Having played green thumb sims since I first got the game, I know that the graveyard death plant will only give you one flower ever. However, if you then plant that flower, you can attempt to revive the plant you grow after you've picked its flower. It doesn't always work, though. Seeds are also a good way to get more deathflowers. I start collecting special seeds as soon as I can, and often have several deathflowers in my inventory at once, for all that you only get one or two flowers per plant. Hope this helps...