Using recycled cardboard to kill your Bermuda lawn, called as sheet mulching, is the easiest and most environmentally friendly way to do it. Unlike St. Augustine or Marathon, which can both be removed with a sod cutter on a large site, or with minimal digging on a small site, Bermuda can’t be easily removed by digging.
Digging can remove it, but it’s not an easy weekend project without 10 friends and a 10 ton low-boy construction debris dumpster. Then you either dig down about a foot, and throw away all the grass and dirt to that depth (why the low-boy is needed), or dig down about a foot, and sift each shovel-full through half-inch hardware cloth to remove the runners, which means ultimately lifting each shovel load at least twice. Until recently, the only alternative to the dig-out or the dig-and-sift methods has been to use glyphosate (Roundup), which, while safer than some herbicides, is not benign.
A few years ago, I was introduced to the idea of sheet mulching. It sounded simple: just smother the lawn in a layer of cardboard and mulch, wait a couple of months, then plant into it. I thought it would work for everything except Bermuda. I kept that thought until I had young environmental activist clients who wouldn’t use glyphosate, didn’t have the finances to hire a crew for several days, and, because the wife was pregnant, weren’t able to do that kind of labor themselves in their very large yard.
I suggested sheet mulching, since their options were limited. They gathered appliance-store and bike-store boxes, and laid them across the yard, making sure there were no openings between boxes that could admit light, and spread mulch over the whole thing. Three months later, I visited the site, and surprise!, there was almost no bermuda left. The few single pieces left were stringy, weak, and easy to remove. Most of the cardboard had degraded under the mulch, and the small cardboard pieces that were left were easy to pull out and throw away.
Cardboard is an excellent light barrier, and I think of it as “mulch pressed into a shape.” It biodegrades, improving the soil as it does so. Adding a layer of at least two inches of shredded mulch on top improves the light barrier which keeps the grass from photosynthesizing. The Bermuda travels under the cardboard, exhausting its underground resources and is unable to replenish them. This is why the process takes time. While the best time to do it is in summer, it can be done at any time.
While wet newspaper or sheets can work as the first layer, I have gotten the best results from cardboard, which is a better physical barrier. Since it is covered with mulch, it looks like a finished landscape and won’t upset the neighbors. While some references suggest using compost under the cardboard to help kill the grass more quickly, compost shouldn’t be used where natives will be planted as they don’t like the high fertility.
Though people love the idea of using black plastic to kill the lawn, it doesn’t work well here. It may work in places where the ground retains water and there is a lot of heat in summer. In those places, the water in the ground cooks the plants under the plastic. Unfortunately, in California the ground is drier. Because plastic isn’t as good a light barrier, the grass can often still get enough light to keep growing. Multiple cuts and tears quickly appear in the plastic, compromising the barrier effect. In the end, cardboard is better.
So, step by step, the first step is to measure the area’s square footage. Gather at least that much cardboard, plus at least 20%. The best cardboard comes from appliance stores and bike stores, since it is thick. Grocery or liquor store boxes may need to be laid in two layers. Step two is to lay out the cardboard, lapping the edges over each other by at least 2”, with no gaps. Cover every gap with another piece of cardboard. When there is a sidewalk or patio, it is best if the cardboard can be laid up over the hard surface and taped down for safety. If the cardboard can’t be laid over the sidewalk, dig at least 6” down, removing as many of the roots as possible. Any roots left in the ground at an edge will be an ongoing problem. Step three is to cover with at least 2” of mulch. The easiest mulch to get is shredded arborist’s waste, which is shredded mixed tree and bark, with some leaves. See the resources page for sources. Then, turn off the water to the area, and leave the whole thing alone for two to three months.
It is easier to cardboard a whole lawn than to only do part of it, since at the edge, a trench has to be dug out at least 6 to 9” deep, and a physical barrier added where the Bermuda will stop. If the lawn area to remain won’t be very big, it might be worthwhile just to remove the whole thing and replant the lawn areas with St. Augustine, than to deal endlessly with Bermuda runners.
When time comes to plant, if the cardboard remains, holes can be cut into it with a box knife and plants can be planted through the cardboard.
If you are planning to plant natives this year, it isn’t too late to cardboard, but it would have to be done as soon as possible, with a plan to plant in January or February.