Getting rid of yellow nutsedge (Cyperus esculentus), is a special problem for the organic gardener who does not want to use chemical herbicides. Although nutsedge yields tiny, football-shaped seeds, it spreads primarily by small tubers or "nutlets" that grow on creeping underground stems called rhizomes, making it difficult to kill. Applying sugar to boost beneficial micro-organisms in the soil is a counter-intuitive but useful way to suppress it organically.
Challenge for Organic Gardeners
Growing stems up to 3 feet high, yellow nutsedge can invade vegetable or flower gardens, forming dense colonies. It is among the most persistent, noxious weeds invading agricultural soil worldwide. Attempting to pull or dig it up is difficult to impossible because its rhizomes spread in the top 12 inches of soil. If you leave a nutlet behind, it will yield multiple plants. Attempting to dry it out doesn't work either because the nutlets store moisture as well as nutrients. Yellow nutsedge grows in U.S. Department of Agriculture plant hardiness zones 4a through 9b.
How Sugar Kills
Sugar feeds micro-organisms in the soil. If micro-organisms receive a sugar boost, they consume more soil nutrients than they ordinarily would. This starves yellow nutsedge and other annual broad-leaf weeds of the nutrients they need to grow. Using white sugar, Australian researchers recorded a suppression of yellow nutsedge and annual weeds and a growth spurt on surrounding, newly sown native plants.
Home gardeners typically use molasses to carry the sugar necessary to suppress yellow nutsedge. They can use molasses they buy in a supermarket, but horticultural molasses from a garden supply center is less expensive. Horticultural molasses, a 1-0-5 fertilizer, provides carbon, sulfur and potash to soil microorganisms. Dry molasses is liquid molasses sprayed on bits of soy meal. The sugar levels in dry molasses found in garden supply centers ranges from 38 to 42 percent.
The best time to apply molasses is in the spring when yellow nutsedge is actively spreading and needs nutrients the most but before it produces seeds. Mix 1 cup of liquid molasses to 1 gallon of water in a backpack sprayer for direct application on 10 square feet of yellow nutsedge. Two or three applications spaced a week apart will slowly kill yellow nutsedge. Spread 5 to 10 pounds of dry molasses on 100 square feet of nutsedge-infested soil. Reapply the dry molasses once or twice a month, which should lead to some results in a few weeks.