The "moss" on rose stems is typically not moss at all, but a gall known as robin's pincushion or bedeguar gall, caused by a tiny gall wasp.
Understanding Robin's Pincushion Galls
Robin's pincushion galls are abnormal growths that develop on rose stems, often wild or species roses, due to the activity of a gall wasp (specifically Diplolepis rosae). The female wasp lays her eggs in the rose tissue, and the developing larvae release chemicals that stimulate the plant to create the gall. This gall provides shelter and food for the wasp larvae as they mature.
Characteristics of Robin's Pincushion Galls
- Appearance: The gall presents as a dense, moss-like or pincushion-like growth on the rose stem.
- Color: It usually has a reddish-pink or yellowish-green color, depending on the age and condition of the gall.
- Texture: The gall is generally hard and woody, composed of numerous small, bristly structures that resemble moss or pins.
- Location: These galls are most often found on the stems, leaves, or flower buds of roses.
- Harmlessness: Robin's pincushion galls are generally considered harmless to the overall health of the rose bush, although they may be aesthetically unappealing to some.
Management of Robin's Pincushion Galls
Because they are generally harmless, control measures are usually unnecessary. However, if the appearance is bothersome:
- Pruning: The affected stems can be pruned off during the dormant season (late winter or early spring). This removes the gall and the wasp larvae inside.
- Prevention: There is no effective way to prevent gall formation, as the wasps are naturally occurring and widespread.
Distinguishing from Actual Moss
It's important to differentiate robin's pincushion galls from actual moss growth. Moss is a simple plant that typically grows in damp, shady areas. It has a soft, green, and velvety texture, quite different from the hard, bristly texture of a robin's pincushion gall. Moss does not typically cause swelling or abnormal growths on plant stems.
In short, the "moss" you see on rose stems is most likely a robin's pincushion gall, a harmless structure created by a gall wasp.
