How To: Grow Soap Nut Trees

A few months ago I wrote a review about soap nuts. These are nut husks (Sapindus spp.) coated in saponin, a natural soap. The image to the right is a grove of young adult Sapindus marginatus though the related Sapindus mukorossi is most commonly used as a detergent substitute. You can use these to wash your laundry without other detergents. The best part is that they actually work really well. I use them myself, and anyone who knows me will till you I am renowned for my subtle yet pleasing odour.

One day I found a soap nut seed amidst the husks. These are supposed to be removed from the husks, but this one made it through. My immediate thought was I must grow my own soap nut tree, and so I began to search for information about how to grow them. I could not find much information available, and what is available is geared towards commercial farmers in Asia. After 3 months, I successfully germinated some soap nuts, and now have living soap nut trees in my garden. I'm no expert at this, but I do have experience in horticulture, so I thought I would share what I've learned with you, gentle reader. These directions are based on reading guidelines given to commercial soap nut tree growers, my experience in growing soap nut trees of my own, and my horticultural experience growing other plants.

1. First you must find a seed. These can be found with the husks occasionally, but you may have to look through quite a few husks to find one. I'm sure this varies from source to source, but by my estimate about 1 seed makes it through for every 50 to 100 husks. Because you use about 5 husks per load of laundry, this means you are likely to find one every 10 to 20 washes. The seed will be charcoal black, about the size of a small grape, and stone hard. If you find the seed after it has gone through the wash, it will still be perfectly good.
2. Secondly, you must scarify the seed. Because the seed coat is so hard, the plant embryo inside cannot breakthrough the seed coat on its own. You must help it by damaging the seed coat. (The evolutionary purpose of this is to stagger the germination span over a period of months or even years, as well as dispersing them geographically so that an entire generation of seedlings cannot be wiped out in one streak of bad luck.) Nature scarifies seeds through harsh weather and by animals eating and partially digesting them. We'll have to be a little creative. One option is to use a nail file and wear down a notch in the seed coat. I found the seed coat to be so tough that sand paper and fine-grained files did not leave a mark. Another option is to hammer the seed. Be careful not to crush the seed; we just want to weaken the seed coat. I gave about a dozen hard whacks to my seed against concrete, and felt like I was weakening it, but did not see any visible change. Thirdly, you may soak it in hot water. Don't use water that is actually boiling, but it can still be very hot. I boiled a kettle, let the hot water sit for five minutes, and then filled up a vacuum-insulated thermos with the seeds and water, and let it soak for 24 hours. The thermos will keep the water quite warm throughout that period. I used all three methods (filing, hammering, soaking) and it worked ok, but I'm sure there are other good methods too. Soaking is particularly important though, as the water is what activates the germination.
3. Thirdly, you need to plant the seed. I would do this in spring or early summer in a pot either outside or in a greenhouse. Choose a pot that is deep, as soap nut trees send down vertical tap roots. If you don't have a deep pot, a 2 litre plastic bottle works well - cut off the top and drill several holes in the bottom. Bury the seed in potting soil (not dirt - use good quality potting/germinating soil) to about three times the seed's depth. Put it in a place where it will not be in direct sun, and where it can catch some rainfall. Water the pot if the soil starts to dry, but don't water if it is still moist - that can promote fungal growth. Also, avoid fertilizing the soil before germination occurs - high levels of nitrogen in the soil can actually inhibit germination in general.
   

4. Fourth, wait. Your seed may take a long time to germinate. It could be 1 month to 3 months, perhaps even more. Not all of the seeds will germinate, but if you follow these directions, you should get 80%+ to grow. Once it does begin to grow, it will shoot up fast. About 1 foot in 1 month should be about right, then it will slow down a little. Give it plenty of full sunlight, and water when soil begins to dry. The image to the right is a Sapindus mukorossi (as far as I can tell) about 3 weeks after germination. It is ready to be repotted in a larger pot.
   
5. Fifth, taking care of the tree... My trees are still very young, so I cannot provide a lot of personal experience. I will be growing mine in progressively larger pots, keeping them on a sunny patio when temperatures are above freezing and moving them indoors when it gets cold. (They may be able to cope with British/American winters, but I need to find out about that - they are mainly grown in northern India and southern China, so they may or may not be able to sustain freezing temperatures, depending on their specific biome.) They appear to be generally quite hardy and should not need a high level of care. From what I can piece together, you should start getting decent crops of soap nuts in about 10 years, but I would not be surprised if you get some significantly earlier or later than this. The tree will start off with a smooth silvery bark, which will eventually become darker and rougher. Some of the largest Sapindus trees are 75 m in height, but that would be quite unlikely out of their natural environment.