Sunflower plants are allelopathic (or phytotoxic) to many species of plants. The allelopathic agent is probably a phenolic compound (guessing) the sunflower manufactures, which makes fewer the number of plants that are able to grow well near it.

Even when sunflower is grown as a crop, it must be rotated to prevent the allelopathic compounds from building in the soil and further diminishing the vitality of the next crop. When crop chaff is worked back into soils to keep it fertile, it would even effect the next sunflower crop if the crop was not rotated. All parts of the plant are allelopathic, including roots and seed hulls. Ever notice the low germination rate of seeds that have fallen to the ground around bird feeders filled with sunflower in the seed mix?

Al

Specifically, they contain phytotoxic

Phytotoxic...what. Phenoloics is the compound that is phytotoxic.

Scanning the literature and the scant threads here, I'll refrain from passing off as an expert but recalling chem classes, phenols are unstable and a hot pile should be able to make the compounds dissociate. If you are doing a cold pile I'd avoid and instead do the standard put out the heads for the birds and chop the residue for where you want weed suppression.

The issue is in the field where you don't have the benefit of high heat and thus rotation is needed, which is why you rarely see sunnies two years in a row in the same field.

My 2¢ & I'll defer.

Dan

The term is "allelopathy" and it is the ability of one plant to influence the growth of others by exuding specific chemicals. A good many plants have this property and it is part of their natural defense against competing species.

While I've not seen any studies that addressed sunflower specifically, the composting process effectively neutralizes this effect with various other allelopathic plants, specifically eucalyptus and black walnuts - two plants that are generally considered to present this allelopathic ability to a severe degree. And to confirm this contention further: "Both abiotic and microbial decomposition will have significant effects on the concentration of allelochemicals reaching other plants. Soil microorganisms ingest allelochemicals as energy sources, and metabolic decomposition can render the chemicals non-toxic to plants. Composting all questionable plant material will reduce the chance of a toxic reaction." Elmer Belssner, TAMU Extension

I'd say go ahead and compost your sunflowers :-)

I routinely compost sunflower stalks, leaves, and flower heads. Our finished compost is used in our garden, and also to amend the soil when blueberry shrubs are planted. In 15 years, I have seen no ill effects from our compost. However, we have an open compost pile, with a pit underneath. The pit is about 5 feet diameter and one foot deep. I move the pile and dig out the pit every spring, in April or May. The finished compost will have been in the pile for about 18 months at that point. Our pile is mostly browns, & it does not get very warm, to my knowledge. I never turn the pile, but I do water it with a hose when there is a drought.

'Phytotoxicity' poisons, 'allelopathy' inhibits is how I remember it. So the question in the OP as GG expands it is whether the sunnies poison and/or inhibit. Eric's cold pile and time adds to and bolsters the statements above that the phenols leave the material (as they are volatile).

We always use our sunnies for fall decoration then chop for mulch for weed suppression, so they don't go in our pile so this is all guesswork on my part...

Dan

It doesn't really matter what the plant does when its alive... once its composted and no longer alive, its not that plant anymore... the material has decomposed back into its base components of carbon and nitrogen and no longer contains the chemical properties of what it once was....

And that sums it up very nicely :-)

And certainly with more brevity than others here *coughDan'lcough*!

Dan

While I have seen much about the allelopathic properties of sunfowers and their seeds I have not seen evidence of that. Except that the wild turkeys scratch vigorously around the feeders, often digging holes in the area, plants of all kinds grow around the feeders including sunflowers from the seeds that do not get eaten.

To all,

Thanks for your input, great info. I will be composting them.

Decomposition of any organic matter is always poisonous.
Even in your (and my) stomach. That is why we have liver. Otherwise we could die in few days.
Poisons in compost is not a problem because those poisons are also organic substances and will be decomosed in further stages of decomposition. Organic poisons can be very strong, such as botulinum. It is normally produced during anaerobic decomposition of cellulose by bacteria botulinun which take place in compost also. So, unfinished compost cam be toxic for plants also. But it's not a problem. Just continue composting untill compost be "finished".
Everything can be composted. Everything toxic in the begining and untoxic (and even healthy) in the end. Healthy because those mycrobes that had destroyed toxins will continue destroy them in the soil also.