No garden should be without columbine (Aquilegia).  The flowers are unique, the colors are exquisite and even the lobed green leaves are attractive.  (In fact, the leaves look good even when they are beset by the great columbine pest, leaf miner.)

But be aware that columbines are full of suprises.  Buy even one this year and chances are, next year you will have many little plants.  Columbine self-seeds with wild abandon.  This isn't really a problem because the little seedlings are easy to transplant.  The colors of the offspring may surprise you, however.  Fortunately most columbine colors harmonize with other columbine colors and you may end up with some serendipitous combinations.

If you like those combinations, they will enrich your garden.  If not, wait a season.  Columbine tend to appear and disappear according to their own rules.  I used to have many blue and blue-purple flowered plants in one bed in my back garden.  The originals seem to have disappeared and now I have a nice clump of them in a nearby bed.  The newcomers bloom later and some of them are pink.  I treasure them right now, but know that I shouldn't get too comfortable with the display.  Next year it is likely to be different.

Columbine is probably not for the person who likes to control everything--unless that person also likes to transplant interloping plants.  For those of us who enjoy an element of serendipity in the garden, they are one of the best choices.