Common Coleus Rainbow mixed - Buzzy House Plants. Sow indoors from March till August at 20º C, in a seedtray. Keep soil moist by spraying water with a plant spray and cover the tray with clingfilm to prevent the soil from drying out. Remove the clingfilm when the first shoots appear. As soon as the height of the plants is 5-6 cm, you need to transplant them into flowerpots. After June you place them outside, if there is a sunny sheltered spot.