionally cut on Derby Day, when whole households were at the races. Trim it any earlier and frost can brown the new re-growth.