Salvia, hardy geranium, rose and catmint in a perennial border; photo courtesy of Flickr cc/ ukgardenphotos
Basic Design Considerations
Site Assessment (for sun and moisture): Is it in full sun (6 or more hours), partial sun (4 to 6 hours) or partial shade (2 to 4 hours)? Does the area retain water in summer or winter? Is there heavy root competition from neighboring trees?
Season-long Interest: Do separate areas of the garden have different seasonal interest, or is multi-seasonal interest incorporated throughout the plantings?
Bloom Time: This is an imperative consideration when combining plants; be creative. Siberian irises and peonies are a classic combination for May into June. Other interesting pairings for peonies, which have wonderful red foliage as they emerge, are early season bulbs such as Siberian squill (Scilla) or miniature daffodils (Narcissus).
Color: There are cool colors (blues, purples, and greens) and hot colors (reds, yellows, and oranges). Colors are affected by their surroundings, including the colors of neighboring plants, textures and light levels. Colors increase in intensity when placed next to colors that are on the opposite side of the color wheel (complementary or contrasting colors) and decrease in intensity when placed next to colors adjacent on the color wheel (analogous colors).
Form and Texture: Form and texture are just as important as color in creating an effective plant combination; they hold the composition together once flowers have faded. Feathery and bold textures make wonderful contrasting combinations that bring out the best in their partners. Play around with floral shapes.
Plant Selection: Research potential choices before purchasing the plants. Cultivars come in many different shapes and sizes: does that perennial sunflower variety grow 4 to 6 feet tall or 6 to 10 feet tall? Is the plant low maintenance or high maintenance? Is it resistant to pests and diseases?
Tips for Creating Successful Plant Combinations
Copy successful and beautiful gardens: Draw on plant combinations that work in other gardens as the basis of your own unique combinations, using different cultivars. For example, daylilies make wonderful partners with hydrangeas. Experiment with different types of hydrangeas such as bigleaf hydrangea (Hydrangea macrophylla) or oakleaf hydrangea (Hydrangea quercifolia) and various daylilies, of which there are over 50,000 cultivars, lending to a seemingly endless number of pairings of colors, sizes and shapes.
Be aware of maintenance requirements: Don’t take on more than you can handle. It will look like a mess and turn into frustration. For every high maintenance plant, there is usually a low maintenance alternative. Delphiniums (Delphinium) can be a lot of work, but monkshoods (Aconitum) are easy. Lupines (Lupine) are lovely but short-lived, while false indigo (Baptisia) will hang around for years.
Simplify: It is tempting to try to fit into your garden each plant you fall in love with at the nursery. Combinations will be more stunning, however, if plants are not competing with one another in an overcrowded space. Less is often more in garden design. Find a balance between variety and simplicity. Planting in large drifts always makes a wonderful statement, where the details of combinations can be appreciated.
Repetition creates unity and harmony: An important tool in creating a coherent design is the repetition of colors, shapes and textures.
Remember that green is a color: Create a beautiful tapestry, particularly in a shade garden, by using different shades of green, from chartreuse to forest.
Work with the site conditions: If the site is wet, plant water-loving shrubs and perennials. If in shade, install plants that don’t need a lot of sunlight. Work with the site conditions for healthier plants and a more problem-free garden.
Use space wisely: Design the garden around the way the space will be used: for entertainment, recreation, sunbathing, relaxing?
Aconitum (monkshood) is an easy, low-maintenance plant
Classic Plant Combinations
Wild ginger (Asarum) and hellebores (Helleborus)
Wild bleeding heart (Dicentra), hosta (Hosta) and Solomon’s seal (Polygonatum)
Catmint (Nepeta) and roses (Rosa)
Peonies (Paeonia) and Siberian irises (Iris)
Astilbe (Astilbe) and hosta (Hosta)
Salvia (Salvia) and hardy geraniums (Geranium)
Scented geraniums (Pelargonium) and herbs
Nasturtiums (Tropaeolum) and Swiss chard (Beta vulgaris subsp. cicla)
Cornflower (Centaurea) and fern-leaf dill (Anethum)
Japanese hakone grass (Hakonechloa) and lady’s mantle (Alchemilla)
Mexican sunflower (Tithonia) and cannas (Canna)
Coneflower (Echinacea) and Russian sage (Perovskia)
Hot poker plant (Kniphofia) and Helen’s flower (Helenium)
Daylilies (Hemerocallis) and hydrangeas (Hydrangea)
The Encyclopedia of Planting Combinations by 
The Winterthur Guide to Color in Your Garden by 
