When the nodes alternate, your plants are ready to flower. You can keep them in the vegatative stage to promote more growth, but keep in mind that plants double to triple in heoght during the flowering stage. To better explain alternating nodes, i have drawn up some diagrams with paint. (im no artist) But check the diagrams below. In this diagram, the plants are NOT ready to start the flowering cycle. In this diagram, the plants IS ready to start the flowering cycle.