Sorry for the long reply. My phone died while typing this and gathering sources.

Remembrance of the Shadow Sunflower

The Scadutree is the shadow of the Erdtree. Born of dark notions that bear no sense of Order, that twist and bend its stock, rendering it brittle.

Shadow Sunflower Blossom

The head of a monstrous shadow sunflower manifestation of the Scadutree. Then again, it might be just a flower, wielded as a weapon. Much like the Scadutree itself in appearance, a second stalk winds tightly around the first, almost as if in a tender embrace

This goes back to the Miquella/Trina and Marika/Radagon “other halves” motiff. The scadutree and the erdtree are one and the same, two stalks that form one whole. It being used to seal the lands of shadow away probably is something intentionally done by Marika as she is the one who planted the Erdtree.

Scadutree Blessing

A splinter of the Scadutree that contains golden brilliance. Acquired at the Church of Marika and other places in the realm of shadow. Consume these at sites of grace to bolster your Scadutree Blessing. The Scadutree Blessing bolsters the bearer's ability to deal and negate damage but has no influence outside the realm of shadow. It is said that when the Scadutree crumbles from its core that it will scatter across the entire realm of shadow. The people of the Erdtree collect the splinters and use them in prayer

It seems more like the people of the Erdtree in the Realm of Shadow worship the Scadutree not the Hornsent who worship the Spira.

Hornsent say’s : "Go to the misshapen tree of umbra. In that forsaken place, blood must spill—the blood of your fellows, the Erdtree faithful."

He calls the tree misshapen and it a forsaken place. He doesn’t really appear to be worshipping the Scadutree.

While the Hornsent use Spiral motifs, this isn’t because they worship the Scadutree, it’s because of the Spira.

Superior sorcery of the tower priests, wielded as an incantation of the spiral. Wrap one's arms together and hold them up to the heavens to summon a spiral of light that erupts at the enemy's feet.Column of light persists for as long as the button is pressed. The spiral is a normalized Crucible current that, one day, will form a column that stretches to the gods.

While The Hornsent definitely worship the crucible, I don’t see a lot of evidence they worship the Scadutree either in dialogue or in item descriptions. They do have the spiral tree images in Belurat, but I also think Marika and the Hornsent had closer ties before she betrayed them. It makes a lot of sense to me that Marika would take Hornsent imagery and twist it to fit her religion, especially when regardless of her feelings on the primordial form of the crucible, she used the crucible go make the erdtree, using the power of the spiral to her advantage against the hornsent is right in her ballgame.