Considered by some people to be the source of the mustard seed mentioned in Scripture, the mustard tree (Salvadora persica) grows wild throughout much of the Middle East and Africa. Generally attaining a height no taller than 25 feet with fleshy, 1 1/2- to 3-inch leaves, the tree takes advantage of damp conditions near rivers and waterholes but can survive on fewer than 8 inches of rainfall per year. It is hardy in U.S. Department of Agriculture plant hardiness zones 10 through 11. The mustard tree is not the source of the seeds used to make condiment mustard; those seeds are harvested from herbs of the Brassica family.
As Keen as Mustard
Desert browsers such as goats and camels gobble up the mustard tree's mustard-flavored and moisture-rich edible leaves, the most tender of which are eaten by people as well. From January through April, the tree produces 2- to 5-inch-long panicles of small, greenish-yellow flowers. After pollination, those flowers set pea-size fruits that ripen to a maroon shade; each fruit contains a single seed. Although sweeter than the leaves, the fruits also have a pungent flavor and can be consumed raw, dried or cooked.
Cutting the Mustard
Mustard tree is also called toothbrush tree because the tender young sticks cut from the tree have been used as antibacterial teeth cleaners for hundreds of years. People wishing to "brush" their teeth generally strip off a young stick's bark and chew on the stick's inner fibers. Those fibers provide the peelu often found in alternative toothpastes.
A Grain of Mustard Seed
Growing mustard trees from seeds begins with soaking a mustard tree's fruits in lukewarm water for one to three days until their pulp has been reduced to a runny texture. Strain the pulp through fine cheesecloth to collect the tiny, brown seeds. While they are still moist, scatter the seeds in a seedling flat or other container filled with damp sand, and press them into the sand's surface. If their pulp — which contains germination inhibitors — has been completely removed, the seeds should begin to sprout within 24 hours. Keeping the container on a heat mat so the sand's temperature stays in the mid-80s to mid-90s Fahrenheit produces the best results. Because mustard trees grow slowly, waiting until your trees are up to 3 years old may be necessary before transplanting them outdoors.
Tree tobacco (Nicotiana glauca), hardy in USDA zones 8 through 11, also is occasionally called "mustard tree," perhaps due to the yellow shade of its tubular flowers. A South American, soft-wooded shrub that can reach 20 feet tall and has 2- to 8-inch, oval, bluish-gray leaves, it is highly toxic and can be invasive.