Does Freshwater Seaweed Make Good Fertilizer?

Freshwater Seaweed

• Any water-dwelling plant may be called seaweed, even though it doesn't come from the sea. But not all water-dwelling plants are considered weeds. Aquatic plants are an essential part of water ecosystems. Unfortunately, many uninvited water plants have invaded freshwater lakes, ponds and waterways. Harvesting these weeds to compost for garden fertilizer benefits your garden and the environment. Aquatic plants can provide valuable garden nutrients. When done properly, harvesting freshwater seaweed can also help control some of these undesirable, invasive water-weed populations.

Seaweed Sources

• Freshwater tides push aquatic plants ashore in the same way plants wash up along seacoasts. In many locations, these weeds can be gathered from the shores to compost for fertilizer. Freshwater weeds can also be gathered from the water through hand-harvesting and other means. Take proper care to identify plants so beneficial plants are not taken, and so the best harvesting techniques are used. Some aquatic plants are annuals, some reproduce from fragments, some by seeds. Informed harvesting protects against inadvertently spreading invasive plants. Before gathering any freshwater seaweed from the shore or the water, obtain permission from private landowners or permits from public officials for public shores.

Preparing Compost

• Freshwater seaweed must be fully composted before added to your garden. Drain or dry the weeds first, then chop them into smaller pieces to hasten decomposition. Compost them as you would high-nitrogen greens like grass clippings. Layer the chopped seaweed with dry brown material like leaves or straw in your compost bin. This keeps them from turning slimy as they decay. In general, aquatic weed seeds are not a problem on land unless handled carelessly. Never compost your weeds or spread the compost in an area where water run-off could result in seeds going back into the water. Be certain aquatic weeds are completely composted and dry before using.

Freshwater Seaweed Benefits

• Compost made from freshwater seaweed results in a well-aerated, organic, high-nutrient fertilizer and soil amendment. It provides major nutrients such as nitrogen, phosphorus and potassium along with many micronutrients. The slightly alkaline compost can be substituted for peat moss to temper acidic soils and improve soil structure and texture. Unlike saltwater seaweed, freshwater weeds don't need washing or leaching to remove salt. Incorporate compost into the soil or use it as a mulch to retain soil moisture, reduce weeds and add nutrients as it decomposes more. If harvesting freshwater weeds from polluted waters, be vigilant regarding potential contamination. Use compost wisely, and test for pollutants if problems exist.