Wouldn't the sunflower and tomato plant be competing for nutrients and water, besides light? It would sort of be like letting a giant weed grow next to your tomato plant.

I have heard about this b4 and had the same concern as you Californian. Shade would be a blessing but think it might not be too good for the tom plant? Linda

"I'm not getting as many tomatoes as I have in the past, though, and I'm still waiting for my first ripe one. Is it the shade, or maybe our wonky weather this year?"

I suspect that you may be getting lower fruitset on your tomatoes because sunflowers do have some allelopathic proerties. Just how much has been widely debated, some people see little effect, and others see more, but it is a known fact. It could be due to whether or not sunflowers have been grown in that location previously or the variety of sunflower being grown.

There is a Colorado State paper by Amanda Broz (2006) on the "Allelopathic potential of sunflower (Helianthus annuus)" that says in part that different varieties of sunflowers exhibit differing abilities to inhibit germination or growth of various plants.

I am including a link to a Google Scholar search for sunflower alleopathy. You can decide for yourself if this is something you want to repeat, especially if you grow your sunflowers in the same location each year as the effect seems to build up in the soil if the plants are planted successively. (Commercial sunflower growers rotate follow their sunflower crops with a different crop each year to avoid the allelopathic buildup in the soil.)

Betsy

Here is a link that might be useful: Sunflower Allelopathy

Very unusual approach.

In addition to the nutrient and water competition, Bets hit the nail on the head IMO with the allelopathic relationship. And it will only increase as the sunflowers age. All in all, not a good choice for tomato supports but I guess you are stuck with it for this season. Good luck.

Dave.

In my opinion, I will tell you based on the words of your post, that your problem is the wacky weather. As Betsy clearly states, there is debate regarding allelopathic theory.

Debate in itself means nobody knows what the truth of the matter is. And that is it. I think the last sentence of your post is the answer.

Just feed the soil a bit more along with water. I have a hard time believing shade is an issue,along with any properties regarding 2 plants grown closely together.

Then again, I have many issues regarding recent studies. We just do not live long enough to know what the truth is.We can try though,and that usually makes us feel better.

Take care,
Travis

For the last 2 years I have grown a mix of sunflowers ( from giants to small varieties)along the highway fence in front of my property. Just on the other side of that fence I grew squash, and melons 2 years ago and last year I grew 35 tomato plants as a crop rotation. Both years, I did not have any adverse reactions from the sunflowers. This year I will be growing my sunflowers again (same spot), but growing only the big giants and the crop rotation this year, on the other side of the fence, will be corn. I use the sunflowers as privacy screen during the summer/fall months and the birds enjoy the seeds all fall/winter until I remove them the next spring. I would think the giant mammoth sunflower would be a very sturdy support for tomatoes by strategically planting the sunflower seeds. Before planting, the entire area was given compost and rotting manure. I had such an abundance of tomatoes last year and the year before squash that kept on growing into November.