The length of time required for fumigants to escape from the soil before plants can safely be planted varies greatly. Typically seven or fewer days are needed under summer conditions, however in the winter and early spring it may take months.
Seeded crops are less susceptible to residual soil fumigant injury than transplanted crops.
In general, fumigants escape slowly from cold, wet, heavy soils. With a trowel dig into the treated soil to, or just below, the depth of application. Use mason jars, wheat germ jars or similar jars with gas tight lids. Sprinkle lettuce seed on the moistened surface of the soil and recap immediately. Lettuce seed will not germinate in the dark.