I don't know anything about Preen, but I have had great results using only mulch. I just use a layer about 2 inches deep, have very few weeds, and my peonies seem to love it and are doing great. When they're mature, they shade the ground below and that helps as well.

I use PREEN, with the fertilizer, in my flower beds. The peonies get better every year!

That's good news!! I love Preen - works great. Thanx! Sue

I used Preen or the first time, last week, and the peonies were already taller than 3 inches, however, some of the leaves are now curling. I've had these peonies for over 15 years. The only difference this year is the preen. I am getting very worried. I wouldn't use it. Let's hope they survive this year and are able to produce blooms and come back next year. Wondering if I should move the plant even.

Most people have had success with using Preen AFTER peonies are 4-6" tall. (source: various horticultural forums) I've tested and used it over non-listed flowers and never found a problem. Pendulum is a fairly broad preemergent that is list for "Chinese Peony". I keep that and oryzalin products (ie Surflan) on hand, and the two of them cover everything in my landscape. Preen is trifluralin, pretty benign, and I use the dry sprinkle for small areas or touch up if I disturb a treated area.

I've used the fertilizer Preen on my sweetcorn, pumpkins and tomatoes for years with zero known problems. I apply it after plants are about 4 to 6 inches tall, and mulch with lawn clippings after. Last year I use Preen on my new peonies ( 8 total ) and it worked as it should without any noticeable problems with the peonies. One of them even got a flower. Today I just finished planting 26 new ponies, and I fully intend to use Preen next spring. The one very important aspect to remember is that you really need to make the correct application and never over use the product, because even the fertilizer it contains, if over applied, will certainly burn and or kill any plant. Any fertilizer is fully capable of killing plants. Best of luck to all.