When a weed invasion is discovered a decision has to be made as to whether to attempt to eradicate it, contain it or do nothing. Ideally, these decisions should be based on a complete benefit-cost analysis, but this is often not possible. A partial analysis, combining knowledge of the rate of spread, seedbank longevity and economic-analysis techniques, can assist in making the best decision. This paper presents a model to decide when immediate eradication of a weed should be attempted, or whether weed control should be attempted at all. The technique is based on identifying two 'switching points': the invasion size at which it is no longer optimal to attempt eradication, and the invasion size at which it becomes optimal not to apply any form of control. It is shown that seed longevity is a critical factor constraining the feasibility of eradicating large invasions.