Why Is My Rose Stem Covered in Brown Crusty Scales?

Rough, encrusted bark on rose stems — with what looks like overlapping brown or grey scales, barnacle-like bumps, or a whitish powdery coating — is caused by scale insects. The most common species on roses in temperate gardens is rose scale (Aulacaspis rosae), a type of armoured scale insect whose waxy protective shell makes it look more like a bark condition than a living pest. Beneath each scale is a tiny sap-feeding insect that is extracting nutrients from the phloem continuously.

Identifying scale insects on roses

Rose scale appears as a greyish-white crust on older stems, particularly on established and sometimes neglected plants. The individual scales are about 2–3 mm across and circular to slightly elongated. When you scrape one off with a fingernail, you will find a tiny dead or dormant insect body beneath. Heavily infested stems look rough and unhealthy, with a distinctive encrusted texture quite different from normal bark. Young shoots tend to be less affected. Soft scale species produce honeydew and are often accompanied by sooty mould on the stems and leaves below.

The damage scale insects cause

Light infestations of scale insects cause little visible harm. Heavy, long-standing infestations weaken stems by continuously removing sap, leading to reduced vigour, poor flowering, die-back of individual canes and, in severe cases, death of whole stems. The plant looks generally tired and underperforms compared to its potential. On soft scale species, the associated sooty mould is unsightly and reduces photosynthesis. Heavily encrusted old canes rarely recover fully and are usually best removed.

Physical removal

For moderate infestations, scrubbing the stems with a soft brush dipped in a solution of water and washing-up liquid removes many scales and the crawlers sheltering beneath them. This is most effective in late winter or early spring before the new generation hatches. Heavily encrusted old canes should be pruned out entirely and disposed of (not composted). Where scale is confined to a few stems, pruning alone may resolve the problem without any chemical treatment.

Oil-based winter washes

A plant oil winter wash applied to dormant rose stems in late winter, when leaves are absent, smothers overwintering scales, eggs and crawlers. Work the spray thoroughly into all crevices and junctions, where scales congregate. This is one of the most effective control measures for established infestations and does not harm natural predators that have yet to emerge. Repeat application the following winter if populations are still present.

Treating the crawler stage

Shortly after egg hatch in late spring and early summer, the tiny mobile crawlers can be killed by sprays of insecticidal soap, pyrethrin or plant oils — treatments that have no effect on mature scales with their protective shells. Watch for the crawlers' emergence, which coincides with settled warm weather in late May or June. Apply spray at this point and repeat a fortnight later. This is the single most effective window for chemical intervention against scale insects.