Answer: A sturdy landscape fabric will block most weeds. Black plastic is not recommended because it does not allow air or water to pass through it to the soil. The fabric will also keep the gravel from working its way down into the soil during the annual freeze-thaw cycles.
To work around existing planting you will have to cut pieces of fabric, fit them around the plants, and then overlap them to cover the seams. If you have any perennials or bulbs in the area, wait to do the work until all the perennials are up, so you don't inadvertantly cover them. Also, be sure not to cover spring bulbs.
The amount of rock will be determined on how deep a layer you desire. Basically, you'll need to measure the area in square feet, then multiply it by the depth. For example, a 10 x 10' area is 100 square feet. If you want a layer 4", or 1/3 foot, deep, then multiply 100 by 1/3, and you get about 33 cubic feet, or about 1 1/4 cubic yards.
I also caution you on several points: The rock heats up in the sun and reflects heat back at your plants. The extra heat can cause your shrubs stress, both in summer and bright winter days. Boxwoods in particular are sensitive to this. Also, stone has no nutritive value and thus does nothing to enrich the soil. A natural mulch such as shredded bark will feed the soil as it breaks down slowly over time. Finally, you will find that weeds will eventually begin growing in the rock because autumn leaves, etc., work their way under the rocks, rot down and form a humusy seed bed on top of the fabric.
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