Can a small plant pull toxic metal out of contaminated soil? Some plants absorb heavy metals through their roots. Thlaspi is one of them. This process is called phytoremediation (using plants to clean up pollution). You plant 100 Thlaspi seeds in soil that contains 600 parts per million of zinc. After 30 days you remove the plants and replant fresh seeds. You repeat this for four cycles over 120 days. After each cycle you send a soil sample to a lab. The zinc level drops slightly with every planting cycle. Four rounds of growth bring the level from 598 ppm down to 565 ppm.