Azolla filiculoides (Water Fern / Grote Kroosvaren)

Azolla filiculoides is a tiny floating fern forming a dense mosaic of overlapping fronds that turns vivid red-purple in strong sunlight or cooler temperatures. It fixes atmospheric nitrogen, reducing fertiliser needs in planted ponds, and provides welcome shade for fish. Widely available in Belgian and Dutch vijver centres as seasonal or year-round cover.

Grows rapidly in still or slow-moving water in full sun. Thin out regularly (weekly) to prevent oxygen depletion underneath. Can be invasive in open water — do not release into natural waterways. Overwinters as spores or can be taken indoors in a shallow tray with water. Sensitive to salt; keep away from marine systems.