Can I Use Black Plastic Bags Under My Raise Garden Bed

Carole Mapes transplants flower seedlings into a field. Credit: Gabor Degre | BDN

Every gardener knows the feeling of being so frustrated with the weeds in your plot that you just want to smother them. Well, good news: You can.

Sheets of black plastic and landscape fabric are two popular ways of mulching for weed suppression. Both involve laying a material over the majority of a garden plot, with openings for where the crops will come through. This either prevents the weed seeds from germinating altogether or smothers them as soon as they crop up.

However, there are distinct differences between the two.

"Landscape fabric is a totally different thing from black plastic and often people mix up the two," said Kate Garland, horticultural specialist at the University of Maine Cooperative Extension.

For starters, Matthew Wallhead, ornamental horticulture specialist and assistant professor at the University of Maine Cooperative Extension, said that black plastic is generally cheaper and lower maintenance than landscape fabric. For example, he said that while black plastic for gardens usually comes equipped with perforated openings for crops, most landscape fabrics require that you cut or burn the holes out yourself.

"The plastic is probably less expensive than landscape fabric, it can be a little easier to handle just as far as actually laying it out in the field," Wallhead said. "There's just a little more labor sometimes involved in the landscape fabric."

Eric Gallandt, professor of weed ecology at the University of Maine, said that one of the primary advantages of black plastic — especially in Maine for warm-loving crops like tomatoes, peppers and cucurbits — is that it warms the soil.

"If you use regular black plastic, you want to make sure the soil you're putting that plastic down is nice and firm and level [so it's] warming from the sun and conducting heat in the soil," he noted.

Garland added that black plastic is effective for retaining moisture, but some sort of irrigation under the black plastic might be prudent, especially during dry years.

"It also makes it harder to water because you have to target your watering into the hole where you plant or count on moisture migrating through the soil where it needs to go," Garland said. "In a normal rainy year, water landing on surrounding soil can migrate in underneath that plastic pretty well."

For the thrifty gardeners, Garland said that you could use a heavy-duty black garbage bag instead of purchasing specialized sheets that are thicker and made for gardening, but read your labels carefully.

"Sometimes garbage bags are lined with stuff like pesticides to reduce maggot growth and stuff like that," she said. "It should be listed on the package itself whether there's any extra product in there."

There are some downsides, though, namely that the plastic is generally thrown away after the growing season is done.

"They're damaging to the environment," said Tom Roberts, owner of Snakeroot Farm. "You've paid somebody to extract oil and turn it into plastic. You're creating demand for plastic [and] you're producing waste."

Wallhead said that he usually opts for the reusable option in landscape fabric, despite the extra effort it requires.

"It does last longer, whereas with plastic you're going to be replacing the plastic every year," he said. "For annual type crops, plastic would be better, [and] for perennial crops; the landscape fabric would be [better] for permanent beds like cut flower gardens."

However, Garland said there are distinct downsides to landscape fabric. After the fabric is laid down, it is often topped with a layer of bark mulch or another organic substrate. Over the years, she said, soil — along with weeds — can accumulate on top of the mulch and fabric as well.

"The roots will grow through the landscape fabric because it's a woven material," she explained. "You end up with this mess where you're pulling up weeds and the landscape fabric is pulling up. It's not fun. Once you've dealt with that issue once, you'll never want to use landscape fabric again."

Still, Garland has found uses for landscape fabric in other parts of her vegetable garden.

"I sometimes use it in between rows in a vegetable garden where I know I'm not going to top it with mulch," she said. "It's a flat material that I could sweep off if [I] happen to get some dirt on it."