Monkshood, or aconite, with spires of blue-purple flowers in summer and early fall, gives an effect similar to delphinium in areas of partial shade. The 3- to 4-foot plants have cut, somewhat ferny leaves and tall narrow stalks. Unfortunately, they are not for children's areas since all parts are extremely toxic.
Plants are always much easier to grow if grouped with others of similar cultural needs, and monkshood is definitely of the group that likes rich, moist (but well-drained) soil and shelter from the strongest sun. In most areas of the country, you'll need to water these plants more frequently than others, so putting them all together fairly close to the faucet is a good idea.
Bleeding heart (Dicentra) is a good companion, as are astilbes, anemones, columbines (Aquilegia), lady's mantle (Alchemilla), hostas, ferns, epimedium, snakeroot (Cimicifuga), brunnera, foxgloves (Digitalis), astrantia, lobelia, lilies (Lilium), primroses (Primula) and spiderwort (Tradescantia).
The blue-purple of Monkshood flowers is a cool foil for almost any other color, so you should have little trouble in color coordination. Daylilies will give you the most choice in color range, from pale yellow to pink and peach and then hot oranges and reds, often blooming at the same time.
Use lots of yellow to brighten the picture, perhaps with variegated hostas splashed with yellow, or shrubs such as aucuba that also have yellow variegation. True lilies also come in yellow, as well as many other shades.
You can also create a bed of cool pinks and blues using bleeding heart, foxgloves and astilbes. Many of these, however, bloom earlier in the year than the monkshood, so you'll need to add some annuals or late bloomers such as Japanese anemone to continue the show.
One quick way to make your bed look professional is to use three or four different textures, choosing from medium, ferny, bold and linear. Monkshood themselves have a somewhat delicate texture, but adding maidenhair ferns or astilbes at their base is still an ideal way to begin.
Daylilies, spiderwort and Siberian (Iridaceae sibirica) or gladwyn iris (Iridaceae foetidissima) all lighten a composition with their fans of sword-like leaves. Smaller plants that provide the same contrast include carex and liriope.
Big, bold textured leaves are another possibility. Use hostas, bergenia or lady's mantle or, for the largest leaves, try bear's breech (Acanthus).
Monkshood in Containers
You'll need a whiskey barrel or other similarly large container to keep aconite in scale, with plenty of lower annuals and perennials to hide the somewhat bare bottom stems. Choose something to trail over the side, perhaps petunias or bacopa, then add a hosta and ferns, astilbes and daylilies for texture and extended bloom. Snapdragons would add some height.
Monkshood in the Wild Garden
A drift of monkshood can be as much a part of a wildflower meadow in your garden as they are in their native Europe and Asia. Lobelia cardinalis likes similar conditions, and the spikes of bright red flowers are a lovely contrast. Ferns are always appropriate. Bee balm (Monarda) and spiderwort would also fit here. Just be sure to choose a moist site where the water will not pool or water frequently.