What earns the space
Cosmos does the same thing and does it on poor soil, which is useful, because rich soil gives you a large green plant and not many flowers. Sweet peas are the most demanding thing on this list and the most worth it, but only if you pick them every few days without fail, because a single pod left to form tells the whole plant the season is over.
Dahlias are the exception worth breaking the rule for. They are not cheap, they need lifting or heavy mulching in cold areas, and they take up real room. They also produce more usable stems per square foot from late summer onward than anything else you can plant, and they keep going right up to frost.
Then there is the part people leave out and then miss: foliage and filler. A bunch of nothing but flower heads looks like a supermarket bunch. One plant of something with good leaves, or a patch of an airy filler, does more for how the arrangement reads than another color of the same flower would.
If you plant nothing else, plant these four:
- Zinnias for sheer quantity from midsummer onward, in colors nothing else quite matches.
- Cosmos for height and movement, and because poor soil suits it better than good soil does.
- Dahlias for the late season, when everything else is finishing and they are only getting started.
- One good foliage plant, whatever grows easily where you are. It is what stops a bunch looking like a handful of flower heads.