If by \"Zygo\" plant you mean a Zygocactus truncates or Christmas cactus house plant, then wait another month before pruning and shaping your winter flowering Zygo. Unlike most cacti this variety appreciates moist soil when it is in active growth and bloom. Let the soil dry out before watering for six weeks in the fall to really bring on the winter blooms. As for using a lawn weed killer, the weed and feed products work best when the temp is above 65 degrees and work really well if you read and follow all the instructions about how much to apply, when to water and what weather conditions are best for application. Keep growing, Marianne Binetti