In terms of sustainability, renewable resources, nourishment and healthy diet, crops are important to the public. Thus, knowledge of crops is needed in order to enable people to participate in public discussions and take responsibility. This is in contrast to former surveys showing that students' knowledge of and interest in plants in general, crop plants and agricultural issues is moderate to little. At the same time, approaches to improving knowledge and interest in school are missing. We initiated and established the Greenhouse Project (GHP) where secondary school students (grades 5-13) get to know crops through cultivating from seed to seed. To investigate whether or not original contact with a variety of staple crops and hands-on activities positively affect students' knowledge, students of two German secondary schools were asked via questionnaires before and after the treatment. Our study was conducted in the cities of Mainz and Wiesbaden which are situated in the German Federal states of Rhineland-Palatinate and Hesse, respectively. In total, 74 students in 6/7th and 11/12th grades took part in this pretest-posttest survey; three additional 6/7th and 11/12th grade classes (i.e. 48 students) were used as control classes, and had no contact with the GHP during this time. We demonstrated that the treatment has positive effects on students' knowledge, and that girls performed better than did boys. Therefore, knowledge of crop plant species, as well as morphological knowledge, improved. A higher level of knowledge cannot only be observed objectively via test scores, but also subjectively via the students' self-assessment of knowledge. In contrast, the students' opinions about and attitudes towards agriculture and crops decreased in the posttest, both in the treatment and control classes.