It's neither beneficial nor harmful. It's caused by the Mossy-rose gall wasp, also called Robin's pincushion gall and bedeguar gall, scientific name Diplopepis rosae.

The Swedish name is "sömntorn", sleep thorn in translation, because it was believed in old lore in to give sound sleep if one slept with a gall under the pillow.

Thank you mariannese - at least I don't have to rush out and cut them all off!

Tricia

I have had these on my R. canina in Tennessee and they did not strengthen the bush. It might be just my conditions, but after the first year, when I thought I had destroyed all of them, more came back and the third year the rose died.

I can't think of anyone within fifty miles of me who grows the old roses that these mossy rose galls seem to prefer, and suddenly they appeared on a five year old canina.

Still puzzling that one.

Hi anntn6b,

thank you for letting me know that - my R. canina (which form part of a hedgerow along with blackthorn, hawthorn and firethorn) were all planted in January 2012, so I think I will cut them all off and bag them and then burn them. They're only on half a dozen plants at this stage (that I noticed) - tomorrow I'll have a closer look and take a bag and my pruners with me when I do my 'inspection'.

Tricia