The Soap Nut or Soap Berry is an extraordinary tree that produces a nut with high saponin content. Saponin is a natural antimicrobial detergent, which lathers when it is wet and rubbed. It is a great environmentally friendly alternative to commercial chemical laundry detergents.
The nuts are simple to use - put 3 to 8 nut shells (depending on how dirty your laundry is) into a muslin bag and throw it in with the washing. Reuse the nuts until they turn dark and soggy, then they can go in the compost. More industrious types might boil up the used shells to make a mild shampoo or liquid hand soap.
The nuts that are available in health stores are imported. The trees do grow here and on the surface, it does seem possible that a commercial operation could be viable.
There are two types of trees available in New Zealand, the Sapindus mukorossi and the Sapindus saponaria.
The mukorossi - commonly known as the Chinese Soapberry, is grown in India and Southern China. It produces the best quality nuts with the highest saponin content; they take around to 10 years to fruit. The tree is deciduous and if grown in warmer subtropical areas it will grow up to a height of 20 metres. Whilst this variety won't tolerate frosts, it is possible to grow it in cooler areas but the tree is unlikely to grow to its potential height. The trees produce large panicles of small creamy white flowers in summer. These develop into fruit which are harvested in winter. Traditionally the nuts are sun dried and deseeded ready for use. Only the outer shell is used for washing.
The saponaria - commonly known as the Wingleaf Soapberry, is grown in the Florida Keys, the Caribbean and parts of Central America. It takes around 3 years to fruit. The fruit is smaller than the mukorossi and the concentration of saponin is lower. The saponaria variety would be fine for home gardeners wanting a fast fix for their laundry blues but the nut quality just isn't there for commercial production. The saponaria is a smaller more compact evergreen tree, growing to around 9 metres. They will tolerate high salt laden winds, infertile soils and drought, making them ideal for coastal gardens. The tree produces large panicles of small creamy white fragrant flowers, which attract bees and butterflies. Like the mukorossi, they develop into fruit which can be harvested in winter.
I am only aware of seeds being available here. They are relatively easy to germinate in the summer months. Lightly sandpaper the seeds to weaken the seed coating. Then drop them into warm/hot (not boiling) water and soak the seeds for 24 hours. It's good to put them into a thermos to keep the temperature constant but not vital.
Put the seeds into deep pots filled with good quality potting mix at 2.5cm depth. Place the pots in a warm spot out of direct sun and keep lightly watered. The seeds should germinate in 1 - 3 months. Once the seedling emerges it has begun to throw down a tap root. It will need re-potting into a larger pot to ensure the tap root is protected. Take care to keep watering the seedlings until they are planted out in autumn.
Plant the mukorossi in rows with 5m between the trees and allow 5m + the width of a tractor between the rows. Make sure you have an excellent irrigation system - these trees will not tolerate drying out. Mulch and fertilise them well. Keep the grass mown between the rows and manage the weeds while they grow.