If you put the slate down without a weed suppressent everything, including weeds, will grow through.

And if you have a weed suppressant the bulbs won't be able to grow through... unless you cut small holes in and then not only will the bulbs come through but the weeds will too!

Well, they're right, the two previous answers. If you've got bulbs in there, personally, I'd wait till spring - then you can see where they are and move them around after flowering if you want to. That's the time to lay a membrane, cutting out holes where you want plants to be, then adding your slate on top.