The red blossom seed is used in the Herblore Habitat. It should be planted in a vine flower patch. They can be bought from Papa Mambo in the Herblore Habitat area.
When planted, certain jadinkos will be attracted to it (along with other plants that are required for that certain jadinko) and will show up in that area. Once the jadinko is in the area, it can then be hunted by either tracking with a noose wand or box trapping with a Marasamaw plant, depending on which kind of jadinko it is.