Peonies need all day sun in Western WA, in an uncrowded bed. Half day sun (morning) is sometimes preferable in other warmer parts of the US, particularly the deep South and southern California.
Bed preparation should be #1 on your list of things to do. The soil on the west side of the Cascades generally needs some amendments such as sand, compost and the addition of new top soil. Avoid manures unless worked into soil a few years in advance of planting peonies. (Even then, in the cool springs and summers of Western WA, there just isn't enough heat consistently to rapidly and thoroughly compost manure atop the soil or even well worked into it, so beware!) Raised beds are best. Don't use wood chips as a mulch on your peonies or anywhere on your perennial bed. Beauty Bark, as it's known in Western WA, is fine for applications around trees and shrubs, but it's been known to kill perennials including peonies.
Some garden lime,
In Western WA your soil will need garden lime. Add three heaping handfuls to each hole and work well into soil before planting the peony root. You won't need the lime, if you live in an area where you have alkaline (sweet) soil.
Any brand of time release fertilizer from 10-10-10 to 14-14-14 will do. About a level quarter cup or less for each well established large plant. Apply early in spring no more than once a year. Use fertilizer in moderation, strictly according to directions. Too much fertilizer stresses the plant, causes spindly growth or invites disease. There are tons of plant foods out there that somebody would like to sell you. On the other hand, some peony professionals would tell you that if need to add plant food to your peonies, you've got them in the wrong place.
The Correct Depth,
Plant herbaceous peony roots 1/2'' deep, presuming the uppermost eyes will not sink below 2'' in loose, freshly cultivated soil. In California or the South, plant the eyes of the roots at ground level. Plant tree peonies deeper. The graft should be at graft should be at least 4 inches underground so the tree peony can start growing it's own roots.
& A Little Patience.
Herbaceous peonies may not bloom the first year after planting and will take about three years to bloom profusely. Tree peonies may take even longer. There's no way to rush it.
A few other things to consider:
The Right Place
Start off in the right place—in full sun; in good, loose, friable soil; and preferably in a raised bed. If the soil’s too sandy, add amendments, such as fresh soil and compost. Excessively sandy soil can sink peony roots and stunt growth. Clay soil should be amended with sand and compost and formed into a raised bed to facilitate drainage. Also, don't plant your new peony divisions under eaves or big trees (too dry) and don't plant too close to shrubs or trees.
Bare root peonies may be planted in the fall, from September through November in Western WA, or even later. If the roots arrive looking dry, soak in water for several hours. Plant roots as soon as you get them. And, though unlikely, if there’s a bit of greenish or whitish mold on the roots when you open the package, simply wash off with water. This mold is not harmful.
Suckers on Tree Peonies
Sucker roots might appear around your tree peony stems in spring. If they look different than the rest of your tree peony, they're likely herbaceous stems from the host root. We've had some luck in swiftly yanking such stems up and off the root. If we're lucky, the stem severs just at the host root junction. To really solve the problem, however, (and avoid this kind of violence to your plant) as soon as you can tell your tree peony has grown some of its own roots, dig it up in fall and cut off all the root below the original graft. Keep cutting back suckers, until you can dig the plant. If sucker roots are left to grow for any length of time, they will diminish your tree peony or worse. Picture: On the left are three herbaceous suckers with sample of herbaceous foliage.
Botrytis is an airborne fungus that can affect peonies growing any where and can especially be a problem in prolonged wet, cool weather, such as we might have here in the Pacific Northwest. Botrytis can virtually be prevented if you effect a spray program before any botrytis damage is done. Younger peony plants and overfed plants are often most vulnerable. Botrytis can also be minimized and controlled even if your plants get a touch of it.
Signs of blight are stems that rot off at the base or dense gray mold on stems or leaves, blackened buds or blooms or totally limp stems. Immediately clean your plant— that is, cut off all affected parts. If using your all purpose garden clippers, wipe with alcohol after peony use.
If only the tip of a leaf is affected, however, only remove the tip, not the whole leaf. Leave as much leaf as you can. Removing infected parts might be all you need to do if you have just a few peonies. Otherwise, a spray program may be necessary. For best results start your spray program when stems are up 5 or 6 inches then continue spraying once every week to 10 days until bloom time. Spray the plant, drip spray down the stem into the ground and spray the ground around the plant.
Fungicides approved for peonies include captan, copper sulphate (try Lilly Miller Micro-Cop), Daconil (chlorothalonil), Mancozeb, Maneb, Bravo or professional fungicides such as Decree, Chipco 26019 or Cleary’s 3336 (thiophanate methyl). A new systemic fungicide is TM85 and another is offered by Cleary's. Follow label directions carefully. Mixing up such sprays carelessly and adding too much surfactant can sometimes cause more damage than the botyritis.
Occurrences of botrytis are a natural part of growing peonies, but we've never had to destroy a peony because of it. It rarely harms the root itself, unless left untreated for years. Steubers in Snohomish, WA is a good source for fungicides locally (360-568-2626). Or find a retailer in your area. Approved fungicides vary from state to state.
Fall Clean Up
In fall the leaves of your peony will pale as the plant enters dormancy. By the end of November cut all stems at ground level and destroy them (do not compost, or you might be saving botrytis in your compost pile). In cold winter areas, all first year peonies should be mulched with clean hay, straw or pine boughs after the first freeze to prevent the ground from heaving. No mounds of pine needles or bark mulch over your peonies, please. Clear away mulches in early April.
If you'd like to learn more about peonies, opportunities abound on the web. The American Peony Society has a website. The Heartland Peony Society has a wealth of information. There's also a chat group with many members, among them some distinquished and respected peonists and peonistas.