Question
Can you tell me some complementing perennials to mix with some of the following plants: Lillies(stargazer and day lillies), Crocosmia(Red), Cannas(red and yellow), Gladeolus(mixed), Iris(purple), Dianthus(pink), Sedum, Hibuscus(pink)...  
I am thinking about texture(I have mostly spikey looking and tall)and color compliments(maybe setcresia or others).  What about some low growers(other than dianthus) to add height contrast as well?
What are the principles for complimentary plant design?
A designer once told me "something spikey, something round,..." then something that I don't remember.

Answer
Hi Adam!

This is a tall order. I'll see what I can do. From the plants you list, I suspect that you are in zone 8 or 9, so I'm going to make recommendations based on that guess.

Lilies are large and round. I like to pair them with Veronica, which is spiky and feathery. There's a white Vernoica that would suit your lilies and daylilies.

Crocosmia has a horizontal effect and is linear, so you want to pair it with something smaller, lower, and less dramatic. Sweet alyssum would be nice whether you have a red or yellow crocosmia. One of the shorter asters would be nice if you like the purple/blue.

Do you have the short cannas or the big ones? I like either of them (and the red, orange, and yellow flowers) with lantana in those same "hot" colors.

With glads, you really want something soft in shape. Calendula would be very pretty and so would zinnias.

If you have Dutch iris (or "bearded"--the fluffy big kind), you could have a very pretty combination with some dwarf sunflowers! If your iris is pale, a white or burgundy sunflower would be nice, and if the iris is deep or strongly colored, yellow sunflowers. With Siberian iris (the boggy kind, small, grassy foliage), the sunflowers would still be nice. Alternatively, ranunculus, or scabiosa.

Dianthus has a small bloom, so you want something big with it. Globe alliums would be fun. White liatris would work well (tall, spiky), poppies, echinacea.

Sedum... which one. Many people have the tall (2' or so) kind such as "Autumn Beauty." I have mine planted with white achillea (yarrow) which ends up making a big landing pad for butterflies. If you wanted something lower, marigolds would be good. Ganzania and zinnia are other options. If you have one of the groundcover sedums, I really like snapdragons or the short "jewel box" celosia.

Hibiscus. They're huge and tropical. Think about fushia, liatris, datura, allium, yucca, flowering tobacco.

For low growers, annual "bedding" type plants are a good answer. Miniature roses are often overlooked as short plants. Annual salvia is spiky and short. If your low growers are growing in the shade of taller plants, you can do a lot worse than violets.

I'm hesitant to list any principles for complimentary plant design. I think gardens are too personal for rules. There are some basic flower shapes, height of flower, and color to consider. Sometimes I like to contrast blooms (something big with something small, something short with something tall--I'm sounding like Dr. Seuss now--and something yellow with something blue) and sometimes I like to match similar looks (round scabiosa with round marigold or pink with pink and yellow with orange). You're going to have to make up your own mind about what you like to put together.

That said, there are a couple of principles most good gardners seem to follow:

For a natural look, plant odd numbers of plants--"drifts" of 3, 5, 15 (depending on the size of your garden), or combine three different plants together. Keep lines curvy and asymmetrical. For a formal look, plant pairs of things (1 blue flower with 1 yellow flower) in straight lines with right angels in symmetrical patterns.

For a long-lasting garden, choose some plants that bloom in the snow, some for late spring, some early summer, some in the middle of summer, and some that will last you into autumn.

There are art "rules" about complementary colors. The idea is, if you imagine a color wheel, the colors opposite each other are complementary and should be, in this case, planted together. In my opinion, this only goes so far. The palette of the garden is different from that of the canvas. For instance, in a painting, white is a "neutral." In the garden, GREEN is the neutral. So, I don't think this rule works very well. However, you will want to pay attention to HUE. Hue is all about the vibrancy or intensity of the color. Pastel pink, blue, and yellow all look good together. Bright red, yellow, and orange all look good together. Peach and bright red look pretty awful together. Colors can contrast, but it's difficult to make mismatched hues work together. When it does work, it's when you have different hues of the same color--bright pink and pastel pink.

I hope this gives you some ideas.

Good luck!