The element of garden design referred to as ‘plant size’ includes many facets, height being just one. Plant spread, density, and footprint are other factors of plant size that gardeners consider when selecting plants and designing their garden spaces. Giving consideration to the whole concept of plant size is critical when selecting plants and planning for a mixed perennial garden, such as Lurie Garden.

The garden designer, like the artist, has a toolbox of techniques and elements they use to create a beautiful scene. Learning about these elements increases appreciation of a garden’s design and, should you be inspired, guides you on how to create your own.

There are typically four elements when talking about plant size:

• Plant height is the measurement of the plant from the ground to the top-most portion of the plant at maturity.
• Plant spread is the maximum width of the plant as measured at its widest part from leaf tip to leaf tip at maturity.
• The density of a plant refers to the amount of open space within the plant’s stem, leaf, and flower growth. A good visual assessment of a plant’s density is to see how much background can be seen through the plant’s growth.
• A plant’s footprint refers to the amount of space on the ground, usually measured in square feet, that a mature plant will occupy.

Next to flower color, gardeners at all skill levels often focus on plant size as a factor when selecting plants and designing gardens. This focus on plant size is natural—size is an easily quantified characteristic, can be readily visualized, and represents a simple mechanism for plant classification.

Plant height can also be used by gardeners to arrange plants to either avoid or take advantage of the effects of shading by taller plants. When you view a garden, your eyes are naturally drawn to unique and interesting elements. Plant size is leveraged by the designer to move your eye through the garden. Watch where your eyes go when you look at the picture below. Where does your attention always end up?

Here the mass of green in the foreground, Allium ‘Summer Beauty’ (ornamental onion), invite your eyes into the garden scene. Your interest keeps moving toward the purple color of the taller blooming Salvia nemorosa ‘May Night’ (meadow sage) that can be seen behind the shorter ornamental onions. Finally, the pop of taller blue in the distance (Amsonia tabernaemontana var. salicifolia, willowleaf blue star) pulls your attention through the entire scene.

Plant Height

Plant height, in landscape architecture terms, forms the foundation of the concept of vertical structure. The vertical structure of a garden draws attention to particular areas of or specific plants in the garden. Three primary plant heights are common in garden design: base plants, seasonal theme plants, and structural plants. Let’s discuss each layer, from shortest to tallest.

Plant height, or vertical structure, helps draw attention to certain areas of the garden by creating transitions from base plants, to seasonal theme plants, and on to structural plants (short to tall). In this picture from Lurie Garden, Sesleria autumnalis (moor grass) forms a solid base plant layer (foreground) of lower growing plants, while Pycnanthemum muticum (mountain mint) creates a quality seasonal theme plant layer (mid-ground), and Amsonia hubrichtii (Arkansas blue star) and Panicum ‘Shenandoah’ (red switch grass) establish the tall structural plant layer (background). The right side of this picture translates plant height layers into mass color blocks representing base plants (brown; foreground), seasonal theme plants (light green; mid-ground), and structural plants (green & black; background).

Base Plants

Base plants are low growing, short stature plants that effectively cover bare ground. This shortest group of plants (typically no taller than 30 cm) are the first line of defense against an invasion of weeds, occupying open soil spaces in the garden and excluding space for weeds. Base plants also help in preventing soil erosion in the garden and aid in soil moisture retention. This group of plants can comprise up to 50% of your garden.

In addition to helping in weed control and soil erosion, base plants are regularly used to create masses of solid color (monochromatic) plantings into which other seasonal theme and structural plants are planted. Think of base plants as the primary wall color of a room—covering lots of space, connecting all four walls of a room, and creating a platform onto which highlights can now be added.

Examples of base plants include: Carex pennsylvanica (Pennsylvania sedge), Sesleria autumnalis (moor grass), Ruellia humilis (wild petunia), Geum triflorum (prairie smoke), and Allium ‘Summer Beauty’. The latter three plants serve a dual purpose as base plants and seasonal theme plants.

Base plants form the primary foundation of the garden, which can be monochromatic or, as here, green with hints of other highlight colors that can be used to mirror colors used in the taller seasonal theme and/or structural plants. In this picture of Lurie Garden, Epimedium x versicolor ‘Sulphureum’ (bishop’s hat; background) is interplanted with Sesleria autumnalis (moor grass; foreground) to form a relatively monochromatic base plant layer. Later in the growing season, seasonal theme and/or structural plants with flower colors that mirror the red highlights seen in the bishop’s hat could be used to connect plants at different heights.

Seasonal Theme Plants

Seasonal theme plants (typically no taller than 90 cm) work hand-in-hand with based plants to fill space in the garden, but add flowering interest throughout the growing seasons. Base plants set the stage for building a scene, seasonal theme plants can be complementary or contrasting beyond base plants that entice your eye to explore. Seasonal theme plants can occupy 30-40% of your garden space.

