I've had bad experiences with fabric weed barriers because weeds can grow in the mulch on top of them. These weeds are difficult to pull because their roots grow through the fabric. The product can be difficult to remove, especially once soil accumulates on top of it. Keep in mind that if you grow annuals that need replanting each year or perennial flowers that need to be divided occasionally, the fabric weed barrier will be an obstruction to digging in the bed. A more appropriate use of the weed barrier fabric is around trees and shrubs that remain in place once planted. Your birch should suffer no ill effects from the fabric. Fabric weed barriers are porous, permitting water and air to move through them. However, avoid using impermeable black plastic as a weed barrier under the mulch. Plastic blocks air and water from reaching the roots. You may want to consider using mulch with no weed barrier under it in your cutting garden. Transplanting and dividing chores will be easier, and as the mulch decays, it will add organic matter to the soil.