The Sapindus mukorossi, or soapberry tree, soap nut tree, washnut, ritha or Chinese soapberry, is native to India and has been used for centuries for its potent natural cleansing properties. Its fruits, known as soapnuts, are inedible due to their high saponin content. The shells of these nuts become an excellent organic detergent, usable in place of commercial products for both laundry and personal hygiene. This tree is also visually appealing and can be cultivated in Italy; in fact, a fantastic specimen already exists within the Palermo Botanical Garden, and some nurseries are beginning to propagate seedlings from its seeds.
Let’s explore all the secrets of this tree and the magical properties of its nuts.
Origins of the Sapindus mukorossi
The Sapindus mukorossi is belongs to the extensive Sapindaceae family and is known by various names worldwide, including soapnut, soapberry, washnut, reetha, aritha, dodan, and doadni.
In reality, the Sapindus genus includes several species, but the most widespread and utilized for its saponaceous fruit properties is the mukorossi. This particular species is native to the upper regions of the Indo-Gangetic plains, Shivaliks, and sub-Himalayan tracts, spanning a vast region that includes southern China and Nepal. The ritha is extensively cultivated at altitudes ranging from 200 to 1500 meters.
Botanical Characteristics of the Sapindus mukorossi
The soapberry is a deciduous tree that can grow up to 20 meters in height and develop a trunk with a diameter of 1.8 meters. Its crown has a globular shape with dense, leathery, and relatively thin foliage. The bark is light yellow in its youth and darker in adulthood. It is smooth, with numerous vertical lines of lenticels and thin cracks that peel off irregular wood flakes. The wood is hard, non-fibrous, and light brownish-orange. The final leaves are long, approximately 30-50 cm, alternate, paripinnate, with a very narrow common petiole and a glabrous surface. The inflorescence is a terminal compound panicle, 30 cm or longer, with pubescent branches. The flowers are about 5 mm in diameter, terminal, polygamous, white-greenish, sub-sessile, and numerous. Each sepal has a woolly scale on both sides. Flowering occurs in late spring.
The most valuable part of the soapberry is its fruit, known as soapnuts. They are roughly the size of a large olive, but they have a dark brown color. When fully mature, they resemble globose drupes, fleshy, with only one seed, about 1.8-2.5 cm in diameter. Sometimes the drupes are paired. Complete ripening and harvesting occur in autumn. The seed inside is approximately 0.8-1.3 cm in diameter, globular, smooth, black, and loosely enclosed within the fruit’s husk when it’s dry. The husk contains over 10% saponins, an extremely high percentage. The fruit is not edible. It is dried to separate the husk and the seed, which are then used in various ways.
Traditional Uses of the soap nut tree
In its countries of origin (India, China, Nepal), the ritha has numerous applications. Firstly, it has been used for centuries in the ancient Indian practice of Ayurvedic medicine. Recently, many of this tree’s pharmacological actions have been studied. These include properties such as:
Triterpenic saponins are found in the fruit husks and are potent natural surfactants. Soapnuts are well-known among the local population for their cleansing and insecticidal properties. They are traditionally used to remove head lice and are also useful in treating skin issues such as eczema, acne, and severe psoriasis. It’s interesting to note that jewelers in India use the fruit husks to restore the luster of tarnished gold, silver, and other precious metals. The most classic use, however, is as a natural laundry detergent.
Uses of the Sapindus mukorossi’s Seeds
The Sapindus mukorossi’s seeds contain an oil that can be extracted and used as a biofuel. Dried and powdered seeds are employed in the treatment of dental caries, arthritis, common colds, constipation, and nausea. Of course, the seeds are also used for plant propagation.
Using Soapnuts as a Natural Detergent
In the Western world, the soap nut tree is primarily known for using soapnuts instead of traditional chemical detergents. As mentioned, the shells are sun-dried and used for washing and cleaning. Placing them in a cotton pouch, they are used directly in the washing machine. When they come into contact with water, they release active cleaning substances. The great thing is that they last a long time, providing several wash cycles. The popularity of soapnuts among eco-conscious consumers stems from the following characteristics:
- Soapnuts are 100% organic and biodegradable.
- They leave no toxic residues in the environment.
- They can be composted or discarded in organic waste when exhausted.
- They are hypoallergenic and suitable for infants and sensitive skin.
- They work on all fabrics, even delicate ones like silk, cashmere, and wool.
- The production process is organic and does not harm the ecosystem.
This is why soapnuts are considered renewable primary products. If you are interested you can find them here.
Cultivating the Soapberry
Sapindus mukorossi can be considered an exotic fruit in every sense. Although it is cultivated at altitudes up to 1,500 meters, it is still in a subtropical region with high temperatures and long rainy seasons. It is a fast-growing tree but delays fruit production. When cultivated from seed, it takes 10 years to see the first nuts. However, it is a very long-lived tree, reaching a century in age. In Italy, there are a few specimens, one of which is kept in the Palermo Botanical Garden. In our opinion, it could be grown in our country like other exotic fruits such as mango and avocado, preferably in southern regions with artificial irrigation.
Germinating the Seed
One way to try to cultivate the soapberry is to obtain some seeds. It’s relatively easy to grow a seed into a seedling. The challenge lies in finding healthy seeds that can successfully germinate. The seeds are medium-sized, black, with a tough protective film. To facilitate germination, you can soak them in warm water for 24 hours. Once the outer part softens, lightly scrape it with a knife. This way, the roots will have an easier time emerging. Plant the treated seed at a depth of 2-3 cm in a 20 cm diameter pot with good germination soil, and keep it consistently moist.
Germination takes time, and it can be 2-3 months without any signs of growth. Of course, not all seeds will have a 100% germination rate, so it’s best to plant more than one. The pot needs to be kept in a mild location, away from direct sunlight. In winter, it’s best to keep it indoors but away from heaters.
Caring for the Sapling and Planting
Once the soapberry sapling has grown, it’s advisable to keep it in a pot for at least the first year, or even longer. Water it regularly and gradually acclimate it to your region’s climate, providing protection during challenging times (excessive heat, hail, heavy rain, frost, etc). The ideal time for final planting in the ground is the beginning of spring or early autumn. The plant can be placed in full sun but preferably sheltered from prevailing winds. Keep in mind that if the Sapindus mukorossi takes root and thrives, it can become quite large, requiring space in terms of width, occupying at least an 8-meter diameter.
Protection Against Pests and Diseases
An interesting aspect of the soapberry is that it is particularly unwelcome to pests. In its native regions, the plant grows undisturbed, without insects or fungi damaging it. We can say that it is an extremely hardy plant that requires minimal care.
Potential Use in Agriculture
All these characteristics of the ritha suggest that it could find a place in organic farming. One of the most commonly used remedies to eliminate pests is potassium soft soap, a highly concentrated yet gentle soap for plants (which you can find here). These characteristics are also found in soapnuts, with the added benefit of their own repellent and insecticidal action. With many fruits available, you would only need to infuse the nuts in water and then use it for direct treatments against pests. We will see if this development happens in the future. The current limitation is the availability of soapnut in large quantities.