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Does Lasagna Gardening (AKA Sheet Mulching or Mulch Garden) Work To Smother Heavy Invasive Grasses?

Lasagna Gardening, also called “sheet mulching” or “deep mulch gardening” is a gardening method where layers of green matter, including weeds, grass clippings and leaves are layered with brown matter, such as sticks, twigs, branches, composted manure, wood chips, and garden soil to create raised beds and fertile soil.

Lasagna Gardens Use Mulch, Compost, & Organic Matter To Build Soil And Suppress Weeds:

This style of gardening has several benefits, such as:

  1. It's easier than pulling grass and weeds. When you are ready to flip your garden bed, you simply cover or weed whack the remaining plants and weeds, and leave them in place forming sheet compost. You also do not disturb the roots, which is certainly easier than pulling them out!

  2. This technique maintains the natural microbes living around roots – called the Rhizosphere. Saprobionts is the general term for the array of soil bound bacteria, organisms and fungi which decompose biological matter, and help plants to derive nutrients from soil. Thus, when you do not disturb the roots of weeds and the prior crop you planted, you do not disturb the Saprobionts. This is a base tenant of no-till gardening, which seeks to maintain the microbe-plant relationship, and to reduce the release of native nitrogen in the soil, which can be disturbed when you till.

The theory is that over time, the layers of organic materials, including weeds and grasses, are composted and decompose. Not only is lasagna gardening effective in many conditions, it is a great way to build vegetable garden beds with less effort than thoroughly weeding between plantings. The question is, can it keep strong invasive grasses out? If not, then the potential benefits are outweighed by the grass root & strangulation pressure on your crop.

Does The Sheet Mulch Technique Work To Build Raised Beds In The Gulf South, Where Native Weed-Grasses Are Unbelievably Aggressive And Dominant?

Accordingly, the question posed in this trial is: Does lasagna gardening work where the grass is composed of aggressive native runner & rhizome weed grasses such as: crab grass, barnyard grass, torpedo grass, goose grass, bermuda grass & nut grass. These grasses are survivors – with runners and rhizomes that can withstand pressures from weeding, drought, heat, and most garden conditions. While in some contexts, you want to keep your enemies close, that is not really the case in gardening!

Our Tiki Farm is located in Pearlington, Mississippi, on the Gulf Coast of the United States. Our area is dominated by the incredibly virile and invasive runner & rhizome grasses listed above. In the past 5 years, we have experimented with different variations of lasagna gardening, raised beds with cardboard at the base to inhibit grass-weed invasion, and complete removal of the weed-grass, runners and rhizomes in the area of the bed. Due to the benefits and comparative ease of lasagna gardening versus full weed/root removal and tilling, we decided to run a side-by-side experiment with two beds in the spring of 2023, to evaluate the relative success of each technique.

Bed 1 was the lasagna garden technique bed (closest one in picture). Here, we simply covered the remaining vegetables, weeds and grass from the prior planting with various lasagna layers to build the new bed. On the bottom, we placed about an inch of pine needles, hoping that the acidity and moisture wicking nature of pine needles would act as a natural weed preventing mat at the base of the bed. Next, we added a 4-6” layer of larger logs to the new garden, followed by layers of leaves, twigs, wood scrap, grass cuttings, and then a mixture of compost, soil, peat moss, rabbit poo, trace minerals, bone meal & fish meal (“Soil Mixture”). The entire new bed reached about 3.5 total feet in height.

First stages of lasagna bed & weed/till bed test

Adding layers to lasagna bed & weed/till bed

Bed 2 was the weed & till bed (furthest one in the picture). In this bed, we painstakingly pulled all of the weeds and native runner & rhizome grasses. Next, we used our Mantis tiller to break up the soil to further seek out the pesky runners & rhizomes from our unbelievably aggressive native grasses. In doing this, we filled a very large Gorilla Cart, which was dumped on one of our compost piles, and mixed with browns (twigs & sawdust) to create soil for future beds. So, one very different consequence of these two gardening techniques is the need to separately compost the weeds & runners you pull from the bed. It also fully removes the runners from beneath the bed, so in theory, the grass weeds will take longer to take over again (they always do!). In the future, we will run another trial using a physical barrier inserted into the ground to further inhibit runner growth. Here, before planting, we added about 10 inches of small twigs and sticks and 2 feet of the same Soil Mixture as used in Bed 1.

Both beds were planted and time passed. The initial productivity of both beds were comparable. After several months, we noticed more grass intruding on the lasagna bed, but not so much to make a huge difference.

Planted lasagna bed - Bed 1 of our Trial.

Planted weed/till bed - Bed 2 of our Trial

Over time, it became apparent that the grass intrusion in the lasagna bed was getting a bit out of control. While there was grass invasion on the weeded/tilled bed, we also found that this bed was much easier to weed, because the new weed grass did not have well established runners/rhizomes and pulled out easier. Alternatively, when weeding the lasagna bed, the runners were larger and went down deeper – seemingly to the deeply submerged rhizomes and larger undisturbed network of still existing runners.

At about 3 months, the lasagna bed was almost completely overtaken by weed grass. We purposefully did not continually weed the two beds so we could evaluate the weed grass intrusion rate at 3 months.

At 3 months, lasagna bed is overcome by invasive grass

At 3 months, weed/till bed has much lass grass invasion

Finally, when the beds finished producing and we were able to get back in there to evaluate what was going on, we found that the lasagna bed created a sub-surface explosion of super-sized runners and rhizome nuts. It appeared that the nutrient rich soil placed on top filtered down and turned the weed grass roots below into super-sized monsters!

Monster runners & rhizomes that grew out of control under the lasagna bed

Our Trials Suggest That Sheet Mulch Gardens Are Not More Effective Than Labor Intensive Weeding Of Invasive Weed-Grasses In The Gulf South.

After considering the results of our bed building trial is has become apparent that while lasagna gardening may be preferable in areas that are not dominated by strongly invasive native runner/rhizome grasses, it is not preferable in the Gulf Coast region where such weed grasses prevail. We have found that for each successive removal of runners and rhizomes, the weed grasses tend to come back slower, and are easier to remove because the network of runners is not as well established. So, unfortunately, in our environment, there is no substitute for the physical removal of such weeds. We really wanted to love lasagna gardening in our environment, but it has not shown to inhibit weed grass growth. In fact, we have found the opposite to occur - the still living runners and rhizomes, now deep below the surface grow stronger with the nutrients reaching down from the new rich soil added on top.

Because the weed removal/till approach does release nitrogen and disturb the Rhizosphere, we believe it is particularly important to implement a companion program of composting and vermiculture (using worms to break down compost) so the soil you use to top off your beds is rich in nutrients and nitrogen. We also alternate vegetable plantings with peas to boost the nitrogen content of the soil (plus peas rock for salads, garnish and purees!).

Hopefully this information is helpful for planning your vegetable & herb gardens! And, sorry for bringing bad news that weeding is still necessary when the prevailing weeds in your beds are runner/rhizome grasses.




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