Water hawthorn overwinters for me here but it gets off to a pretty slow start in the spring. I drop its pot into the 4' section of the pond to keep it through the winter. I think it is on the edge of its hardiness zone here.
I collect the large seed pods when they are very fat and almost ready to burst. The seeds are easily germinated in a jar of water in a sunny window sill. Germination is excellent, have to be careful potting them up. After several tries and much error I now just put some soil in a pot when they get to be of handle-able size, fill partially with water and float the seedlings. As long as the water isn't too deep they seem to root easily into the soil on their own. Then I gradually top up the water (or gradually move the pot into a progressively deeper part of the pond) until they are big enough to go into the position I want. I've given lots of these away.
This is a picture of newly germinated water hawthorn seedlings from my first attempt; toothpick on top shows scale:
Love your pink blooms Annette. Mine are white.
The blooms are very fragrant, almost like vanilla. In my pond they bloom early in the summer and then the leaves fade away; just a few small ones remaining through the summer. When the temperatures cool later in the summer they resume blooming.
emm