Planting in spring isn't wise, as peonies bud at this time. Bare-root tubers or overwintered transplants can be planted in spring, but they won't flourish in the first year.
Plant peonies in early fall when the soil will be firm to prevent heaving during frosts. Planting early will help them succeed, as their feeder roots will have time to develop.
Rootstock Quality
Check the roots for the number of eyes (pinkish-red protrusions), eye-to-root length, and overall quality. Avoid pre-packs of soft, squidgy, or wrinkly rootstocks.
Choose roots with three to five eyes, or you can divide them. The crown needs at least two roots that are ½ inches across and 6 inches long, and check for fungal decay or hacking.
Too Deep
Peonies planted too deep can result in a season of no-shows. The root crown should be nearly flush with the soil, with the upward-facing eyes buried at ½ to 2 inches.
Plant tree peonies at 4 to 5 inches to promote a new root system. Bury intersectional peonies at ½ and 1½ inches in warm and cold climates, respectively, and seeds a ½ inch deep.
Nursery owner Billy Carruthers says, "The mistake people always make is to keep them [soils] too dry." They need moist, well-draining, rich soil but not too much water.
Loamy soils with a pH level of 6.5 to 7.0 are best. To improve water absorption, enrich clay soils with compost or pine bark, opt for raised beds, and add lime to low-pH soils.
Avoid planting peonies too close together, as overcrowding creates leggy stems that don't complement the sumptuous blooms. Try to space the plants 3 or 4 feet apart.
Peonies dislike sharing light, water, and nutrients, so follow the same spacing rule for companion plants. Lilacs absorb the same nutrients, and black walnut trees release toxins.