According to Traditional Chinese Medicine, winter is the season of the Kidneys. The Kidneys are at their most vulnerable this time of year so it is the perfect time to nourish them. Here are four herbs to Tonify the Kidneys.

Walnut - Especially good for cold and weak back and knees. Also good for frequent urination, coughs/wheezing that is worse with exertion, and constipation. Note that eating in excess may cause canker sores.
Ashwagandha - Normalizes hormones including thyroid, testes, and adrenal glands. Also helps with anxiety, fatigue, muddled thinking, stress, low libido, lowers Blood sugar, insomnia, etc. A calming adaptogen. Note that it takes about a month to build up in your system so the effects of this herb won’t generally be noticed for a while.
Fenugreek - Especially useful when there is coldness in the body from a weakness of the Kidneys such as abdominal coldness and pain, pain and cold sensations in the knees and feet. Also helpful for fevers, asthma, emphysema, hernial pain, nervous exhaustion, anemia, and pain in the testicles. Note that it should not be taken during pregnancy.
Damiana - A Kidney tonic especially useful for decreased libido and impotence. Also used as an aphrodisiac. A nervine and mild antidepressant per David Winston it is useful for elderly people with mild depression. Note that this should not be taken during pregnancy.