משנה על סוטה 9:3

One who suspends vines over papyrus-ropes [of a trellis], he may not bring seed beneath the remainder [of the trellis]. However, if he did so, he has not caused them to be prohibited. If new [tendrils] spread [over the remainder], that [which was sown under the remainder] is forbidden. Similarly, when one hangs vines over some part of a non-fruit-bearing tree.