A surprising number of them produce seeds, which is often given the disparaging label of "going to seed" but which is actually a very useful thing ... if you want seeds.
Our nasturtiums, for example, produce many large seeds, easily picked up off the ground around the plants and saved for next year. I've been experimenting with seed from cabbage, rhubarb, bluebells and others.
Today I learned what happens when you let cilantro go to seed - you get coriander!
I was thinking of saving some to plant next year, but coriander is just such a useful spice that I'm afraid we're going to use it instead. This must be the sort of dilemma that faced real farmers; thank goodness we have the luxury of choosing either path!