Black plastic mulch is a valuable tool for those who wish to grow tomatoes in cooler climates. In areas where the growing season is short, tomato plants often fail to produce fruit. Black plastic mulch raises the soil temperature from 3 to 5 degrees Fahrenheit, giving the plants the heat boost they need to produce fruit all season long. You should avoid using black plastic mulch in hot, humid climates because it will trap too much heat in the soil, killing the tomato plants' roots.
Use a hand tiller to loosen the top 6 to 8 inches of soil in the garden bed. Remove and discard any rocks, loose roots and other debris.
Sprinkle a layer of garden fertilizer onto the prepared soil. The standard amount of required fertilizer is four pounds for every 100 square feet of garden space. Mix the fertilizer into the soil with the hand tiller. Soak the whole bed with water from a garden hose.
Dig two 4- to 6-inch-deep trenches along opposing edges of the garden using a shovel or hoe. Unroll the black plastic mulch slightly, placing about three inches of the end into one of the trenches. Fill that area of the trench up with dirt to anchor the black plastic mulch in the ground.
Unroll the sheet of mulch across the length of the garden, stopping at the other trench. Slice the length of mulch from the roll with a sharp knife, leaving about a 3-inch tail on the end. Lay the tail in the trench and cover it with dirt to hold it firmly in place. Repeat this process, slightly overlapping the lengths of plastic mulch, until the entire garden is covered.
Cut 6-inch slits in the plastic, spaced two to three feet apart. Reach through each slit with a small spade and dig a hole that's about twice the size and depth as the seedling's root ball. Slide a tomato seedling into each hole and fill the hole with soil.
Water the plants lightly each day by sliding the hose underneath the plastic mulch through each seedling slit. Tomato plants grown with plastic mulch will not require intense watering, but they do need a bit of moisture each day. If the soil under the black plastic mulch dries out, it can be very difficult to reinvigorate.
Leave the black plastic mulch on the garden bed until after the growing season has ended.