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Purple loosestrife is an erect, summer-green perennial herb that grows 1-2m tall. The woody stems are square in cross-section. It has narrow leaves, which are usually paired and are heart-shaped at the base. From December to February it produces 20-25cm long spikes of purple-magenta flowers. The flowers are followed by blackish seed capsules that are 3-5mm long.
Purple loosestrife is a highly aggressive invader of damp ground, wetlands and shallow water. It over-tops native species with its dense, bushy growth and is long-lived. It tolerates hot or cold conditions and low to high nutrient levels, though it is intolerant of salt water.
Purple loosestrife reproduces mostly by seed but re-growth from plant fragments has also been observed. It produces millions of long-lived, highly viable seeds that can remain viable in the soil for many years. Seeds submerged in water can remain viable for 1-2 years. For germination, disturbed sites or open, moist substrates are preferred.