Perhaps rubber tree plants (Ficus elastica) were common in 1960s student digs because they tolerated limited light and stale air -- and possibly because the rugged plants were readily available in variety stores. Later, numerous NASA studies would find that they help clean indoor air, too, no doubt a boon to academic concentration. When leaves begin dropping from the bottom of these sturdy trees, the problem might lie in the soil below.
About Rubber Trees
Rubber tree plants are indeed trees, growing to 50 feet or more in their native range from the Eastern Himalayas down the Malaysian peninsula to Sumatra. Hardy in U.S. Department of Agriculture plant hardiness zones 10 to 12, they are grown primarily as a houseplant in the U.S. and valued for their thick evergreen leaves. Breeders have expanded the species to include plants with purple or red leaves or highlights along with the original foot-long green leaves. Although the plants might attract the usual houseplant bad guys -- scale, spider mites, mealybugs, aphids and thrips -- these pests typically attack tender foliage, which would be near the top of the plant.
Bottom leaves drop as a rubber tree grows older. Like other ficus, rubber plants grow Banyan-type aerial roots to age into a multiple-trunk tree with roots along the surface of the ground. While this genus-specific type of growth may not be likely in a rubber plant that spends its life indoors, it’s not out of the question for one that spends most of the year in a planter outdoors in dappled shade to consider acting like one of its tropical forebears.
Bottom leaves that turn yellow and fall off may be a sign of cramped roots or shortage of nutrients. Rubber trees must be repotted periodically into the next size pot or their roots will begin to grow around and around the root ball and out the drainage holes, eventually strangling the plant’s roots, blocking its ability to absorb nutrients. Plants need slow-release or half-strength liquid houseplant fertilizer monthly while they are actively growing. Rubber plants seldom bloom indoors, so nitrogen, which provides food for foliage growth, is the probable need; shortages of potash lead to necrotic leaf edges and leaf drop at the bottom of the plant.
Both overwatering and underwatering might cause lower leaves to drop. Rubber tree fleshy leaves are akin to succulents that store water in their leaves, so too much water will cause leaves to become spongy and fall off. Continual overwatering can lead to edema and death; underwatering dehydrates the tropical plant. Rubber plants can be allowed to dry a bit before water is added to encourage use of reserves. Another common mistake involves placement of plants near floor vents that create a draft of hot air in winter or cold air in summer. Rubber plants do best in warm temperatures -- 75 to 80 degrees Fahrenheit during the day and 60 to 65 degrees at night -- without drafts.
- Missouri Botanical Garden: Ficus Elastica
- University of Florida Horticulture: Ficus Elastica -- Rubber Tree
- New York Botanical Garden: Clean Air With an Indoor Garden
- Purdue University Cooperative Extension Service: Houseplant Problems
- Clemson University Cooperative Extension: Houseplant Diseases and Disorders
- Clemson University Cooperative Extension: Rubber Plant