If they are silk flowers, I'd think they would just stick them in the top of the cake. That's what I did when I used silk flowers on the top of a birthday cake recently. I just washed the stems and stuck them in right before delivering the cake.
If I was doing this cake ......
I would lay the bottom row of roses on the cake, then prop a couple of roses on top of the bottom row. With the stems cut really short, there would be no cake-penetration.
If your flowers will be sitting for more than a few hours, you can create an arrangement for the top.
Get a 6" round, clear plastic plant tray that one can put underneath planters/pots to catch water than drains through. Any home improvement store should carry these--usually about 50 cents each. They also come in 4" and larger sizes, too.
Cut a chunk of floral Oasis to fit in the plant tray, and stick flowers into the oasis. Insert several horizontally, to cover up the plant tray. Water the oasis. It will hold plenty of water with none standing or spilling.
Then, you can just set the small arrangement on top of the cake once the cake is in place.
OK, that sounds great, too. The party is at 12:30, so I am hoping to set up around 11 or 11:30
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