Tangled lignum (‘lignum’) is a dioecious, multi-stemmed woody shrub that is common in flood-prone areas of inland Australia, including the Murray–Darling Basin. It is often leafless during dry periods, but maintains vegetative growth by stem layering, and responds rapidly to rainfall or flooding by production of shoots, leaves and flowers. This study considers the viability of lignum seeds (contained in achenes) under various conditions of temperature, light, moisture and storage or burial. The seeds are not innately dormant, and germinate within 14 days under ideal conditions. From 66 to 86% of fresh and dry-stored seeds germinate in fluctuating temperatures (15°C/5°C, 24°C/10°C, 31°C/15°C), and optimally at 24°C/10°C, given moisture and light. They also germinate in water (56% success), and remain buoyant for 5–25 days. Germination is inhibited by constant temperatures of 12 and 24°C (4.0–4.8% success) and continuous darkness (6.0–56.0% success), but increases on return to light. Seed viability is depressed by 10% after 70-day dry storage and by 48% after 92-day burial in soil over winter. In one year’s (2002) observations of a population on the River Murray floodplain near Morgan, South Australia, winter- and spring-seeding plants produced viable seeds 14–30 days after anthesis, and although rainfall in winter (July) produced a pulse of seedlings, none became established. Achenes were shed soon after maturation, but soil samples revealed very few germinable seeds. It therefore appears that the seeds do not persist for long on the mother plant or in the soil. The persistence of lignum in environments prone to erratic droughts and floods appears to depend mainly on its capacity to tolerate drought, maintain vegetative growth and respond quickly to watering.