Using black plastic on the soil when you plant your flowers will benefit them in several ways. For example, it keeps weed growth down. It prevents the soil from becoming compacted and hard. Not having to pull weeds and hoe means there's less risk of injury to the flowers' roots. Black plastic retains heat to help warm the soil and promote growth. It takes time and effort to put this gardener's friend into place. But the effort will save you time and money in the long run.
Test the soil to determine what type, and how much, fertiliser should be used in the flower bed. Evenly apply the fertiliser, then work it into the soil with a spade.
Water the soil evenly and thoroughly if it's dry. Cut a piece of the black plastic to fit the length and width of the flower bed.
Dig a 3-inch trench around the perimeters of the bed. Stretch the plastic out flat across the bed so the edges are in the trench. Fill the trench back in with the dirt you removed. Make sure to cover the edges of the black plastic completely to weight them down.
Find black plastic at garden supply stores, at plant nurseries, and through mail-order catalogues. Cut X-shaped slits in the plastic to plant the flowers in the soil. To further weight the black plastic down, place small rocks on it after the planting is done. After all the flowers have been planted in the bed, you can cover the plastic with a thin layer of sawdust or mulch to make it look more appealing.