pre flowers are a sign of sexual maturity on your plants and signifies that the plant is ready to flower. you are always in control of when you switch your plants to flowering. do you have a measuring spoon set or a syringe? you can begin to introduce 1/4 teaspoon or 1.5 millilitres per gallon of all 3 of your nutrients to your larger plants now, just watch the leaf tips for yellowing or browning. if there is no yellowing or browning of the leaf tips you can increase this to 1/2 teaspoon or 2.5 millilitres per gallon of all 3 of your nutrients the next watering to your larger plants. Top dressing or placing new soil on top of the plant pots is an acceptable practice to add soil and nutrients to old soil.