How to Plant, Grow, and Care For Itoh Peonies

Peonies enchant with petal-packed semi-double and double blooms, full-bodied and ruffled with a sweet fragrance to match their loveliness. They bloom once a season, from late spring to mid-summer, with a huge show among herbaceous perennials. Peonies can live for a century, often persisting in historic gardens and old homesites with little care. 

Itoh peonies are intersectional hybrids that bring the longest bloom time of all the peonies. Their variations in color and floral form bring dramatic interest and decadent appeal in the warm season. With color flares and overlays, the multitones delight on single or semi-double petals. They bring colors not found among other herbaceous peonies, including the brightest yellow in cultivation.

Cold hardy and with hybrid vigor, Itoh are exceptional performers in the peony realm. With sturdy stems, good disease resistance, and hefty flowers, the prized selections are collector-worthy additions to the early summer display. Let’s explore how to grow itoh peonies!

What Are Itoh Peonies?

Itoh peonies, also called intersectional peonies, are hybrid crosses between the common herbaceous peony (P. lactiflora) and the tree peony (P. suffruticosa, P. x lemoinei, and others). The name comes from its creator, Japanese plantsman Toichi Itoh. Itoh made the first successful cross of the pairing in 1948, among the reportedly 20,000 attempts during his career.

After Itoh’s death in 1956, American enthusiast Louis Smirnow brought his plants to the U.S. a decade later. He registered them as Itoh Smirnow hybrids. Breeders went on to make more successful hybrids with these as a foundation and changed the peony landscape. Itoh peonies are seeing a rise in popularity due to their decadent blooms, extended flowering, cold hardiness, and robust forms. Because of their challenges in propagation, these intersectionals are more costly than other types.

Peonies are a fit for the cutting garden and perennial bed. While they steal the show, they also make good partners with plenty of companion plants. Pair them with geranium, agapanthus, salvia, and spring-blooming bulbs. They are mildly toxic to dogs and cats, so keep them out of range of curious pets.

Characteristics

Peonies bloom at different times in the season, depending on the variety. For a full-color display, select early, mid-, and late-flowering cultivars. Intersectionals fall into the “late bloomer” category and flower later, beginning in early summer. The enormous blooms reach up to eight inches across. Itoh has narrowly lobed leaves in rich green, finer than herbaceous varieties. With good disease resistance, they remain attractive throughout the growing season.

The intersections represent the best of the herbaceous (common garden) and tree peonies. Like herbaceous types, they have a bushy form and sturdy stems that die back to the ground in winter. Like tree peonies, they also have woody stems and dramatic flowers. Compact at two to three feet tall with strong stalks, they need no staking.

In addition to its well-branched form, Itoh stands out with one primary bud and three side buds that lead to several weeks of blooming as they open in succession. The main bud opens first, followed by the side buds, which may be even larger and more full than the first.

The hybrids enter winter dormancy, dying back to the ground with heavy frost. They grow from tubers (fleshy, starchy, enlarged roots) and resprout beneath the soil surface in the spring, where tender new shoots are protected from freezing.

Native Area

Peonies are native to the northern hemisphere, particularly Europe and Asia, with two species native to Western North America. They flourish in a Mediterranean climate, where summers are warm and winters are cool and wet. In hot climates, they’ll struggle to achieve the winter chill they need to thrive and face heat and humidity stress.

Itoh’s parent species experience warm, sunny days and cool nights in their natural growing areas. They grow best in moderate climates with plenty of precipitation and sunshine.

Planting

Plant peony tubers in the fall for best success. This timing gives them a chill blast while allowing growth before summer’s heat. Peonies are slow to develop and, once planted, don’t respond well to disturbance. Nursery starts are further along, usually around two years old, and do well with a fall or early spring planting.

Peonies require good air circulation between plants, so they need proper spacing and pruning. Allow three to four feet between Itoh and other specimens.

Position tubers one to two inches beneath the soil surface. If they’re planted too deeply, they’ll fail to flower. For nursery starts, plant the crown level with the soil surface as it is in the pot. The buds on tubers should stay at one to two inches deep.

Tubers and leafy specimens also grow in containers large enough to support their mature size. Itoh has deep roots and prefers to be in a permanent location. Ensure pots and potting media are well-draining and plant as you would in-ground specimens.

Transplanting

September and October are the best times for transplanting peonies. The move or division may delay booming a year or two.

To move them, cut stems to the base. Capture and lift the roots by digging a berth of six inches around the crown. Transplant immediately to the new garden location and water in well.

How to Grow

During the growing season, Itoh peonies store energy to continue root development for future seasons. Regular moisture, good soil drainage and aeration, and airflow are essential to overall health and flowering.

Light

Itoh prefers full sun for the biggest and best blooms. They thrive with six to eight hours of sunlight daily. In climates with intense summer sun and heat, offer afternoon shade protection. Blooms also last longer with a bit of shade protection. 

While they tolerate partial shade, the sun-lovers become less vigorous with fewer flowers without adequate sunlight.

Water

Peonies rely on regular moisture. They withstand periods of drought but won’t flower under stress from hot or dry spells. Prolonged drought hinders growth and causes them to enter conservation mode and drop leaves. Provide supplemental irrigation during dry spells to retain green, leafy vigor and budding.

Consistent moisture usually means watering once or twice weekly in the growing season. About an inch of water per week, whether from rain or supplemental irrigation, is sufficient. Itoh doesn’t tolerate saturated soils. Ensure good drainage and increase frequency during dry spells.

