A backyard pond might start to become choked by dense bulrush (Scirpus spp.) growth. Digging up the plants might be effective with small patches of bulrush, but you may have to turn to chemical weedkillers for dense, aggressive bulrush growth. There are no biological control methods for bulrush.
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Growing in U.S. Department of Agriculture plant hardiness zones 3 through 11, bulrushes spread through aggressive rhizomes. The rhizomes can be pulled out of the ground, but new plants can grow if you leave a piece of the plant behind. Bulrushes grow in the shallow water along the edge, as well as the wet soil surrounding the water. Their usually hollow stems are mostly leafless except for a few papery leaves at the base that form a sheath. A large, spiky panicle with red-brown flowers appears atop the stems in midsummer. Seeds develop on the panicles at the end of the flowering season. New plants can sprout from these seeds.
Mechanical control is not always effective but is worth a try before resorting to chemical control. Grasp the bases of the plants as far below water as possible. Pull up on the plants slowly so you don't break off the rhizomes. Alternatively, cut the plants as close to base the possible with pruning shears or loppers. Soak the pruning tools in solution of 1 part bleach and 9 parts water before use. Whether you pull or cut the bulrush stems, these methods alone cannot remove all the rhizomes, so you must dig in the soil to remove them. This isn't always easy when the rhizomes are under water, but the rhizomes are buoyant and float to the surface if you loosen them with a shovel or garden rake.
About Chemical Control
Glyphosate herbicide is the most effective herbicide for bulrush control. Unlike contact herbicides that work on the surface of a plant, glyphosate is a systemic herbicide that absorbs inside the stem and is translocated to the rhizomes to kill the entire plant and not just the stems. This is a rather slow process that is made much faster with the addition of a non-ionic surfactant which helps the product coat the stems and penetrate more easily. Chemical control is most effective when you apply the herbicide immediately after flowering but before seeds develop. It takes roughly two weeks for the glyphosate to desiccate the stems and completely kill the bulrush.
Applying Gyphosate to Bulrush
While many weedkillers contain glyphosate, only use a product labeled for use on aquatic plants for bulrush. Mix the glyphosate in a garden sprayer at a rate of 2 ounces to 1 gallon of water, or according to the label's instructions. Add a non-ionic surfactant at a rate of about 1 teaspoon per gallon of water. Spray the herbicide along the length of the stems until they're well coated, but not so heavy that the liquid runs off the stems into the water. Treat no more than one-fourth of the bulrushes in one session, particularly if there are other plants or aquatic wildlife in the pond because the dying plants can deplete the oxygen in the water, killing fish and other creatures in the pond. When the bulrushes have died, remove them from the pond.