Combatting
Witchgrass
with Mulch

in asparagus, grapes, rhubarb, blackberries and other perennials
and
anywhere else it's not wanted.

by Tom Roberts, November, 2008

Click on photos to enlarge.

In case you are not familiar with it, witchgrass is everpresent in Maine, especially in pastures and fields. It spreads via rhizomes, or underground stems, which sprout up new spears of grass every few inches. It can withstand frequent cutting, grazing or mowing and keeps coming back. Like most grasses, it loves cool wet weather.

When the gardener tries to pull it up, the rhizomes break off in the ground, making it very difficult to eradicate that way. Tilling breaks the rhizomes into hundreds of small pieces, each of which sprouts a new plant, although any left entirely on the surface in dry weather will dry out and die in a few days.

When tilling witchgrass in the garden, it is advisable to go very slowly, thus breaking the rhizomes into the smallest possible pieces. That way, although there will be more sprouts of grass emerging, there will be less energy per rhizome piece, so subsequent control become easier. When the new sprouts have exactly three leaves on them, the rhizome has put out a maximum amount of energy without getting much back yet. This is the point at which pulling or re-tilling works best.

But what about witchgrass in perennial crops where tilling is not an option? Over the years we have discovered several mulching methods which do not work. These include mulching with new grass clippings, hay, leaves, pine needles, or wood chips. They do not work because the energy reserves the witchgrass allow it within a few months to grow its sharp pointed rhizomes upward to the light, and then sprout leaves of grass to the point that it is now growing stronger than ever.

But that doesn't mean that mulching doesn't work, it just means that a simple layer of mulch doesn't work. What we have developed over the past ten years is a method that does work, and we use several variations of it.

The secret is to cover the witchgrass with a layer of cardboard (or many layers of newspaper) and then cover this with a layer of hay, leaves, grass or wood chips, which will serve not only to hold the bottom layer in place, but to help it biodegrade over time as well as improve the appearance of the mulched area. In the photo to the right, which is a closeup of the photo at the very top of this page, I first added a layer of compost to the blackberry beds, then covered this with cardboard (either cereal box cardboard or corrugated) then added a layer of wood chips. Before putting down the compost, I spent some time pulling all the witchgrass rhizomes out that I could, just to increase my chances of success. In many places it was just like digging into a plate of spaghetti, the roots were so thick.

You will find the process much easier if it is done in late fall or early spring when the grass is not growing. Alternatively, during the growing season you can mow the grass very close, then use the mowings as part of your top mulch. (The fresh tops won't root; only the underground rhizomes will propagate the plant.) Next, lay down overlapping layers of newspaper or cardboard. If using newspapers, use about 20 sheets per layer—we use folded newspapers about a quarter inch thick and overlap them by at least six inches. The work goes much easier if the newspaper or cardboard has been left out in the rain or soaked in a tub overnight. This keeps the wind from blowing them as you work, and removes any stiffness, which makes it easier to cover irregularities in the ground.

In the above photos, Jill is putting down an overlapping layer of newspapers on mowed witchgrass in the rhubarb patch in late May, then covering these with leaves. Note how the newspapers are tucked in under the rhubarb plants, and how much the newspapers are overlapped. At right, in the same rhubarb patch the following November, you can see why the newspapers need to be overlapped more than was done in May. The grass was able to find its way between the papers and emerge, weakened, but ready to catch up. So remember to overlap your bottom layer quite well. You will note that the second bed over has much better witchgrass control, since the newspapers were overlapped more.

Another thing we have learned is that, altho the two layer system works well, the fact that the newspaper layer is biodegrading at the same time the witchgrass is trying to poke through it means that sometimes some of the witchgrass will get through. Other times witch grass will be invading from the edges of the mulched patch. This will be evident within a few months, and usually happens only in a few patches here and there. What we have learned to do is to apply the very same method again the following year. The second time, the grass is weaker and sparser, and the mulching even more effective. Once you've killed the witchgrass, it is gone, and won't be coming back. However, it may be forever invading from the edges of your planting, so re-applying the two-layer mulch may become an annual or biennial necessity, at least in certain patches.

In the photo to the left, you can see the grape arbor as it appeared in early November. The area in the foreground was mulched with hay-over-newspaper in the spring of the previous year, and now eighteen months later the grass is well on its way to recovering. Further back, you can see where we applied the hay-over-newspaper again in the spring of this year, and there is essentially no witchgrass in that section. Prior to our initial application of the mulch two years ago, the grass was as thick as I've ever seen it, very much enjoying the leaves-only mulch I had used several years previously.

We are always saving pizza boxes, cereal boxes and any cardboard boxes we come across. Mulching the eighty foot long Blackberry Garden this October used up two years worth of cardboard hoarding. Better to use too much than too little. Last year we were fortunate enough to have someone deliver us a van of bundled newspapers, and to date we've used about a third of those.

What is our preference for the top layer? Generally whatever is available. During the summer months we are mowing the fields every few weeks so we have a good supply of young hay. Sometimes we have a large pile of wood chips that we make ourselves or is delivered to us. And as you can see in our Using Mulches article, we get pine needles, hay and leaves delivered to us every fall by the good residents of Pittsfield. For each of these we have other uses as well, besides using them for the top layer in our witchgrass battle. So whenever we are applying our two-layer mulch, we look around to see what's in surplus for the top layer.

Chips or sawdust as a top layer will last the longest, although for mulching witchgrass it is the bottom layer, not the top layer, which determines effectiveness. Leaves, pine needles and old hay are the second longest lasting, but loose leaves have a tendency to blow off exposing the bottom layer if they are in a windy location. Old hay may itself contain grass seeds (but not witchgrass seeds, for witchgrass produces few if any viable seeds). Young hay and grass clipping are the shortest lived, completely disappearing by the following year.

The lack of longevity of the bottom layer is surprising to most people. Newspaper and cardboard when kept moist and in contact with the soil will biodegrade almost completely during the first season with the help of micobes and earthworms. Meanwhile, if applied thickly and overlapped enough, it will have done its job. Then bottom of the top layer will then begin to decompose, slowly feeding your plants.

Adding a layer of compost prior to the application of the newspaper or cardboard layer is a way of feeding your plants an even better diet than the decomposing mulch would be. Compared to simple layer of compost on the top of the ground, compost under the mulch will remain moist decomposing and combining with the soil more completely.

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