Water at the base around the perimeter of the leafy canopy to reach all the roots. Water deeply to reach the extensive root system and moisten the surrounding soil.

Soil

Paeonia prefers organically rich soils. Slightly acidic to neutral soils with a pH between 6.5 and 7.0 are prime. They tolerate poorer soils but may show slower growth and flowering.

Amend the soil with compost, leaf mold, or aged manure at planting. Itoh relies on essential elements and minor nutrients to store energy for flowering. In poor soils (sandy or clay), incorporate two to four inches of compost derived from completely broken-down plant material. Compost helps with aeration, moisture retention, drainage, and nutrition. 

Just as good drainage is essential, aeration is important in supplying oxygen to respirating roots. In compacted soils or aged potting media in containers, peonies may show stress. They have extensive root systems and healthy soils up to two feet deep ensure the best foundation.

Temperature and Humidity

These flowering perennials grow best in temperate to cool climates. On average, 55-60°F (13-16°C) night-time temperatures and 70-80°F (21-27°C) daytime are ideal. In hot climates, they may not thrive. Provide shade protection from the midday heat and keep them out of drying winds. Offer plenty of moisture to those in more sun in warm growing areas.

High humidity may lead to fungal problems like stem or crown rot and leaf spot. These may be more of an issue in spring’s cool, damp conditions. Ensure good air circulation with the proper spacing.

Mulching helps regulate temperatures in spring and summer by keeping roots cool and retaining moisture. For overwintering protection, add more mulch before cold winters, especially in zones 3 and 4.

Fertilizing

Nutrient-rich soils ensure health and vigor all season long, but the showstopping blooms require a lot of energy in a single season. Consider an all-purpose organic fertilizer as new growth emerges in spring. A slow-release granular 5-10-10 or 5-20-20 organic fertilizer promotes blooming without sending too much nitrogen into leaf and stem production. 

If not using a slow release, provide three applications of a low-grade supplement. The first is as new growth springs up, the second as buds form, and the third in late summer to support energy stores in the tuber. This can be from kelp, seaweed, fish emulsion, alfalfa, or a phosphorous-rich formula.

Maintenance

Regular deadheading promotes a tidy look. The robust blooms also make gorgeous fresh arrangements.

As Itoh peonies die back after fall’s heavy frost, remove brown and dry stems and leaves. This cutback prevents fungal spores from developing or overwintering to affect the next season of blooms. Bulk up the mulch layer for seasonal insulation.

Propagation

Itoh peonies propagate through tuber division. The cultivars are sterile and don’t produce seeds. While they don’t need it regularly, divide peonies to expand the colony or if crowding becomes an issue. They’ll show reduced flowering when it’s time to divide.

With their extensive roots, peonies are sensitive to transplant disturbance. It will take some time to restore energy for flowering, likely a year or two.

Division

In the fall or spring, dig tubers using transplanting guidelines, capturing roots and tubers. Divide them using a clean, sharp blade. Tubers need “eyes” (buds) to sprout; try to capture three to five bud eyes per piece. Smaller portions will take longer to develop.

Popular Varieties

Choosing your new Itoh is the most exciting apart (aside from its stunning blooms to follow). With colors unique to the intersectionals and a later bloom time, pair them with other Paeonia favorites to start the show earlier.

‘Bartzella’

‘Bartzella’ is one of the most distinctive yellow Itoh cultivars with soft lemon petals in double and semi-double arrangements. A scarlet blush highlights the centers. The flowers are uniformly spaced atop deep green feathery foliage, and mature specimens are prolific producers.

A sweet fragrance accompanies the pastel hue. The massive blooms measure six to eight inches across. Its robust growth and flower characteristics earned ‘Bartzella’ the American Peony Society Gold Medal and the Royal Horticultural Society Award of Garden Merit. Mature colonies produce up to 80 blooms.

‘Hillary’

‘Hillary’ is another Gold Medal and AGM award winner with watery petals in pink shades. Light silvery and deep tones intermingle on each petal with richer cerise interiors. Fuzzy yellow centers highlight the centers.

Outer petals age to creamy pink for color variation per bloom on each plant. ‘Bartzella’ is one of the seed parents, contributing to its vigor and glowy ivory-yellow tones as flowers mature.

‘First Arrival’

One of the earliest to bloom among Itoh cultivars, ‘First Arrival’ is a mid-season peony that welcomes early summer in the prettiest of pinks. Tissuey petals with lilac overtones and deep magenta centers have ruffled edges for a crape paper likeness.

To match its coloration is a rose fragrance, adding further dimension to the large, semi-double flowers that reach eight inches. Established plants yield 30 to 40 blooms. Yellow stamens draw pollinators as petals open fully. ‘First Arrival’ is another Award of Garden Merit recipient.

Common Problems

Fortunately for the beautiful bloomers (and their gardeners), pests and diseases aren’t usually issues for Itoh. Botrytis and powdery mildew are occasional problems.

Planting in full sun with plenty of air circulation and regular moisture helps stave off issues.

Pests

You may notice ants on the buds. They’re frequent but harmless visitors with no ill effects. Ants feed on sweet nectar from the plant and may deter other pests from invading.

Diseases

Botrytis is a fungal disease especially prevalent in high-humidity conditions with limited air circulation. Brown spots and “fluffy mold” on leaves and stems are indicators. Take care not to overwater or overfertilize peonies, which weakens the plant. Avoid overhead watering or prolonged moisture on leaves if botrytis is an issue.

Powdery mildew is another common fungal disease indicated by a gray-white powdery substance on leaves, stems, and buds. Leaves may distort and drop.

Frequently Asked Questions