Here are some other options, depending on the context:
* A bed of roses is the most traditional and common.
* A bouquet of roses is used for a group of roses that have been arranged.
* A cluster of roses can be used for a small group of roses growing together.
* A thicket of roses can be used for a dense group of roses growing wild.
* A profusion of roses can be used for a large and abundant group of roses.
Ultimately, the best collective noun to use depends on the specific context and the way the roses are being presented.