The garden peony is a popular, long-lived perennial that provides beautiful flowers in spring and handsome foliage throughout the growing season. If left undisturbed, a peony plant may flower for 50 or more years. Most garden peonies grow 2 to 3 feet tall. Mature plants may be 3 to 5 feet wide. Peonies bear 3- to 6-inch-diameter, fragrant flowers in May or early June. Flower colors include white, pale yellow, pink, rose, and red. Garden peonies are classified into five types based on flower form: single, semi-double, double, Japanese, and anemone.
Double-flowered peonies are the most popular. The stamens of double-flowered peonies do not bear pollen and resemble the wide outer row of petals. Single-flowered peonies have one row of petals surrounding a cluster of yellow stamens. Semi-doubles have multiple rows of petals surrounding a cluster of yellow stamens. Japanese peonies have five or more large petals surrounding stamens that bear no pollen. Anemones are similar except the stamens are wider and more petal-like.
The best time to divide and plant peonies in Iowa is September. Planting later may not allow enough time for the plants to become established before winter. Plant peonies in a sunny, well-drained site. Do not plant peonies near trees or large shrubs. The shade cast by the trees and shrubs, together with the competition for water and nutrients, will discourage plant growth and flowering. Root rots may be a problem in wet, poorly drained soils.
When dividing peonies, cut the stems near ground level. Carefully dig up the plants and wash or gently shake off the soil. Divide the clump into sections making sure that each section has three to five eyes (buds) and a portion of the root system. Divisions with fewer than three eyes may take 3 to 5 years to produce flowers.
Prepare the site by digging a hole 1 to 1 1/2 feet deep and 1 1/2 to 2 feet across. Mix compost or peat moss into the soil. Also, add 1/4 cup of 5-10-5 fertilizer to one-half of the amended soil. Place the fertilized soil mix in the bottom of the hole.
When planting, position the peonies so that the eyes are 1 to 2 inches below the soil surface. Planting too deeply will inhibit flowering. Fill the remainder of the hole with soil amended with compost or peat moss and water thoroughly. Water regularly during the first fall and spring of establishment. Peonies should be spaced about 3 to 4 feet apart.
Mulch newly planted peonies with several inches of straw in late fall. Mulching will prevent freezing and thawing of the soil that may heave and damage young plants. Remove the mulch as growth resumes in the spring. Take care when removing the mulch as the new shoots will be brittle.
Peonies may not bloom the first spring. In fact, it is advisable to remove flower buds that develop the first spring to promote root and foliar growth. While the plant may produce only a few blooms by the second year, flower numbers should increase rapidly by the third and fourth years. Once completely established, full flower production should continue for many years.
Ants are frequently associated with peony flowers. They neither help nor hinder flowering. Ants are attracted to the sweet nectar produced by the peony flower buds.
Fertilize plants in early spring as the new shoots begin to emerge from the ground. One-fourth cup of 10-10-10 fertilizer should be lightly cultivated into the soil around the crown of the plants. Avoid getting fertilizer within 6 inches of the crown. Peony crowns and young shoots are very susceptible to fertilizer burn. Water the area immediately after the fertilizer application to dissolve the fertilizer and move the nutrients into the soil.
Many peonies, especially the double-flowered varieties, need to be staked/supported to prevent them from flopping over when in bloom. The best way to support peony plants is to place metal hoops or rings over the plants in early spring. The flowers of single-flowered and Japanese varieties are lighter in weight and less likely to flop over when in bloom.
Cultivate around plants to control weeds and break hard soil surfaces that may slow water penetration to the roots. Do not cultivate deeper than 1 to 2 inches to avoid damaging the root systems of the peonies.
Water peonies when the weather is dry. This is extremely important during bud formation and flowering. Buds may fail to enlarge and open if the plants are dry. Water by thoroughly soaking the soil to a depth of 12 inches.
Remove the spent flowers (deadhead) to improve the plant's appearance and prevent fruit formation. Fruit development reduces the amount of food the plant is able to store in its root system and may result in fewer flowers the following spring.
After a hard freeze in the fall, cut off the foliage at ground level and discard the plant debris. Removal of the peony foliage from the garden helps to control leaf blotch and other fungal diseases.
While garden peonies are the most popular, other types of peonies can also be successfully grown in Iowa.
The fernleaf peony (Paeonia tenuifolia) has finely divided foliage and produces single, dark red flowers. They are also one of the first peonies to bloom in spring. Plants commonly grow 15 to 18 inches tall. The foliage dies back by mid-summer. 'Rubra Plena' has double, dark red flowers. The culture of fernleaf peonies is the same as for garden peonies.
Tree peonies are actually medium-sized shrubs. Plants seldom grow more than 4 to 5 feet tall. Unlike garden peonies, tree peonies have woody stems that do not die back in the fall. Flowers may be single, semi-double, or double and are commonly 6 to 8 inches wide. They are also available in a wide range of colors. Flower colors include white, pink, and red. However, they are also available in the more unusual colors of yellow, purple, and green. Tree peonies grow rather slowly. They often take 4 or more years to bloom well.
Intersectional hybrids are produced by crossing herbaceous (garden) peonies with tree peonies. These hybrids possess the hardiness of herbaceous peonies and the attractive flowers and foliage of the tree peonies. Intersectional hybrids are available in many flower colors. However, yellow is the most common color. The stems of intersectional hybrids die back to the ground in fall.