Abstract: A planting unit lockup assembly extends between upper and lower links of a parallelogram linkage. A lockup link is pivotally coupled to the upper link and extends downwardly to the lower link. The lockup link is also provided with a slot having a hook. A slide member is mounted to the lower link and engages the slot. A control link is pivotally coupled to the lockup link by a loose pivot coupling. The control link is biased towards the lower link by a spring. The control link is provided with a trapping element that engages the lockup link. The trapping element of the control link engages the lockup link in one of two positions, a normal position and a lockup position. In its normal position, the control link biases the hook away from the slide bushing member. In its lockup position, the control link biases the hook toward the slide bushing member.