The red mangrove (Rhizophora mangle) is a tropical tree that grows in saltwater right on the edge of the ocean. Mangrove tree seeds germinate, split open and grow a 6- to 12-inch-long cylindrical root called a propagule while still attached to the tree. These then fall, stick into the mud and grow. It is easy to replicate this at home and grow a mangrove tree as a houseplant by starting with these seedlike propagules after they have fallen, but before they have sprouted. In nature, propagules can float on the ocean currents for up to a year before landing and sprouting.
Select a propagule that has not sprouted any roots or leaves. It should be about as thick as your finger, shiny green and relatively straight. It may have a red tint on the fat end. Avoid shrivelled propagules or ones with visible scars and broken tips. The thin end should taper to a green point. Propagules can be obtained by mail order or in some aquarium shops. They can also be legally collected in many tropical regions.
Fill the plastic flower pot with the soil to the top of the rim. Gently firm it up by pressing down on the surface. The surface of the potting mix should be about a quarter of an inch below the pot rim after firming.
Poke a hole in the centre of the soil with your finger. It should be about 1 to 2 inches deep.
Insert the bottom of the mangrove propagule into the hole. The bottom is the fatter part that is usually slightly reddish. The top is pointed and slender.
Firm the soil around the mangrove propagule so that it stands up right on its own without support. If it leans or falls over, then pull it out and bury it a little deeper.
Water the mangrove thoroughly so that water drains out of the bottom of the pot.
Place the mangrove in a bright location, but not in direct sun which may overheat it. It should sprout in a week or two, and the first set of leaves will develop on the top. Keep the soil moist at all times. Mangroves do not like to dry out. As the plant grows it can be moved into a sunnier location.
Buy a mangrove that has not sprouted any roots or leaves yet. They are more easily adaptable to your conditions if planted before they start to sprout. Any type of potting medium will work as long as it holds moisture. Periodically wipe the leaves off with a damp paper towel. As mangroves grow, they expel salt from glands around the leaves that would naturally wash away in rain. This needs to be manually removed when indoors. After several sets of leaves have grown and the plant is well established, the top set of leaves can be pinch off to cause branching and bushiness. This helps keep it small indoors.
Mangroves are tropical plants and are damaged by frost and killed by freezes. They should only be placed outdoors in the summer in temperate climates. Never let the soil dry out. Mangroves normally grow directly in water or in very moist soil. Do not water it with saltwater. Mangroves can only tolerate salt if they have been acclimated over a long period of time.