Architectural plants provide a bold, symmetrical look

Cordyline ‘Burgundy Spire,’ is surrounded by a mixed border with phorimum ‘Black Adder’ plants.
Plants are used in landscapes to soften the hard edges of buildings and hardscapes. Often, a landscape needs some form of architecture to help define and add symmetry to naturally soft textures.

Garden art and hardscapes such as walls and benches can help define these spaces, but as designers, gardeners and growers are finding out, there is a wealth of unique plants that can give bold lines and symmetry to the landscape as well.
 
The term architectural plants can be loosely defined as “spiky plants” or plants with defined lines and a naturally upright habit. Architectural plants are a natural fit for the center of a mixed container, as container specimens or anywhere a dramatic landscape focal point is needed. Architectural plants can fit a spot where something like a specimen tree will not work. 

The dracaena spike (Draceana indivisa) is a classic architectural plant. Now a mainstay in mixed combinations, the dracaena spike has led the way for many new alternative plants that offer bold lines and unique foliage colors.Cordyline ‘Torbay Dazzler’

Cordylines offer red tones
Cordylines are directly related to dracaena and sometimes the two genera are lumped into the same genera. Both are Southern Hemisphere natives. Cordyline, in particular C. australis-type hybrids, have the same upright growth as dracaena but with deep-red foliage.

Cordyline ‘Pink Passion’‘Red Sensation’ and ‘Red Star’ are two of the more common cultivars. Both are regularly available from tissue-culture labs and young-plant companies.
 
Some new exciting variegated cultivars are ‘Torbay Dazzler,’ a white-and-green variegated selection, and ‘Electric Pink,’ a clumping selection with deep-pink-striped leaves.
 
Cordylines prefer high light and moderate temperatures. Plant cordylines in a well-drained medium and avoid excessive drying. Cordylines are hardy to USDA Hardiness Zone 8, depending on the variety. No pests are overly troublesome. When grown in cold climates some cultivars such as ‘Red Sensation’ can show symptoms of edema.

Phormiums move over from New Zealand
Originating in New Zealand, phormiums have been popular landscape plants on the West Coast and in milder climates worldwide for some time. There are two species in the Phormium genus: tenax and cookianum.

PPhormium ‘Alison Blackman’. tenax varieties are notable for their bold upright spiky foliage. Plants generally get very large and can produce leaves up to 4 inches wide. P. cookianum varieties are generally smaller in stature, and the leaves have a tendency to droop at the ends. Both species have produced a wealth of hybrids with foliage in every color imaginable.
 
Some challenges that have occurred with phormiums, especially the variegated selections, are high rates of reversion back to foliage color of the original species, or the foliage turns a muddy brown color. Some varieties, like ‘Cream Delight,’ ‘Tricolor’ and Pink Stripe,’ seem to be more stable than others. There can also be issues with the variety names. Some are sold under multiple names. There are also selections that may be named, but are not the true variety.
 
There are many phormium selections with solid-leaf coloration varying from lime green to almost true black. There are also a number of variegated selections with bold striping. The variegated selections have proven to be the most popular with consumers.
Phormium ‘Black Adder’ 
Phormiums prefer full sun to partial shade and a well-drained medium. Phormiums are susceptible to crown rots, requiring that the crowns be planted above the soil line. Mealy bugs and spider mites can be problematic pests and should be scouted for on a regular basis. Plants are hardy to approximately Zone 8.
 
There are more than 50 cultivars available. ‘Rainbow Sunrise,’ also known as ‘Maori Sunrise,’ has terra-cotta and golden variegated leaves with strong upright foliage. It grows 36 inches tall.
 
‘Cream Delight’ is a bold, drooping-leaved selection with cream-colored striping. It grows approximately 36 inches tall by 48 inches wide.
 
‘Tricolor’ is similar to ‘Cream Delight’ but has creamy-white striping with green and small lines of pink coloration. ‘Tricolor’ is a unique specimen that reaches similar final dimensions as ‘Cream Delight.’
Phormium ‘Margaret Jones’
‘Margaret Jones’ is a newer selection from Green Fuse Botanicals. It is notable for its deep-peach and watermelon striping on dark foliage. The leaves droop at the ends.
 
‘Black Adder’ is a new selection from Ireland, introduced by Anthony Tesselaar of Australia. This variety is notable for its glossy deep-purple-black foliage. Leaves are strongly upright and droop at the ends. It is a great component for focal points in mixed containers.

