Growing peonies in Arizona is a challenge in the desert areas of the state, such as Phoenix and Tucson. The summer temperatures get too high, over 100 degrees, for peonies to thrive. Flagstaff, Payson, Prescott, the high country, White Mountains and Rim country have mild summers, so it's possible for peonies to grow in those locations. Peonies come in every color except true blue. Plants range from 6 inches tall to more than 4 feet. The flowers range in size from 2 inches to 10 inches in diameter.
Select the location. Peonies need full sun for the majority of the day. Where summers are on the warmer side, plant where the peony will receive mid- and late-afternoon shade. The soil should be rich and not boggy. Clay soils produce more flowers but slower growth. Well-drained soils produce faster growth but more greenery and less blooming.
Prepare the soil. Dig a hole 2 feet deep and 2 feet wide. Mix the soil with compost or organic material and add fertilizer as the package directs. Replace most of the soil, leaving a trench about 4 inches deep.
Plant peony crowns in the fall before the last frost. They may still be planted in late fall if the ground isn't frozen. Place the peony crowns in the trench with the pointed ends up and the root ends down. The pointed ends contain the growth for the new season. The tips of the pointed ends should be no more than 2 inches underground.
Plant peony plants in the spring and summer. The soil should be at the same level on the peony stem in the garden as it was in the pot. If you place the plant too deep, the plant may not thrive, or it might even die.
Water well after planting, during the flowering season, and immediately after flowering when new shoots are forming for the following year. Peonies in the desert areas of Phoenix need water every day. Supplement rainfall in the higher elevations of Arizona, if needed.
Cut the plant to the ground when it turns brown in late summer. Remove the foliage and throw it away. Don't use it in the compost heap or as mulch, since it may contain fungi.
Cover the peonies with a 4-inch layer of mulch in Flagstaff and in the higher elevation areas of Arizona for protection during the winter. Remove the mulch in late spring, being careful not to break off the new growth.