I have not been able to find a prayer of blessing for flowers to be distributed on Mothering Sunday. It is a very lovely tradition I have encountered in many parishes, and I do think we should make more of the "blessing" part of this custom.
I composed the prayer below with a little inspiration for the first part from the Blessing of Herbs on the Assumption of Mary (pre-Vatican II Rituale Romanum).
Please, do feel free to use it in your churches.
Loving God,After this, the Priest sprinkles the flowers with holy water [and censes them].
who created all things visible and invisible,
out of nothing through your Word;
and in your motherly care for us
have provided us with plants,
flowers and fruits to serve our needs,
Bless these flowers
which we are donating to our mothers,
let them be a sign of your love for them
and a reminder of the beauty of your creation.
We ask this through Jesus Christ, our Lord.