Unlike other citrus, the simple thorny key lime tree is most easily and successfully propagated by seed. You can go to all the trouble and personal anguish of air layering or struggling with cuttings if you must. But believe it or not, those seedlings won’t be as vigorous and hardy as babies raised from seeds. So don’t torture yourself. Knock on your neighbor’s door and ask for one of his or her ripe, mature key limes ready to fall from the tree.
Choose a ripe, unblemished thorny key lime that has turned completely yellow on the tree and is ready to fall to the ground between late October and late December. If you grasp it lightly and give just a gentle little tug, it will drop into your hand.
Peel the key lime. Don’t cut it with a knife or you risk damaging the seeds. Break the fruit into individual segments. Squash one of them between your fingers, and seeds will pop out. Rinse the seeds under cool running water and drop them into a glass of water to soak overnight.
Fold a paper towel in half, half it again into fourths, and then once more into eighths. Thicker, soft towels work best for moisture retention. Spread several seeds under the top folded edge, and cover them with it. The seeds should be spaced as far apart from each other as possible.
Moisten the paper towel completely, but not so much that it’s wet or soggy. Place it in a plastic sandwich baggie, leaving the top open. Set the miniature greenhouse on a very warm windowsill with lots of indirect sunlight. If a windowsill isn’t an option, a well-lit spot above a water heater or on top of the refrigerator will work, too. Your thorny lime seeds will begin sprouting in 7 to 21 days.
Dampen the paper towel when it begins to dry out, but just so that it’s uniformly moist. Check on it every couple of days. The miniature greenhouse will transform this moisture into necessary humidity for the developing seedlings.
Plant the seedling in a 2-inch pot when the stem, two leaves and a root have developed. Use a very well-draining, sandy potting medium, and keep the surface uniformly moist. Sprinkle some gravel into the bottom of a shallow bowl and just cover it with water. Set the potted seedling on the gravel and place in a warm spot with lots of very bright, indirect light. The seedling can be moved outdoors when the weather warms up and overnight temperatures don’t drop below 50 degrees F.