Frost includes the direct crystallization of water on exposed objects and, of more importance, the occurrence and effect of subfreezing temperatures on plants and crops. Damage to crops is caused by freezing of the aqueous solutions in plant, cells causing these to burst and destroy the plant partially or completely. The occurrence of frost determines the length of the growing season in many regions. This may reach critically short proportions for most usable crops in some regions. Citrus fruits are particularly sensitive. In colder regions, frost action is exceedingly important in weathering and erosion, in disturbing vegetation (destruction of root systems), and in engineering problems. In such regions, frost causes the ground to heave, disturbing foundations and landscaped surfaces.