thumb|275px|[[Wyethia helianthoides or mule's ear wildflower (on right) showing fasciation]]
thumb|200px|A "crested" [[saguaro cactus (Carnegiea gigantea), resulting from fasciation, located at Saguaro National Park (West), Arizona, U.S.]]
Fasciation (pronounced , 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, bacteria, fungal, viral and environmental causes.
Cause
Fasciation can be caused by hormonal imbalances in the meristematic cells of plants, which are cells where growth can occur. Fasciation can also be caused by random genetic mutation. Bacterial and viral infections can also cause fasciation. and in lilies (Lilium longiflorum), but many fasciated plants have tested negative for the bacteria in studies, hence bacterial infection is not an exclusive causation.
Additional environmental factors that can cause fasciation include fungi, mite or insect attack and exposure to chemicals.
Occurrence
Although fasciation is rare overall, it has been observed in over 100 vascular plant families, including members of the genera Acer, Aloe, Acanthosicyos, Cannabis, Celosia, Cycas, Delphinium, Digitalis, Echinacea, Echinopsis, Euphorbia, Forsythia, Glycine max (specifically, soybean plants), Primula, Iochroma, 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.
thumb|200px|A "cockscomb" strawberry ([[Fragaria × ananassa)]] Many strawberry (Fragaria spp.) cultivars are susceptible to fasciation, often in response to environmental stresses. As the fleshy edible portion of strawberries arises from the receptacle (a portion of the floral stem) rather than the ovary, strawberries that develop from fasciated stems also become fasciated, taking on a wedge or fan shape. Extreme examples are sometimes called "cockscomb" berries for their resemblance to a rooster's comb. Fasciated strawberries may be considered less marketable, but may also grow to extraordinary sizes: in 2021 a fasciated strawberry of the Ilan variety grown in Kadima-Zoran in central Israel was weighed at .
Some varieties of Celosia are raised especially for their dependably fasciated flower heads, for which they are also called "cockscomb".