Digging In - Advice on Pruning Rubber Tree, Coaxing Flowers From Crape Myrtle

By Scott Aker
Special to The Washington Post
Thursday, January 29, 2009

Q Nine years ago, I got a rubber tree that was just 10 inches tall. Now it stands about 11 feet high, and it towers over my living room. How much can I prune this plant, and when will I see new leaves growing? Also, can I use the pruned branches to make new cuttings?

A Prune your rubber tree back to about a foot from the soil surface. Late spring is the best time to do this, when more daylight and warmth will help spur new growth.

In May, when temperatures have warmed, you can place the plant outdoors in a shaded location to promote maximum leaf growth before bringing it indoors in early fall.

Rubber tree cuttings will root, but they are rather slow to do so. Take a tip cutting (or several if you wish) of about six inches, remove the lower leaves and place the cutting with its remaining leaves in a zip-top bag with a few drops of water. Close the bag as much as possible while allowing the cut end to stick out. This will allow the cut to dry. After a day or two, remove any latex that exudes from the cut. Dip it in water and then in rooting hormone. Make a hole in a pot of perlite or sand and place the cutting in the hole. Loosely cover the pot with a zip-top bag and mist it several times a week. Keep the pot out of direct sunlight. After four to six weeks, roots should grow, and you can move the rooted cutting to a container with potting soil.

Rubber trees can also be shortened and propagated through a technique called layering. Score around the circumference of the stem about six inches below the lowest intact leaf. Dust some rooting hormone on the resulting wound and cover it with damp long-fiber sphagnum moss. Place a piece of plastic wrap around the lump of sphagnum, securing each end with a twist-tie. Finally, wrap a small piece of foil around the plastic to keep out light, which would suppress the formation of roots.

After one or two months, roots should be apparent in the sphagnum. Detach the layer from the rest of the plant and pot it to make a new plant.

Five years ago we had a tall crape myrtle tree planted. It is now 20 feet tall but barely flowers. The tree appears healthy but really is a disappointment. How can we get it to bloom? Should we prune it aggressively this spring?

Your crape myrtle may not get enough sunlight, or it may be getting too much fertilizer and water to produce anything other than vegetative growth. If the problem is shade, you may be able to remove or thin branches of nearby trees to elevate light levels. Cut back on watering and discontinue any fertilization. Pruning will only disfigure the tree and will not stimulate flowering.

Crape myrtles prefer a somewhat heavy soil with a fair amount of clay in it. The clay holds a tremendous reservoir of nutrients and water, and crape myrtles can easily get by on little water and no fertilizer in clay soil. They don't perform as well in sandy soil, where they may benefit from a light application of fertilizer in the spring as growth begins.

Scott Aker is a horticulturist at the U.S. National Arboretum.

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