The following plants have parts which can be used for shampoo:
Bracken, a decoction of chopped roots

Birch: birch sap

Chamomile, an extract of the flowers

Juniper, a decoction of the branches as an anti - dandruff shampoo

Chestnut, an extract of the leaves and peels of the nuts

You can customize special shampoo for your hair type. Chamomile soothes and lightens blonde hair, sage for dark hair and parsley enhances dull hair.
Bring 1/2 cup of water to a boil, pour it over the herbs and let it soak 20 to 30 minutes.
Sieve the infusion. Add stirring soap, and some cider vinegar.
You can also (boiled down to 1/4th) add beer or vodka to shine, aloe vera gel for oily hair or jojoba oil for dry hair.

I saw someone with green hair. If I had green hair I would let it paint. (Herman's Hermits)
The best remedy for hair loss is a baldy. (Telly Savalas)
The international situation is sometimes so complicated that even the hairdresser knows no solution anymore. (Marco A. Almazan)

Shower
Showering can quickly and environmentally conscious with e.g. a camping shower, a black water bag that accumulates solar heat. Thereto a tube with a nozzle.
Or use even a small watering can with a sprayer. Put some hot water in a bucket and fill it with cold to an ideal temperature. You 'll be surprised with how little water (and energy) you can still have a nice shower. Can also be fine with 2.

Liquid cleaners as shampoo, conditioner, soap and detergent, you can top up with water. Especially when the bottle is (almost) empty. The diluted product is still excellent.

With a low flow showerhead you use 4 to 9 l / min instead of 10-15 l / min. That's 50% less water and at least € 25 less gas per year.
If you heat your water through a (gas) water heater, you may probably not be able to use a low flow showerhead. The flow rate must be high enough to strike the water heater (= the tap limit).

If you are showering or taking a bath you can still use the water to rinse the toilet, wash the car or to mop.