Gardening in the summer offers an amazing range of flower colors and textures. When combining plants for a flowering gradient, gardeners like to focus on color combinations that they prefer. While this certainly makes sense, floral architecture can add additional interest to any combination of plants or landscape scene. Floral architecture refers to the shape of the flower, which can range from daisy and trumpet-shaped to spiral or spherical. All of these shapes, aside from the color they offer, create additional interest in the garden.

Globular flowers in particular offer strong structural interest and tend to make the eye pause in a composition where they are prominently repeated or perhaps repeated across the room. These flowers typically consist of inflorescences that are arranged in a spherical format. Spherical flowers with their strong symmetry stand out strongly in the flowering period and look noticeable near contrasting flower shapes such as towers and daisies. Three excellent plants with spherical summer flowers are listed below, but there are many more options among annuals, perennials, trees, and shrubs.

The unique flowers and the mint green color of the rattlesnake master offer an interesting contrast to leafy and dark leafy plants. Photo: Mark Dwyer

Rattlesnake Master

Eryngium yuccifolium, Zones 3–8

This North American native prefers poor soils that are on the dry side and is quite drought tolerant once established. In fact, you should avoid soil or shade that is too rich in nutrients as it will get too high and limp. The 1-inch diameter flowers appear in summer clusters over yucca-like, blue-green foliage. Plant height is typically between 4 and 6 feet, although the ‘Prairie Moon’ variety reaches the top at 3 feet. These flowers have significant pollinator value. Rattlesnake Master is perfect for mixed perennial plantings and native meadows.

South globe thistle
Southern globe thistle is a charming European species with velvety-looking flowers above deeply incised foliage that resembles arugula. Photo: Mark Dwyer

Globe thistle

Echinops spp. and cvs., zones 3-8

The golf ball-sized flower heads of the spherical thistle float over the silvery-gray foliage in summer. Extremely drought tolerant, it develops a taproot and slowly recovers when moved or divided. Avoid soil that is too rich in nutrients and prepare for possible staking out to support the floating flower balls. The most commonly available species can be the southern globe thistle (E. ritro, Zones 3–8), which are 3 to 4 feet tall during flowering. Globular thistle includes many different species and hybrids, the flowers of which range from white to all shades of blue. Be sure to note the mature height of the one you are considering. Regardless of which one you choose, you will notice the seemingly floating flower balls of the spherical thistle.

Button bush
After Buttonbush’s white flowers fade, green, spongy seed heads are left to provide structural interest. Photo: Mark Dwyer

Button bush

Cephalanthus occidentalis, Zones 5–9

This native shrub is becoming increasingly popular and available for landscape use. Buttonbush prefers moist soil and tolerates very wet conditions and average garden soil. The native species grows to be around 12 feet tall or taller and is categorized as a large shrub. However, there are now many compact varieties available that reach heights between 5 and 8 feet. This shrub prefers full sun to partial shade and blooms in summer with fragrant, spherical flower heads that have a pincushion-like appearance. Buttonbush will flower poorly in both full shade and dry soil. Many pollinators and other wildlife are fans of this plant, which also has very few insect or disease problems.

—Mark Dwyer, former director of horticulture at Rotary Botanical Gardens in Janesville, Wisconsin, runs Landscape Prescriptions by MD.

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