One-of-a-kind plant is OK when used correctly

Contrary to its popular musical identity, one is not the loneliest number but rather the ultimate prime number. Unless you have a specimen garden composed of one of everything (a recipe for disaster except under the most skilled eyes and hands), one plant will read as either a specimen or a unifier. If you’re going to use a single plant as a specimen or focal point, be sure its qualities are strong enough that it can stand on its own.

Two plants signal formality

Like the guards at Buckingham Palace, the number two conveys formality. Two plants tend to divide your eye, so this number works best when used like sentries to mark both sides of an entrance or passageway. Trees and shrubs are easy to use in pairs, but perennials are more challenging. Some of the larger perennials and ornamental grasses work well in pairs, as do some annuals and tender perennials.

Though three can be tricky in human relationships, this number fits expertly in a landscape. The trick is in the arrangement. Three plants in a row is dull because you know what to expect. If you have enough space, group them in an equilateral triangle. This looks particularly good with mounding or vertical plants. Be sure to leave some space between the plants, especially if they are three different kinds. You can even use a garden ornament, like a birdbath, to count as the third item in a set of three.

Four plants work best when divided

Four can be divided in various ways. Avoid planting two and two, even on the sides of an entry, because it rarely works. It feels off-kilter, always calling for more to make it three and three. A good way to use four plants is by putting one in each quadrant of a circle or square, either planting four distinct specimens or repeating the same plant. Another device that works well is dividing 4 into 3 + 1, positioning three on one side of a path and one on the other. Because of their strong weight, this is especially effective with evergreens.

Five plants is pleasing to the eye

Five is a number that is used often in designs. A classic example of five is to set up two parallel rows with three in one row and two in the other. This works best in rectangular beds but can work in irregularly shaped beds too. That is not the only way to use five plants. I have found that positioning two plants on one side of a path and three on the other does not work (especially with mounding plants), but having four balanced by a fifth feels right.

Six plants should be divided into two sets of three

Seven plants and more becomes a mass

With seven plants, you achieve enough mass to start making a strong visual statement. Once you get as high as seven, you have some leeway to either plant in masses or plant the same plant in groups of 3 + 3 + 1. Avoid 3 + 4 because it feels unbalanced. Even numbers higher than seven can be divided into two sets of odd numbers, such as 12 divided into sets of 5 and 7. Odd numbers higher than seven can be separated into groups of odd numbers. For example, 3 + 3 + 3 = 9 works, but avoid breaking up an odd number of plants into an odd-numbered group and an even-numbered group, such as 5 + 4 = 9. At a certain threshold, roughly around a dozen depending on the type of plant, the eye can’t tell whether you have a mass of 12 or 13 plants. Once you reach a critical mass, you don’t need to worry about counting anymore.