Even flowers can be used for the Dhamma as objects of concentration, in this case not for their beauty, but for one-pointedness of mind, for seeing transiency, Anicca, Dukkha and Anatta (Impermanence, Suffering and Egolessness).
Water Kasina, Fire Kasina, Wind Kasina, Earth Kasina are natural. When it is the time for contemplation, we offer flowers to the Buddha. We place the flowers in the vase, close our eyes and then open them. Look at the Buddha’s image, then close the eyes. Soon we will remember everything and it becomes a meditation device which makes our mind peaceful for a moment. In the same way we can visualise flowers, and after more practice many flowers will become just one with the same colour. In the end there will be tranquility and illumination.
When that happens we should try to look at ourselves. If we do not form any attachments to outside objects, those objects of concentration will bring wisdom. Consequently, concentration will occur and the Dhamma will be revealed. We will have attained a certain stage of Dhamma. Whatever we use as a meditation device, we must not overlook ourselves. Any illumination from different signs (Nimitta) should be in our heart, our face and our whole body.