Yang was initially symbolized by the warmer, southern side of the mountain, a sunny bank, later the mountain as a whole. Subsequently Yang represented everything including light and bright day, summer and sun. Also dryness, air (wind) and fire were assigned to Yang, as well as increasingly warm days of spring, the warmer season’s flourishing and revival of nature, all that is active, creative, invigorating, expanding, shiny, external. Yang corresponds to the hard and masculine.
Yang manifests itself in odd numbers and is represented by a solid line, for example | or also

yarrow stalks

For the I Ching various divination methods exist. When, for example, divination is done with yarrow stalks, fifty dried stalks are selected from a bundle and taken in the left hand. Then one yarrow stalk is put away. The remaining 49 yarrow stalks are divided into two piles and, taking four yarrow stalks from each pile, are counted according to a complicated system until the result is either 2 or 3. This process is repeated three times and the sum is added up. The values 6 and 8 represent a broken line (yin) and 7 and 9 a solid line (yang). This process is repeated six times until the hexagram is complete.
The values 6 and 9 are regarded as changing lines, i.e. they change into their respective opposite: 6 (Yin) becomes 7 (Yang), 9 (Yang) becomes 8 (Yin). This transforms the original hexagram into another of the 64 hexagrams.
In this context the material – yarrow stalks – certainly also has symbolic significance: The stalk of the yarrow is hard on the outside, but hollow and soft on the inside, thus symbolizing the opposites of yin and yang. Yarrow itself is a remedy that unfolds partly opposite effects in the human body, depending on which effect is required for healing, which in turn refers to its balancing effect on the relation of Yin and Yang.

Read more: https://en.wikipedia.org/wiki/I_Ching_divination#Yarrow_stalks