Maybe you are watering too well. Usually yellow leaves are one of 3 things--too much or too little water or lack of fertilizer. Since you say you water and fertilize it is more than likely too much water.

Petunias like to dry out between waterings. Stick your finger into the soil. If it feels wet half way down your pot then don't water. If you use water soluable fertilizer water and then fertilize. Fertilizing a dry plant can burn the roots

No, too much fertilizer.....you are burning the roots and they can't take up the water.
Lay off the fert and water water water to flush the excess out of the soil.
It's impossible to water a plant in a cocoa fiber hanging pot too much....unless you have lined the fiber with plastic.
And petunias do NOT like to dry out between waterings.
Linda C

I beg to differ lindac. For years I was an annual expert on Allexperts. If I had a nickle for every newbie gardener who overwatered their petunias and killed them I would be rich. Every expert there said don't water too much, and they are all very experienced gardeners. A coca fiber liner is made to keep plants from drying out and watering every day is too much water, unless you have excessivly hot weather.

lizkuntne--A week to 10 days is the time between fertilizing. If you have been following that then you have not been giving them too much fertilizer.

There is one way to tell if there are root problems, and that's to just take a look. If you are over-watering you will find skimpy, stringy roots and they'll separate easily from the old remnant of root ball easily. If it's not a root problem, then the roots should be hairy, light, and numerous and you can lift the whole plant out by the stem root-ball intact.

You don't say what you are using as a soil medium. Peat mix? Real soil? Heavy? Light? That'll play into how and when you water as much or more as what kind of container you have them planted in. Are the leaf vein really brown, or is it more a purple tinge? Have you been cutting these back? If so, what does the new growth look like?

Cocoa fibre is sharp draining, and you do often have to water more often than in a plastic or earthenware pot. I agree with Linda in that respect. But, you should only water when the plant tells you it needs water. And over-watering a petunia is a slow, but certain death and I agree with oilpainter on that one. More are killed with over-watering than anything I can think of. By overwatering, I don't mean too much water, you should always water a mature plant until it comes out the bottom (that's what drains off excess salts from fertiliser), but if you water too frequently, and never allow the soil to dry out, you lose the aeration in the soil, and the roots will drown and eventually rot. If you go too long between watering, however, to the point the petunia wilts to a stringy mess, you will get ugly foliage. It will usually, however, grow nice new leaves if pruned back.

That's why I asked if you are pruning back the stems occasionally. It's normal for the lower foliage of petunias to go yellow as they age.....from the bottom, up. To keep vibrantly green foliage and stubby stems, most petunias need to be taken back routinely. It causes them to break new stems and get bushier. When I sell a petunia basket in May, unless my customers do this through the summer and fall, they will no way, ever look as good again.

I have dozens of petunia urns outside in my gardens now. They all are getting the stringy look and as soon as I get a chance, I'll be just taking a walk with a hedge clipper.

Well said calliope!!

Since I have never had coco fiber pots I only had what others have told me and that was that they retained moisture. I bow to your knowledge and I'll know different now

lizlcuntne; You would do well to listen to calliope's advice. I agree 100%

Mine are yellowing, too. Not only that, but I had a beautiful blue and yellow mixed basket and I've lost all of my blue blooms. I'm kinda bummed about that.

Supertunias need more water than regular petunias. I have a summer home just down the road from the man who developed the supertunia Bordeaux and have purchased his baskets yearly. Here is the link to his site..lots of good advice on how to keep these beautiful.

www.petuniabordeaux.com