what can i put under roses to supress the weeds

Why not underplant with perennial geraniums. There are plenty to choose from to match or contrast with the colours of your roses. Heucheras are another good choice but slower to cover the area.

Hoe or dig out what you can first, apply a mulch of soil conditioning compost or composted animal manure - roses benefit from humus rich soil. Bark chips will do, but having bought bark chips recently, I wouldn't recommend those unless you buy play bark. I got a refund on mine because, by and large, the contents of the bags certainly weren't bark, they were large and small lumps of old wood such as packing crates, trellis pieces, etc.