You want to wait. My suggestion would be to plant them up now in containers and grow them on before planting out into the garden. Cannas need heat to do well - both in soil temperatures and ambient air temperatures and neither of those are quick to arrive in the NW :-) Growing them on indoors in containers will alleviate the delay you'd experience in outdoor conditions and give them a jump start on the season. Same thing with dahlias. And they are not the slightest bit invasive, although in time can develop rather startlingly large tubers.

Thank You gardengal.

I will do as you said.
Are they bothered by slugs a lot?

Not particularly in my experience, although slugs are much like deer and will sample anything if determined. And hungry :-)