9 Perennials That Are Famously Difficult to Grow

There’s a certain triumph in the success of growing a finicky perennial, whether it’s luck and circumstance that it survives in our garden (usually) or tending to its every whim. Some do best if you don’t look at them too often or too closely, while others need prodding to establish.

And this is the joy, seeing what thrives and running with it. It’s a happy surprise when an experimental perennial takes off, and a bit defeating when one that “should work” fails to flourish.

Delphinium

	botanical name Delphinium spp.
	sun requirements Full sun
	height 1-6 feet
	hardiness zones 3-9

Native wildflowers like Delphinium are sometimes difficult perennials due to their specific cultural set and also a lack of availability in cultivation. North American species are less commercially available than those of European and Asian origin, bred for their reliable bloom, colors, and performance.

Elegant and stately, Delphinium boasts large, deep blue bell-shaped flowers on tall stems, with cultivars in a range of hues. The glorious bloom spikes make it well worth a try despite its finicky nature.

Delphinium grows best in cool climates. In warm climates, perennialization can be challenging. Because larkspur dies back in the summer heat, it performs best as a cool-season annual in spring. 

Delphinium is susceptible to a host of diseases, especially crown rot and mildew, if conditions are too damp. They rely on moist and very well-drained soils. Full sun and ample air circulation help stave off powdery mildew. They benefit from deadheading the spent blooms, and tall varieties may require staking.

Tall Garden Phlox

	botanical name Phlox paniculata
	sun requirements Full sun to partial shade
	height 2-4’
	hardiness zones 4-8

Phlox is a classic garden mainstay and staple in the border with domed bloom clusters that rise in saturated hues. But, it’s also a difficult perennial for many of us to grow due to its susceptibility to mildew and rot diseases. The showy, fragrant native attracts butterflies and hummingbirds and has an extended flowering season through frost, making it well worth a try.

Since no border is complete without tall garden phlox, aim for a disease-resistant variety in climates with humidity or high rainfall during the growing season. ‘Jeana’ takes top place in Mt. Cuba Center’s phlox trials, showing exceptional powdery mildew resistance, a long flowering time with a spectacular display, and a big pollinator draw.

For best growth, phlox needs organically rich, well-drained soils. Deadhead spent blooms to encourage reblooming. Ensure good air circulation, and cut back and remove plant debris in late fall to prevent harboring fungal spores over the winter.

Peony

	botanical name Paeonia officinalis
	sun requirements Full sun
	height 2-4’
	hardiness zones 3-9

It’s no wonder peonies are a temptation with their decadent, fully petaled blooms and handsome, bushy foliage. They make an attractive backdrop when not in flower, and when they are, their sheer beauty and fragrance take hold of the display.

Peonies are the joy of cool-climate gardeners with mild summers, where the perennials are long-lived. They require a chill period in winter to flower in spring, and need summer temperatures below 80°F (27°C) for full vigor. Their extensive roots are slow to establish, and tubers planted a mite too deep (beyond one or two inches) show hindered growth. Once they’ve settled in, peonies are sensitive to transplant disturbance, which can delay blooming for a couple of years.

More heat-tolerant and adaptable varieties like ‘Festiva Maxima’ show improved growth and flowering in warmer regions, but are iffy in hot, humid growing areas with warm winters. Diseases like botrytis and powdery mildew are a concern, as are sap-sucking pests and Hoplia beetles. Common garden peonies need full sun to flower, with afternoon shade protection from intense summer rays. Regular, even moisture is ideal during the growing season.

Dahlia

	botanical name Dahlia spp.
	sun requirements Full sun
	height 3-5’
	hardiness zones 7-10

Dahlias are prized the world over for their dramatic blooms in an array of colors and petal arrangements, from pompom to cactus, and from petite to dinnerplate. Their nearly perfect petal arrangements create intricately symmetrical and varying forms. Stunning in the summertime garden, dahlias also make standout cut flowers.

With a low threshold for cold temperatures and extreme heat, and with a long development time, dahlias can be high-maintenance. They benefit from lifting and storing the tubers in cold climates to ensure reliable performance. Cool nighttime temperatures and warm days are best for them to grow and flower. On average, 55-60°F (13-16°C) nighttime temperatures and 70-80°F (21-27°C) daytime are ideal. 

Dahlias do well as annuals where they are not hardy, growing quickly in a single season. In late fall, dig up the tubers and store them over the winter for replanting in the spring, or start with fresh selections. They can take up to 100 days to flower from planting. In climates with short growing seasons or to get a jumpstart on development, try pre-sprouting the tubers for earlier blooms.

Dahlias grow best in full sun with evenly moist, well-drained soils. While they tolerate partial shade, more sun increases their vigor and flowering.

