This arrangement of four shrubs can be visually divided into equal halves by the mind.
Since this arrangement of five shrubs cannot be divided into equal halves, it has a more unified, natural appearance.
This is a useful design rule to keep in mind because most homeowners are not trying to create a formal aesthetic with their landscape design. The terms “naturalized”, “flowing”, and “softened” are common adjectives used to describe many homeowners’ landscaping objectives, all of which are reinforced with an odd numbered design approach. This rule of working with odd numbers does not just apply to plants. It also works with hardscape elements as well such as accent stones, planter pots, and elements within focal features.
Here is a side-by-side comparison of a home’s foundation planting using two trees versus three. The two-tree arrangement reads as halves, while the three-tree arrangement appears more balanced and unified.
Scale is a factor to consider as well. Arrangements of threes, fives, sevens, and nines are the quantities you’re dealing with for small scale designs. For large scale designs, it’s more about the overall quantity of drifts and masses – three, five, seven, or nine different visual masses of plants. Whether or not the mass has 54 or 55 of a particular plant is irrelevant, but whether that mass is paired with one, two, or three other species masses is important in maintaining a naturalistic aesthetic.
So, keep the rule of odd numbers in mind as you’re designing your landscape, purchasing plants and pots, or dividing perennials. Unless you are intentionally trying to create a formal design, you're better off sticking with odd numbers.