What Causes Trees to Grow Haywire?

What Causes Trees to Grow Haywire?

Winter is the perfect time to spot odd growths


are enlarged areas of leaves, branches, or trunk, caused by insects or plant pathogens. If the overgrowth is caused by enlargement of individual cells, it is called hypertrophy, and if it is caused by an increase in the number of cells, it is called hyperplasia.

Insects can induce rapid cell division or growth by stimulating or releasing plant growth hormones (such as auxins) through egg-laying or salivary secretions during feeding (poplar twig gall, far right). The plant isolates the toxins, forming the mass of tissue. Gall formation benefits the insect via shelter and an abundant food supply.

Like insects, pathogens induce formation of galls through the synthesis of auxin. Some pathogens can insert a plasmid in plant cells’ DNA that induces the formation of "tumors" (crown gall on apple, above left). As the cells divide, the tumor-inducing plasmid spreads.

A fasciation

occurs when the growing point (apical meristem) of a twig or stem enlarges and flattens at the same time, causing what appears to be a fusion of many flattened stems. Fasciations can occur on many different types of plants, and usually appear on only one part of the plant. Identifying the cause of the fasciation is not always obvious. Most of the time it is a genetic mutation or herbicide injury, but some insects, mites, fungi, or bacteria can also disturb the growing point.

Witches' brooms

consist of a mass of short, stubby twigs growing very close together. The name of this malformation dates back to medieval times, when mysterious occurrences—like strange tree growths—were blamed on witchcraft, and the growths in the trees resembled actual brooms used at that time. Diseases or parasitic plants are the primary causes of witches’ brooms.

In the mountains of Utah, hikers may encounter two common causes of brooms: dwarf mistletoe (a parasitic plant, left) or spruce broom rust (a fungus, right).


Some brooms, however, are caused by genetic mutations. Mutations like these are more common in conifers, and will only occur in one area of the tree. Seedlings or cuttings taken from these brooms have led to a number of dwarfing conifer varieties for use in landscapes.




are prized by woodworkers for their intricate grain patterns. The exact cause of burls is unknown, but they are formed when dormant buds grow inward, twisting and turning under the bark, and never emerging as branches. Most often, burls do not negatively affect tree health.