A simple sheet of black plastic can thwart invasive weeds, keep garden soil moist, and even allow you to plant earlier. Yet black plastic isn't always the best mulch choice, especially if you grow perennial vegetables. Understanding both the pros and the cons of the material helps you determine whether it's the best mulch for your vegetable garden, as well as how to maintain it properly.
Weeds can't penetrate plastic, making black plastic mulch one of the best options for weed control in the home garden. Although weed seeds may germinate, they cannot breach the plastic or receive sunlight, which causes them to die before they can root or produce more seeds. You must cut holes in the plastic for desired plants. Some weeds may grow near the base of the plants in the holes, but prompt removal prevents them from overtaking the garden plants.
Black materials absorb sunlight and heat while retaining the warmth in the soil. Applying black plastic to a garden bed in spring helps warm the soil earlier, resulting in the ability to plant earlier and in healthier initial plant growth. However, plastic left on vegetable beds into summer can absorb too much heat and bake plant roots. Covering the plastic with a thin layer of straw can help keep the soil cooler during hot weather if high temperatures become a problem.
Evaporation causes quick moisture loss from garden soil, especially during hot and sunny weather. Black plastic prevents evaporation so that the soil retains moisture longer. Although the plastic retains moisture, water cannot penetrate it from above, meaning standard overhead watering practices are insufficient. Rainwater may pool on top of the plastic as well. You must water vegetable plants through the planting holes or install a drip irrigation system on top of the soil but beneath the plastic. You can poke holes so that some moisture drains into the soil, but weeds may grow through these holes.
Plastic mulches typically only last one year before they begin to weaken and tear. This may not pose a problem in vegetable and annual beds that require yearly replanting, but this short life span makes plastic unsuitable around most perennial fruits, vegetables and herbs. Anchoring the edges of the plastic down well and only cutting the number of holes necessary for your plants can help extend the useful life of the plastic, but it still rarely survives for more than a year.
The unattractive appearance of a plastic-covered bed makes this mulch unsuitable for most ornamental gardens, so it's primarily used in vegetable beds. You can cover the plastic with a thin layer of a more attractive mulch, such as wood chips, but then both the chips and the plastic require removal and replacement at the end of the year. Covering plastic with an organic mulch reduces its heat retention properties, and weeds may root above the plastic in the organic mulch material as it breaks down.