An seeder includes a frame on ground wheels defining tool bars for seeding elements spaced across the frame. Each element includes a pair of parallel arms connected to the tool bar bracket forming a parallel linkage for up and down movement of a support arm connected at a rear end and carrying the packer wheel. A spring applies a downward spring force on the parallel linkage to apply a downward force on the packer wheel so that the spring force increases as the parallel arms pivot upwardly and decreases as the parallel arms pivot downwardly. A control device is arranged to raise and lower the tool bar in response to a detected value of a sensor so as to change the angle of the parallel linkage and change the spring force. A cam controlled double rack adjustment of the height of the opener allows the user to change seeding depth.