As seen on Gardeners World by Monty Don.

It seems to work. I planted a clove by each rose about 2 yrs ago and left
them to grow.

Last year there were no greenfly on any of the roses, and no problem with
them anywhere except in the greenhouse. I noticed that there were no
ladybirds at all, then realised that there was nothing for them to eat ;-)
The theory is that the rose sap tastes of garlic which the greenfly don't
like.

I hope that this year it will be the same, the garlic have now grown into
quite large clumps which I have thinned.

Marina
E. Sx