This plant is one of the few to flower in the late summer before the Autumn rains. It grows from a very poisonous, large bulb which can be up to 20 cm wide and weigh a kilogram. Several bulbs may grow in a clump and are usually just beneath the surface of the soil. In the spring, each bulb produces a rosette of about ten leaves each up to a meter long. They are dark green in colour and leathery in texture. They die away by the autumn, when the bulb produces a tall, narrow raceme of flowers.

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This species has been used as a medicinal plant since ancient times. It is noted in the Ebers Papyrus of the 16th century BC, one of the oldest medical texts of ancient Egypt. Pythagoras wrote about it in the 6th century BC. Hippocrates used it to treat jaundice, convulsions, and asthma. Theophrastus was also familiar with it. Its primary medicinal use was as a treatment for edema, then called dropsy, because of the diuretic properties of the cardiac glycosides. A solution of sea squill and vinegar was a common remedy for centuries. The plant is also used as a laxative and an expectorant.

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The plant has also been used as a poison. It is very bitter, so most animals avoid it. Rats, however, eat it readily, and then succumb to the toxic scilliroside. This has made the plant a popular rodenticide for nearly as long as it has been in use as a medicine. The bulbs are dried and cut into chips, which can then be powdered and mixed with rat bait. The plant was introduced as an experimental agricultural crop in the 20th century primarily to develop high-toxicity varieties for use as rat poison. Interest continued to develop as rats became resistant to coumarin-based poisons. It has also been tested as an insecticide.

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Pythagoras and Dioscorides hung the bulbs with sprouted leaves outside the door in spring as protection against evil spirits.