Was thinking about this last night. I am not an expert on flowers and such. But id plant what looks good and does good, I know more about veggies but yet again no expert here.
I was thinking of two of my more hardy (or hard to kill) plants one coming from both fields. Swiss Chard this stuff is very outgoing. I plant VERY early usually by broadcasting and walking away. It will grow all year, I still have some growing. This plant looks good in flower beds just as it will in you veggie garden. Another good on for me is Borage I planted some seed in '09 the first and only time I have planted seed for it. The first year it took over it's area no matter how much I tried to tame it. This year it came back on it's own. Same mission, to take over. It spreads outside my garden and gets mowed regularly yet still comes back. Still growing in the garden at this time.
Another crazy one is marigolds. I started seed inside again in '09 and planted a few here and there. This year I started a few but ended up with probably thousands. both in my flower garden and around my mailbox. The came in like wildflowers. I actually planted a few other flowers around my mailbox that were totally choked out by the marigolds. It was nice not to have to do much work and have so many flowers. In reality I didn't plant hardly anything this year in my flower garden but it was overflowing with life.
Gotta love self seeding.
What do you have that is very hardy? Veggie or flower.