Seasonal theme plants can often serve a dual purpose as seasonal flowering interest and base plants. In this picture from Lurie Garden, Salvia x sylvestris ‘Blue Hill’ (meadow sage; dark green) and Sesleria autumnalis (moor grass; light green) form a non-flowering low base plant layer. Flowering Salvia nemorosa ‘May Night’ (meadow sage; dark purple) forms the seasonal theme layer, while Baptisia ‘Purple Smoke’ (false indigo; light purple) brings taller structural plant highlights. The image inset shows major layers color-blocked to demonstrate how massing plants of different heights helps draw the viewer’s eye through the garden from base plants (dark and light greens), to seasonal theme plants (dark purple), and to key structural plant elements (light purple).

Year round planting design is a true four-season display, with various seasonal theme plants providing visual treats throughout the year. For example, in the early spring hybrid tulips (Tulipa cultivars) provide seasonal theme interest, which gives way to a late spring display of Zizia aurea (golden Alexander), which in-turn gives way to summer seasonal theme plantings of milkweeds (Asclepias tuberosa, A. incarnata), and so on through all seasons. This concept of a rolling bloom across multiple seasons combines the ideas of plant size with timing.

The visual effect of plant height does not have to be spread over a large area; its impact can be seen within a small area too. In this picture from Lurie Garden of a small 1-square meter spot, blooming Geum triflorum (prairie smoke; mid-ground) and fading early-spring Camassia leichtlinii ‘Blue Danube’ (quamash; foreground) comprise strong base plant and seasonal theme plant layers, while a developing Silphium laciniatum (compass plant; background) brings taller structural plant interest.

Structural Plants

Structural plants are the tall, standout plants of the garden (typically taller than 120 cm). This group of plants, often the tallest plants in the garden, bring distinctive interest and attention to specific areas of your garden. Structural plants should be used sparingly to prevent a visual sensory overload in your garden. This group of plants should represent only 5-15% of your garden.

Structural plants create dramatic, tall visual points of interest in the garden. In this picture, the winter Lurie Garden is dotted with the dried remnants of Echinacea ‘Virgin’ (white coneflower) and Eryngium yuccifolium (rattlesnake master). These striking and tall points of interest help move your attention around the garden and add whimsy in winter by collecting and ‘suspending’ snow mid-air.

Structural plants can be used to create consistent points of visual height interest across multiple seasons. For example, Amsonia hubrichtii (Arkansas blue star) blooming in early spring is, for that time, one of the tallest plants in the garden. Come summer, A. hubrichtii remains one of the tallest plants in the garden and, even though not flowering, provides a visual highlight with its soft-appearing foliage. In fall, this same species brings a bright golden yellow to the garden while still being one of the tallest plants in the garden.

Other examples of structural plants include: Panicum ‘Shenandoah’ (red switch grass), Echinacea pallida (pale coneflower), Echinacea ‘Virgin’ (virgin cone flower), and Rodgersia pinnata (featherleaf rodgersia).

Similar plants of different heights can work together to create great vertical structure in the garden. In this picture from Lurie Garden, Sesleria autumnalis (moor grass; foreground) forms both low base and middle-height seasonal theme plant layers while Panicum ‘Shenandoah’ (red switchgrass) and Echinacea pallida (pale coneflower) combine for tall structural interest.

Plants serving as base plants in the spring can transition to seasonal theme plants later in the growing year. In this picture from Lurie Garden, the low growing spring foliage of Allium ‘Summer Beauty’ (ornamental onion) serves as a base plant layer (foreground) with Salvia nemorosa ‘Amethyst’ (meadow sage; right) and Amsonia tabernaemontana var. salicifolia (willowleaf blue star) (back, left) serving as taller seasonal theme and structural plant layers. Later in the growing season, the tall flowers of Allium ‘Summer Beauty’ will replace those of S. nemorosa ‘Amethyst’ as the seasonal theme layer in this location.

Things to remember when considering the role of plant size in garden design:

• When experiencing a garden scene, be aware of your senses. For example, consider where your eyes go and what did the garden designer do to draw your attention there?
• When planning your garden, be aware of the full and mature height of the plants you have selected. A simple approach is to stage plants with tallest in back, moving to shortest in front.
• When buying plants for your garden, remember to think percent coverage and purchase accordingly. A good rule of thumb is that base plants should occupy up to 50% of the space, seasonal theme plants represent 30-40% of the space, while structural plants should occupy only 5-15% of your garden.
• Remember that each plant type and color family can be spread across multiple height layers. For example, plants in the base plant layers and seasonal theme plant layers can be the same or similar color families.
• Plant size is just one of many considerations for organizing garden designs. Starting with one basic element, such as plant size, is a great way to get to understand plants grow and how they will fair in your garden.
• Gardening is a relationship between you and your plants. Like all relationships listening to how your partner is responding to you is important and a very rewarding process!
• Do not be afraid to actively experiment in your garden. All gardeners kill plants…and it is ok! Gardening is always an experiment and there are always more plants.