Box problems

Box in the garden

Among the thousands of plants the RHS Advisory Service receives enquiries about every year, box sits up there in our top 20 – evidence of just how popular a plant it is with gardeners.

However, it is troubled by a number of pests and diseases, not least the dreaded box blight which continues to cause problems for those growing box in the UK.

Diseases of box

Box blight (Cylindrocladium buxicola and Volutella buxi): Watch for leaf spots, stem dieback and bare patches.

RHS research into box blight

• Identification into the cause of box blight
• Studies on the control of C. buxicola
• Evaluation of fungicides for the control of C. buxicola in the field

Both new and well established plants may fail completely for no obvious reason. This may be due to a soil-borne disease called Phytophthora root rot.

Box rust is caused by a fungus called Puccinia buxi. This is not particularly troublesome and seldom causes serious problems.

It can be seen as thickened rusty blister-like pustules on both sides of the leaves. The fungus forms only one type of spore which develops in autumn and winter and breaks through the epidermis of the leaves in spring. New leaves are infected in the spring and early summer. The fungus continues to grow during the summer and autumn and as a result the infected spots become thicker. The fungus does not have an alternate host unlike other classic rusts.

To control it either clip off the affected shoots or spray with one of the fungicides labelled for rust diseases. The fungicides difenoconazole (Westland Plant Rescue Fungus Control), myclobutanil (Bayer Garden Systhane Fungus Fighter and other formulations), tebuconazole (Bayer Garden Multirose Concentrate 2) and triticonazole (Scotts Fungus Clear Ultra, Roseclear Ultra) are approved for the control of rust diseases of ornamental plants. The formulation of tebuconazole (Bayer Garden Multirose Concentrate 2) contains deltamethrin and some formulations of triticonazole (Scotts Roseclear Ultra and Scotts Roseclear Ultra Gun) contain acetamiprid to control insect pests.

Pests of box

Box sucker
This pest causes the leaves to become cup-shaped and there may also be splashes of a waxy white material apparent on the foliage.

Box tree caterpillar
This newly introduced pest of box is not common but should be looked out for.

Box leaf-mining gall midge
The box leaf-mining gall midge, Monarthropalpus flavus is an uncommon pest in the UK but sometimes heavy infestations occur. This tiny fly deposits its eggs in the new leaves during late April - May. The larvae feed inside the foliage, causing a yellowish discoloration on the upper leaf surface. The lower leaf surface develops a slight swelling in the area affected by the larva's feeding. The yellow larvae are up to 3mm long and they feed inside the leaves during summer - winter before pupating within the mines in spring. Several mines can develop in a leaf and heavily damaged ones often drop off.

It is not easy to control the older larvae and pupae as they are protected within the foliage. A systemic insecticide, such as Bayer Provado Ultimate Bug Killer (thiacloprid), Westland Plant Rescue Bug Killer Ornamental Plants (thiamethoxam) or Scotts Bug Clear Ultra (acetamiprid), applied in early May may prevent the young larvae developing. At that time these systemic insecticides are more likely to be absorbed by the softer young foliage.

Mussel scale on Box
Mussel scale, Lepidosaphes ulmi, is a sap-sucking insect that attacks a wide range of woody plants, including box, apple, hawthorn, ceanothus, cornus and cotoneaster. The blackish-brown shells or scales are shaped like mussels, up to 3mm in length, and are attached mainly to the bark but sometimes they spread to the foliage. Heavy infestations can result in plants dying back.

These can be controlled with systemic insecticides, such as Bayer Provado Ultimate Bug Killer (thiacloprid), Westland Plant Rescue Bug Killer Ornamental Plants (thiamethoxam) or Scotts Bug Clear Ultra (acetamiprid), or a contact pesticide, such as Bayer Sprayday Greenfly Killer (deltamethin). The best time for control measures is generally early June, when the more vulnerable newly hatched scale nymphs are likely to be present. Dead scales can remain attached to the stems for a long time so there will not be any immediate change in the appearance of the plants. The new growth should stay free of infestation if the pest has been controlled.

Box red spider mite
A fine whitish mottling on the foliage of box plants is caused by the box red spider mite, Eurytetranychus buxi, which is a specific pest of box. This tiny creature feeds by sucking sap from the undersides of the leaves, particularly during spring and early summer. By late summer the infestation has died out and the pest overwinters as eggs, which are laid on the stems and underside of leaves.

Spider mites are difficult to control. Their numbers can be reduced by spraying the undersides of the foliage thoroughly with plant oil sprays, such as Vitax Organic 2 in 1 Pest & Disease Control or Scotts Bug Clear for Fruit & Veg, or fatty acids, such as Bayer Organic Bug Free or Doff Greenfly and Blackfly Killer. Applications on three or four occasions at about five-day intervals when signs of renewed feeding damage are seen on the new foliage may be necessary to control newly hatched mites and break the pest’s life cycle. Fortunately, although the mottling may be considered unsightly, this pest does not cause serious damage to the plants and so it is something that can be tolerated.
 

Environmental stresses

Yellow leaf tips/margins and bronzing/orange foliage:  when the leaves take on distinctive yellow tips or an orange coloration, it suggests the plants are under environmental stress.

Brown leaves on woody plants: many shrubs can suffer brown leaves. Trying to diagnose the problem as soon as possible may help save the plant.
 

Box plantings to see at RHS Gardens

Harlow Carr

• Main Border: 36 clipped box (Buxus sempervirens) cones can be found in the Main Border, some forming an attractive end to the curved wooden seating
• Glasshouse (next to the Bath House): 1m (3¼ft) high box hedge that runs alongside the glasshouse
• Gardens Through Time, Garden 4: small box plantings edging a bed. 

Hyde Hall

• Farmhouse Garden: four beds edged with Buxus sempervirens and balls either side of the Farmhouse door
• Eastern Courtyard (Visitors’ Centre): eight 1.2m (4ft) unclipped specimens of B. sempervirens
• Southern Courtyard (Visitors’ Centre): groups of three box balls of B. sempervirens in bed corners
• Herb Garden (next to the Dry Garden): four cone-shaped individual specimens of B. sempervirens at each corner
• Upper Pond area: many box balls marking the ends of beds
• Tea Yard: B. sempervirens 'Suffruticosa' hedging along walled beds. 

Rosemoor

• Alpine Terrace: see the variegated form of box, Buxus sempervirens 'Variegata', planted here
• Modern Rose Garden: a hedge of a more unusual box, B. microphylla 'Faulkner'
• Model Garden 1: examples of B. sempervirens 'Suffruticosa' hedging
• Mediterranean Garden: see a specimen of B. balearica.

Wisley

• Bowles' Corner: a large 3m (10ft) unclipped specimen of Buxus sempervirens ‘Bowles’s Blue' is sited behind the bench
• Country Garden: many beds edged with B. sempervirens glaucous
• Family Garden and Enthusiast's Garden (Model Gardens): various examples of small edging in box, including geometric shapes planted in gravel in the Enthusiast's Garden
• Herb Garden: glaucous-leaved B. sempervirens 'Wisley Blue' edges the ‘Hazardous Plants’ bed
• Witan Street (near Orchards Café): in demonstration garden ‘Intersection’ designed by Catherine Heatherington, see blocks of B. sempervirens clipped in geometric designs. The handsome blue-leaved hedge running parallel with Witan Street is another example of B. sempervirens 'Wisley Blue'.