If it's not natural senescence (aging) of the individual blooms, it's probably that you're growing in full sun? Blooms are often less vibrant under full sum conditions.

There is another possibility as well. Bloom colors are largely determined by pH at the cellular level. Without going into great detail, some cultural change (fertilizer, soil deficiency or excess of a particular nutrient) can have an impact of both bloom color and vibrancy.

Al

Should have been ... "on" both bloom color and vibrancy. And yes, remove the unsightly stems. They are of no value to the plant & the plant will just shed them eventually anyway.

Al

Hmm, maybe I'll try moving them out of the sun, although the tag on the Dahlia's says full sun. The color was beautiful when I purchased them, a deep pretty pink, now they're looking terrible.

Tag says full sun, but does that mean that the plant's o/a vitality is best in full sun, or bloom color is most vibrant in full sun? See where I'm going on this? Tip: Never get dahlia or dahlietta blooms or loose buds wet. Rot fungi take a near immediate toll on petals. I'm very careful not to get the top of these plants wet while watering.

Al

I've got three dahliettas (Patricia, plus a red) in a windowbox with two small chrysanthemum multicaule. It's a 3' long windowbox, probably 6" deep and high. They're all doing poorly. The dahlietta flowers are smaller than when I first bought the plants a month ago, and the chrysanthemum seems to be dying in a dessicating manner, even though I've kept the soil moist. I have them in partial sun (1 pm - 7 pm), and put them in a rich well-draining soil mix that included greenmarket compost, peat, topsoil, perlite, and hydrosorb crystals. I originally mulched the plants with cocoa shell chips until I read here that they are susceptible to rot. Sure enough all the rain had started some mold growing on top of the mulch so I got rid of the mulch entirely. What do you suggest? Should I repot the dahlietta/chrysanthemum? I have larger planters (1'deep x 10" high x 2' long) that get less sun since they're on the ground instead of hung on my deck railing like the windowboxes are. Or should I try and give the current windowbox planting some plant food?

I had one dahlia in a pot last year and the blooms did just fine with occasional fertilizer (whenever I remembered, once a month maybe?). I had it in full sun with well draining soil. To prevent myself from needing to water every day, I planted Million Bells in the container too, to keep the direct sun off the surface soil. This year, I have the same arrangement with two dahlia plants. Each bloom "spike" will send up three buds. I routinely cut one of the bud stems off and know people who cut two off, so that the energy of the plant is concentrated into one flower instead of three.

If you have mold, they're probably getting too much moisture, not enough air movement, or not enough sun. How many drain holes are in the underside of the windowbox?

Unless it's a plant that likes to be potbound (like African Violets), I tend to give plants much more root room than they need. So I would probably put them in the bigger planter, without mulch, and with more drain holes.

If the plants look like they're suffering, I wouldn't give fertilizer on top of it. Can you remove some mulch and stick your finger in the pot to see how wet the soil is, a few inches from the top?

I have seeds for the smaller dahlias, but haven't grown them yet. Btw, zinnias fade like that also. It's probably normal for dahlias and just means the flowers are past their peak.

Greenfreak, the windowboxes have the standard three holes, and are placed within a larger wooden windowbox which has one drainage hole.

I think I'm going to move the dahliettas to my larger planters. I got some portulaca and melampodium which might tolerate the windowbox better.

Linda if you can move them around a bit. They may not like the spot you put them in (mine went from hanging on the patio fence to sitting on the patio). Dahlias love hot sun. Mine looked sick for a couple of months. My parents live in Maine. Have you been having the wet cool weather too? Just water and feed them and see what they do. It is now hot (80's) and dry and sunny here and mine have perked up and look healthy now. Mine get sun from about noon to 8pm.