A weed mat is a water permeable barrier made of a heavy duty woven fabric type material that blocks growth of plants from beneath. It does allow warmth and water through the material for plants that are placed in the bed through the barrier. The success of a weed mat depends on the quality of the product and proper installation; paying attention to doing it right from the beginning is very important.
Prepare the area you are using for your bed by scraping off the top layer of vegetation with a spade or shovel. You can dig up the area to a depth of about 4 inches, and turn it over, but make sure you pull out the weed plants and shake the soil from their roots as you do so. Rake the area smooth.
Measure the area you need to cover. The weed mat should overlap where pieces need to come together by 3 to 6 inches in order to make sure no gaps allow weeds to grow through. You can also choose to add a 6-inch border to the mat to bury in a small trench to secure the mat if you prefer and are not using edging around the bed. Allow for a 1-to-3-inch border overlap if you are using edging; this gives you excess to tuck beneath the edging to hold the mat down.
Cut the weed mat to fit the spaces with the utility knife taking care not to rip the fabric. Weed mat generally comes in rolls so you will have to plan your pieces using the width of the roll you have purchased.
Lay out the fabric in the location you are covering, being sure the edges overlap and the outside borders are adequate. Plot out your plantings by placing the plants still in the containers on top of the weed mat. Adjust and move them until you are satisfied with the placement.
Cut an "X" in the weed mat where the plant will be just large enough to accommodate the plant base. You don't need to cut a hole or make a large space -- this will allow weeds to creep in around the plant -- only make an "X" big enough to slip the plant through into the ground beneath. Place the plant in the ground, and tuck the weed mat in around the base.
Add anchor pins every 3 to 5 feet if you feel you need them. Normally the ground cover you are going to use and the plants in the bed will secure the weed mat. However if you have a large open area or are on a slope and have concerns about the weed mat moving, drive an anchor pin into the ground with your foot until the head is all the way down to the surface of the weed mat.
Spread the ground cover over the weed mat once your plantings are in place. Cover all exposed areas of the weed mat, as this type of fabric will age and deteriorate in sunlight. Use a good layer of the ground cover, whether rocks or wood chips. Apply according to the recommended depth, but you need at least 3 inches of material minimum to cover weed mat.
Install your edging and secure the border of the weed mat beneath it. The border is one area weed mat can fail by being pushed up or blowing up, so this step is important. If edging is not being used, dig a small vertical trench around the outside of the bed, and tuck the weed mat into it. Bury the edge of the mat securely and tamp down.