Thanks for all your responses. Just to clarify, I do accept she is grieving and has every right to. Also, as I said, I will not object to the planting she has already done, but my fear is she will 'take over' his memorial bit by bit. In my 23 years experience with her, she tends to make 'statements' like this to her claim that he was 'Her Dad' over and above anything else and even to the exclusion of her own brother.
Also, although I won't likely care once I'm in that hole my children will and they will want to have their own input.
The memorial is there for anyone who feels the need to visit and they are at liberty to leave whatever tributes they want, but a planting is more permanent and I feel if I don't nip it in the bud (excuse pun) it will esculate.
I'm sorry if this is coming across as non-empathetic, but in my gut I feel it is a control thing.

Thank you all for your input.
Another bit of background about 'Daisy'.
Roughly this time last year, DH was rushed into hospital. It was indirectly connected with his cancer, but impacted on his diabetes - all to do with his intestinal system. He had been in bed for a few days (Daisy had been to see him once). She went off on holiday for 5 days on the Monday - didn't call in before she went. So later that day DH gets rushed to hospital in a critical condition because - despite me calling the doctor out the day before and being assured it was just a tummy bug - his blood sugar was up in the 30s.
Once he was in intensive care I phoned Daisy. I said 'Your Dad is in hospital. He's critical.' She said, 'What shall I do then? Shall I come Back?'. WTF!!!! I said 'That's up to you.' Anyway, she drove back that evening, leaving her DH and children at the holiday centre. She visited DH briefly before going back to her house to sleep. In the morning she went back on holiday without visiting DH before she went.
That night, DH said to me, 'Wasn't that kind of Daisy to come all the way back from her holiday just to visit me'.
I could have screamed. But I wasn't surprised because DH was no use to her while he was in bed - unable to look after her kids, take her shopping, give her lifts late at night after she'd had a drink, do her odd jobs etc etc.
DH was in intensive care for 12 days and in the ward another few days. I did not miss a single day visiting - and nor should I being his wife, but Daisy visited maybe 4/5 times altogether, timing it so she avoided me but making sure other members of his family saw her there.
There is more to tell over the final months of his life, but I won't go into here. Suffice to say that every move she made (makes) is nothing short of a manoeuvre, carefully thought out to show herself in a caring light. She was never outwardly rude or disrespectful to my face. She was a lot cleverer than that. She would plot, plan and scheme to keep her father away from me and in her pocket as much as possible, using her kids when she could to do it.
Their relationship was covert. She was co-dependent on him and he needed her for a continual supply of attention, praise, neediness and validation, and they managed to pass this unhealthy sham off as a loving, close father/daughter relationship to those not in the know - which was almost everyone.
This is why I am suspicious of her motive for the 'planting'. She has already let MIL know she has planted a rose for her dad, knowing MIL will tell all his siblings.
This is aimed to put her in a good light (because some of her BS has been revealed to his family of late) but where was she when it really counted???