I am going to venture an educated guess based on my experience with dune sunflowers. Just my hunch. Years ago I lived on the water where the soil was almost pure sand. The drainage was perfect. The duneflowers thrived without irrigation and bloomed ten months of the year. Even there, they looked a bit ragged if we had long bouts of very rainy weather. I loved those dune sunflowers and have tried to replicate that success. I no longer live on the water and my soil, although started out very poor, through adding organic material, has improved. I tried them again this year and although during the dry season they looked pretty good (not great, great), in the last month they are turning brown in areas and the flowers are small. I am about to pull them completely. Mind you, they are in an area that does not get irrigated and is an elevated bed.

I would guess that yours are having similar problems. They are getting much more water than they like (rain and humidity, maybe even irrigation), making them prone to diseases and pests. The tilling temporarily disturbs the natural drainage created by creatures that live in the soil (earthworm tunnels, etc.). The tarp increases the moisture in the underlying ground. I don't think any amount of spraying will make them better.

I think dune sunflowers need very consistently poor, dry soil with excellent drainage and as much sun as they can get. They may look good at the beginning, but consistent moist, rich dirt will make them sick and pest prone. Just my hunches. Maybe someone else has other ideas. They sure are pretty and worth trying.

Anna

there are two subspecies of h. debilis, h. debilis debilis and h debilis cucumerifolius. i can never remember which is which, but one is erect and one is more prostrate. the erect one tends to be perennial, and the groundcover one tends to be annual. i grow the annual ssp. this time of year, it always starts to die back. it reseeds prolifically in my yard, esp in mulch but sometimes in grass.

in other words, if yours is dying, it's probably because it's been blooming its head off for months and now is the time for summer blooming annuals to die.

i'm sure anna's right and that there are some cultivation issues involved. but i've never kept a single one of the hundreds of my plants alive through winter.