hi katie welcome im sorry about your loss.i dont see that there would be a problem with mixing your mums ashes with compost.after all you dont know whats in it anyway.i dont think it is morbid at all.you can talk to her when you water the plant.i do to mine. sorry again and good luck.

Hi Katie and welcome to GoY, so sorry for your loss - I see no reason why you can't mix the ashes with compost to grow hybiscus .

I think thats a comforting idea.

... and a welcome to GoY from me too.
I don't think it's morbid Katie. When my Mam died, we dug up her favourite rose from her garden, put her ashes in a hole we'd dug on Dad's grave (she didn't want to be buried) and planted her rose on top of her ashes.

just wanted to say a huge thanks to all who answered my question

Katie, I'm not as sure as the others - for one thing, do you mean, by Hibiscus tricolour, the tender one grown in America, here as a houseplant? Or do you mean a variegated version of Hibiscus syriacus, which is hardy but requires a sheltered spot outside?
If you mean any form of houseplant, the problem is the ashes will be too rich in nutrients in a small to medium pot of the size we use indoors.
If you mean for outdoors, in a larger pot, I suggest you don't buy or plant it until Spring, as the plant is starting to grow, when the excess of nutrients might be something the plant can utilise, and therefore less likely to be harmful.
Were you to use the ashes in open ground, say, at the bottom of a planting hole, it wouldn't be such a problem, but in a pot, it's a bit concentrated, and I'm sure you don't want any plant you pot up with your mother's ashes to not thrive.

i agree with bamboo . the thing is you could add a little bit at a time perhaps . personaly i think its a better idea than a grave . ive infact planted a lovley weeping willow on some common land near a river wear i used to play and still go with my dogs and thats wear im going and so are my dogs so as far as im concerned its a cracking idea under the right plant . id definatly chose a very long lived hardy plant considering the cercamstances myself . sorry for your loss ake care bye for now .

So sorry for your loss Katie and I think planting your Hibiscus in the garden is a lovely idea, maybe you could put the ashes in a tin container(or the container that you have been given) and then plant the plant on top, this will keep the ashes contained should you want to remove them one day, say if you moved house or something like that. I hope the chosen plant brings you comfort.