Bearded Iris

	botanical name Iris x germanica
	sun requirements Full sun
	height 1-4’
	hardiness zones 3-9

If you have a healthy crop of bearded iris, you may not believe that it makes the list, but the Mediterranean perennial is prone to certain pests and diseases like iris borer, rot, and leaf spot. Irises tolerate various soil types, from acidic to clay to sand, with amendments for drainage. They grow best in well-drained conditions that aren’t oversaturated. Aim for even moisture during the growing season, but avoid too much water, as they are accustomed to drier summers.

Bearded iris bears large, ruffly flowers in a rainbow of colors in spring and early summer, with a repeat flush in fall. The three upper and three lower petals (falls) may have frilled edges and a wash of color. Centers often contrast in bright white or yellow.

Skip the mulch for irises to avoid prolonged moisture retention. Grow them in full sun for the best disease resistance and flowering. If you’re ready to divide your colony or give bearded irises a try, August is one of the best times to plant and divide them.

Gardenia

	botanical name Gardenia jasminoides
	sun requirements Full sun to partial shade
	height 4-8’
	hardiness zones 7-11

Gardenias persist in old southern homesteads, growing carefree as fragrant woody shrubs with year-round flowers. But outside their specific conditions of moist, sandy, acidic loams and warm, humid weather, they can be picky. They flower in late spring and early summer with milky white blossoms bathed in an intensely sweet fragrance. 

Where not hardy, try growing them in pots. Or opt for more cold-tolerant varieties, depending on your climate and growing zone. ‘Jubilation’ has whirling blossoms on a rounded form and is more cold-hardy than others. ‘White Gem’ reaches one to two feet tall to fill a small space with high fragrance.

Bring potted specimens indoors to overwinter, trying a spot near a bright window. Gardenias are finicky if conditions aren’t quite right inside, so plan to move them back outside as soon as temperatures warm in spring. Outside, they appreciate consistent moisture. A common occurrence is yellowing leaves, remedied by a fertilizer for acid-loving plants that contains iron.

Bleeding Heart

	botanical name Dicentra spp.
	sun requirements Partial to full shade
	height 9-18”
	hardiness zones 3-8

Bleeding heart is easy to grow given optimal growing conditions, including soils that remain consistently moist in summer and dry in the winter. Balancing these needs, as well as moderate conditions, can make it somewhat difficult in hot climates. With species native to eastern North America and to Asia, Dicentra and its cultivars adapt well to dappled and canopied situations, but struggle in the face of damp winter conditions and dry summers.

Bleeding heart emerges in spring and enters dormancy in the summer heat. Puffy, heart-shaped blooms hang delicately in pink, fuchsia, and white from slender stems. It self-seeds where it is happiest, but is slow to establish, taking two to three years to flower from seed.

The seedlings are sensitive to transplant disturbance, and moving them can delay flowering by another year. It’s essential that soils are well-draining and that they’re not overly saturated during winter months. But don’t allow them to dry out during the growing season, which hinders vigor.

Look for wild bleeding heart (Dicentra eximia), a North American native, with suspended pink hearts. The finely cut foliage is ferny and feathery and withstands drier conditions once established. 

Azalea

	botanical name Rhododendron spp.
	sun requirements Partial to full shade
	height 2-10’
	hardiness zones 4-8

Like gardenias, azaleas present challenges outside their warm, humid range due to their specific needs. They prefer sandy loams, an acidic soil pH, and partial shade, depending on the growing area and variety.

Durable and carefree in their ideal conditions, azaleas bring high spring color with trumpet or fringey blooms that attract pollinators early in the season. North American native azaleas hail from various woodland environments, with cultivated varieties from Asian species possessing traits like reblooming and large ruffled blooms in rich colors.

Indoor or florist potted azaleas can be difficult, too, to transition from a commercial greenhouse to the home environment, where they’re short-lived houseplants. These are primarily from two species of azaleas, pruned and forced to flower in winter to brighten the season.

Azaleas grow best in rich, well-drained, evenly moist soils. Try a native species suited to your growing zone and conditions for a hardy twist on the cultivars.

Staghorn Fern

	botanical name Platycerium bifurcatum
	sun requirements Partial to full shade
	height 3’
	hardiness zones 9-13

Staghorn ferns are stately tropicals with the capacity to live for decades under the right treatment. The blue-green, handsome fronds are broad and deeply lobed, resembling forked antlers. Staghorns develop copper papery shields around frond clusters. The shields multiply and overlap as the fern grows. Its stately form and long-lived nature earned the species the Royal Horticultural Society Award of Garden Merit.

Like orchids, staghorns have epiphytic roots that absorb water and nutrients. Their shields capture nutrient-rich debris from the tree canopy that breaks down for the plant to use in growing. Consistent moisture is best, though mature specimens tolerate dry spells. Allow slight drying between waterings.