Dianella has flowers, too
Dianella is another genus from the Southern Hemisphere that is related to phormium. Dianellas have become popular for their ornamental attributes and for their uses as a durable landscape plant.
 
Dianella is similar in overall look and habit to phormium but with smaller overall sizes and different foliage coloration. Plants can be variegated, solid green or have blue-green foliage. The leaves are generally stiffer than those of phormiums and many species have serrated edges.
Phormium ‘Jester’
Dianellas also have small flowers that can be very showy when in full bloom. Often the flowers are followed by small, black berries. Plants can be very drought tolerant and do best in dry, shaded locations. Plants prefer a well-drained medium. Dianellas are hardy to USDA Hardiness Zone 9.
 
D. ensifolia ‘Silver Streak’ is a white-variegated selection with gray-green foliage. ‘Silver Streak’ is a very attractive variety that can be used in mixed containers or as a focal point in the garden. It grows to approximately 24 inches tall by 36 inches wide. It does best in a partial-sun location with good drainage. Mass ‘Silver Streak’ for a dramatic effect.
 
D. caerulea ‘Cassa Blue’ features gray-blue foliage on a stout habit that reaches approximately 24 inches tall. Plants are topped with deep-blue flowers in spring and summer.
Dianella berries
Yuccas are Southwest staples
This genus of woody lilies is a staple landscape plant in the arid Southwest. The Joshua tree (Yucca brevifolia) evokes images of the classic desert landscapes. There are many variegated selections available that give an unusual architectural look to water-conscious gardens. Though very drought and heat tolerant, yuccas adapt themselves to traditional garden conditions. They do excellent in containers and can be kept relatively contained in small gardens.
 
Yuccas require good drainage and don’t like a saturated medium. Plants prefer full sun and warm conditions. Most varieties are hardy in USDA Hardiness Zones 7-8; however, select varieties are hardy to Zones 4-5. Often under lower temperatures, the plants respond with pink to red coloration in the foliage. Protect yuccas from frost in the coldest climates.
 
Y. Dianella caerulea ‘Cassa Blue’ flowers
Photo courtesy of Ozbreed Ltd.gloriosa ‘Walbristar’ Bright Star is a new selection from Tim Crowther from the United Kingdom. It was discovered as a sport of Y. gloriosa, a native to tropical areas of the Southeast United States. Bright Star is a striking form that produces leaves with gray-green centers and bold-golden margins. The gold variegation dominates this plant. It definitely glows from a distance. Bright Star should grow to about 24 inches tall and wide and is hardy to Zone 7.
 
Y. filamentosa ‘Color Guard’ is a softer-leaved form that has leaves that droop slightly as they age. ‘Color Guard’ has deep-golden foliage with green margins. The leaves are accented with deep-red blushing in cooler months, giving it a tricolor look. ‘Color Guard’ is becoming readily available from tissue culture. It grows to about 20 inches tall and 24 inches wide and is hardy to Zone 5.
 
Y. recurvifolia ‘Monca’ Banana Split is a soft-leaved yucca from Monrovia. Drooping leaves have a center band of banana yellow with dark-green leaf margins. This variety clumps with age and makes a dramatic container specimen. Banana Split is hardy to Zone 7 and grows 4-6 feet tall and wide.Yucca gloriosa ‘Walbristar’ Bright Star

Astelia is great for mixed containers
Astelia chathamica ‘Silver Spear’ is a genus of evergreen clumping monocots in the lily family. They are notable for their wide upright spiky foliage similar to that of phormium.
 
Astelia leaves are covered in small silver hairs that give the plants a metallic look. This foliage color makes astelia a great item for mixed containers. Astelia is hardy to Zone 9. ‘Silver Spear’ grows to approximately 36 inches tall. Plant it in sun to partial shade and provide good drainage and moderate moisture.

More plants to come
The list of genera and cultivars that fit into the category of Dianella ensifolia ‘Silver Streak
architectural plants continues to expand. Genera such as Agave, Fucraea, Manfreda, Mangave and Hesperaloe are becoming more accessible to gardeners due to production advancements, including tissue culture.
 
Many of these plants are not winter hardy but can be grown as annuals or protected as greenhouse specimens in colder months. Most have basic production requirements and don’t have any special landscape needs.