Floral design is based on balance. There are 2 types of balance in floral design, physical balance and visual balance.

Physical Balance

Although balance is important in visual design, it is also crucial for physical balance.

Unbalanced arrangements are more likely to fall over.

For example, a top-heavy arrangement in a thin vase will surely fall over and this is definitely not optimal in any situation, especially a wedding where guests could get a little tipsy and bump things.

By using different shapes, you can increase the visual appeal of your floral arrangement without compromising its structural integrity.

Visual Balance

And then there is the Visual Balance of your design. People like or dislike something because of how things look through their eyes.

When your design is visually unbalanced they may comment that the flowers were ok but I wouldn't have that design.

They don't know why but the design just feels wrong to them. It is creating a negative emotional response.

With visual balance there are 2 types: symmetrical and asymmetrical balance.

Symmetrical balance is achieved by having the same quantity of similar materials on either side of the central axis.

This type of floral arrangement is usually used in formal settings.

We call this design evenly balanced or symmetrical.

Asymmetrical balance is achieved by having different elements arranged around an imaginary axis but with equal visual weight on either side to maintain the sense of balance. Like with a moon design

This type of floral arrangement is usually more stylish and informal.

Proportion and scale are closely related to each other.

While scale focuses on the size of the floral design relative to its settings and surrounding area, proportion focuses on the size relationship between one part of the floral design to another.

Placing one large flower in the middle of a design that is full of small intimate flowers can look out of place or out of proportion. 

For example, the amount of floral material should be proportionate with the container or the height of the arrangement should be proportionate to the width.

Most floral designs should have a focal point that is usually the main feature which our eyes are easily drawn to.

Dominance or emphasis can be achieved by placing sharp contrasting materials in the form of  size, colour or texture.

With this Bouquet the dominate flower was a beautiful pale peach silk peony with smaller flowers surrounding.

The orange ribbon was used to emphasis the burnt orange, green and natural theme of the wedding. 

In floral arrangement, rhythm refers to the visual flow or movement in an arrangement.

First the eye should be attracted to the focal point in the design. Then carried throughout the entire arrangement, giving a sense of movement or motion that connects the floral design from one point to another and back. Most people when they see this movement are drawn to it, without even realising the feeling that they are experiencing. 

They are feeling happy and relaxed when they are viewing the design.

Rhythm is a dominant visual path through a design. It is achieved by the use of gradation and repetition in a linear direction.

In floral arrangement, rhythm refers to the visual flow or movement in an arrangement.

First the eye should be attracted to the focal point in the design. Then carried throughout the entire arrangement, giving a sense of movement or motion that connects the floral design from one point to another and back. Most people when they see this movement are drawn to it, without even realising the feeling that they are experiencing. 

They are feeling happy and relaxed when they are viewing the design.

Rhythm is a dominant visual path through a design. It is achieved by the use of gradation and repetition in a linear direction.

Contrast is the use of opposite or unlike elements to emphasise differences and add interest.

This helps to emphasise their differences like with the flowers, bridesmaid's dresses & Amy's wedding dress. Amy is standing out, as she should. After-all it is her wedding day. 

Similar to the dominance, contrast can be applied to many design elements  such as forms, colours, sizes and textures of the flowers and foliage.

The last principle is more of an outcome if the above 6 guiding principles are being considered for the floral design.

Harmony can be achieved when the combination of the flowers, foliage and accessories provides a pleasant visual experience.