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Alexandra Botanic Gardens of Wellesley College: Sheet Mulching Update

By Tricia Diggins

Ten years ago I wrote an article about Sheet Mulching that highlighted a project in the Alexandra Botanic Gardens of Wellesley College.  I was asked to revisit the subject and immediately welcomed the chance to evaluate what happened to the main project featured in the article. 

Our project was to suppress turfgrass and create forest floor soil conditions by layering cardboard, wood (carbon) based compost, and then a layer of fresh wood chips under a hickory (Carya sp.) and a black walnut (Juglans nigra) tree. The sheet mulching was completely successful in killing the turfgrass. As noted in the original article, the goal at the time was also to have a low maintenance wood chipped tree circle aesthetic. It was not too long before the weeds got ahead of us, and we went from early succession dandelions to mid succession goldenrods, asters, Virginia creeper (Parthenocissus quinquefolia), and some milkweed (Asclepias syriaca)  the succession stage at which it remains today. It is not the clean tree circle look of the rest of the Wellesley College campus, but I have developed an aversion to weeding any native plants that provide services to the ecosystem even when the plant fits the definition of a weed because it is a plant in the wrong place. 

The sheet mulching project at Wellesley College taken 10 years after it was first installed to suppress turf grass June 2020 shows a thriving plant community.

I have observed a few other things about sheet mulching from other projects in the intervening years. Cardboard is the most important part of the system for the initial weed suppression it provides. When using cardboard from boxes, take off every bit of the plastic tape and packing lists. We all know how bad the plastic problem is, and you don’t want to add it to your garden. I don’t use highly colored cardboard because I assume it contains toxic inks. Ants like to live under cardboard, especially in dry spots. Some ants are harmless, but I did have ants kill a couple of tomato plants that were too close to their nest.

The area sheet mulched under Juglans nigra (eastern black walnut) Carya laciniosa (shellbark hickory)is now an understory of shade tolerant meadow plants.

In our initial sheet mulching project, we soaked the cardboard first. If the cardboard is wet and stays damp by being covered with mulch, then it will probably be sufficiently decomposed to plant into (vegetables or perennials for example) in as little as a month in warmer weather. I will put cardboard down dry, with a mulch layer, if it can stay in place over winter. This strategy works well for killing lawn or on a vegetable garden after harvest. If there are existing desirable plants in the sheet mulching area, then the cardboard layer can’t be allowed to dry out since it will create a barrier to rain or irrigation water soaking into the ground. I have not used a soaker hose but if I tried I would put the cardboard down wet and put the soaker hose on top of the cardboard layer but under the mulch layer. Sheet mulching on a slope is very possible though sometimes landscape pins are needed to hold the cardboard or, even better, logs and branches can be used to weigh the cardboard down and slow stormwater to help control erosion.

The compost and mulch layers on top of the cardboard can vary. In the initial article, I emphasized the benefits of sheet mulching for feeding the soil food web and in turn “improving” the soil. That works for most plants humans like to grow; however, there are plant communities, like drier meadows, that would not like added compost providing nutrients to more aggressive plants not normally found in the community.

The sheet mulch created a native ecosystem of goldenrod, milkweed, asters, and Virginia creeper that remains ten years later.

Most of us are having more of our needs and wants delivered to us in cardboard these days. I save nearly all my cardboard for sheet mulching, which doesn’t make me feel virtuous but rather serves as a reminder that I am likely consuming too much. But if we all used our cardboard to kill our lawns and grow goldenrod and milkweed instead, perhaps we could feel a little virtuous.

About the Author

Tricia Diggins was Senior Gardens Horticulturist at Wellesley College Botanic Gardens for nearly 30 years but is currently spending her gardening energy at home.