Why are delphiniums such great plants for cottage gardens? Well, consider the following scene:
You're walking along a lonely stretch of road. You round the bend in the road and behold a cottage not far away. A white picket fence on this property starts at the road and runs through the side yard. Planted on one side of this white picket fence is a garden of perennial flowers.
Layering plants in rows is an effective way to handle such a perennial garden. Tall perennial flowers will fit in best in the row up against the fence. The shortest plants will not be displayed to best advantage if grown anywhere other than the front row. The rest will compose a loose middle row.
Of course, when we speak of tall perennials, medium-sized perennials and short perennials, we are using relative terms. Most delphiniums are among the tallest perennials (although not as tall as real freaks like plume poppy!). Since cottage gardens are known for their rambling, riotous design, with perennial flower spikes sticking out here and ornamental grass seed heads poking out there, a plant of not insignificant height is frequently needed for the back row.
Enter the delphinium! And the 'Black Knight' cultivar will be especially valued if you are a connoisseur of purple flowers.