Fasciation (from the Latin root meaning "band" or "stripe"), also known as cresting, is a relatively rare condition of abnormal growth in vascular plants in which the apical meristem (growing tip), which normally is concentrated around a single point and produces approximately cylindrical tissue, instead becomes elongated perpendicularly to the direction of growth, thus producing flattened, ribbon-like, crested (or "cristate"), or elaborately contorted, tissue. Fasciation may also cause plant parts to increase in weight and volume in some instances.The phenomenon may occur in the stem, root, fruit, or flower head. Some plants are grown and prized aesthetically for their development of fasciation. Any occurrence of fasciation has several possible causes, including hormonal, genetic, bacterial, fungal, viral and environmental causes.

Although fasciation is rare overall, it has been observed in over 100 plant species, including members of the genera Acer, Aloe, Acanthosicyos, Cannabis, Celosia, Cycas, Delphinium, Digitalis, Euphorbia, Forsythia, Glycine max (specifically, soybean plants), Primula, Prunus, Salix and many genera of the cactus family, Cactaceae. Cresting results in undulating folds instead of the typical "arms" found on mature saguaro cactus.

Some varieties of Celosia are raised especially for their dependably fasciated flower heads, for which they are called "cockscomb". The Japanese fantail willow (Salix sachalinensis 'Sekka') is another plant that is valued for its fasciations.

Fasciation that is caused by damage to genetic material and by bacteria can be controlled by not using fasciated plants and disposing of fasciated plants. Avoiding injury to plant bases and keeping them dry can reduce the spread of bacteria. Avoidance of grafting fasciated plants and the pruning of fasciated matter can also reduce the spread of bacteria.