In the fields of horticulture and botany, the term deciduous (/dɪˈsɪdjuːəs/; US: /dɪˈsɪdʒuəs/)^[1] means "falling off at maturity"^[2] and "tending to fall off",^[3] in reference to trees and shrubs that seasonally shed leaves, usually in the autumn; to the shedding of petals, after flowering; and to the shedding of ripe fruit.

Generally, the term deciduous means "the dropping of a part that is no longer needed" and the "falling away [of a part] after its purpose is finished". In plants, it is the result of natural processes. "Deciduous" has a similar meaning when referring to animal parts, such as deciduous antlers in deer,^[4] deciduous teeth (baby teeth) in some mammals (including humans); or decidua, the uterine lining that sheds off after birth.

In botany and horticulture, plants, including trees, shrubs and herbaceous perennials, are those that lose all of their leaves for part of the year.^[5] This process is called abscission.^[6] In some cases leaf loss coincides with winter—namely in temperate or polar climates.^[7] In other parts of the world, including tropical, subtropical, and arid regions, plants lose their leaves during the dry season or other seasons, depending on variations in rainfall.

The converse of deciduous is evergreen, where foliage is shed on a different schedule from deciduous trees, therefore appearing to remain green year round.^[8] Plants that are intermediate may be called semi-deciduous; they lose old foliage as new growth begins.^[9] Other plants are semi-evergreen and lose their leaves before the next growing season, retaining some during winter or dry periods.^[10]

Many deciduous plants flower during the period when they are leafless, as this increases the effectiveness of pollination. The absence of leaves improves wind transmission of pollen for wind-pollinated plants and increases the visibility of the flowers to insects in insect-pollinated plants. This strategy is not without risks, as the flowers can be damaged by frost or, in dry season regions, result in water stress on the plant. Nevertheless, there is much less branch and trunk breakage from glaze ice storms when leafless, and plants can reduce water loss due to the reduction in availability of liquid water during cold winter days.^[11]

The conditions that trigger leafout in spring can vary depending on the species or genera of plant.

Most herbaceous perennials and some woody plants are triggered to leaf out by warming air or soil temperatures, for example birches (Betula) and willows (Salix

Leaf drop in the fall months is based on photoperiod and varies by genera and species.

Walnuts tend to drop their leaves early while some trees such as Norway Maple and willows have extremely late leaf drop, often in the middle of November.

Leaf drop or abscission involves complex physiological signals and changes within plants. The process of photosynthesis steadily degrades the supply of chlorophylls in foliage; plants normally replenish chlorophylls during the summer months. When autumn arrives and the days are shorter or when plants are drought-stressed,^[12] deciduous trees decrease chlorophyll pigment production, allowing other pigments present in the leaf to become apparent, resulting in non-green colored foliage. The brightest leaf colors are produced when days grow short and nights are cool, but remain above freezing.^[13] These other pigments include carotenoids that are yellow, brown, and orange. Anthocyanin pigments produce red and purple colors, though they are not always present in the leaves. Rather, they are produced in the foliage in late summer, when sugars are trapped in the leaves after the process of abscission begins. Parts of the world that have showy displays of bright autumn colors are limited to locations where days become short and nights are cool. In other parts of the world, the leaves of deciduous trees simply fall off without turning the bright colors produced from the accumulation of anthocyanin pigments.

The beginnings of leaf drop starts when an abscission layer is formed between the leaf petiole and the stem. This layer is formed in the spring during active new growth of the leaf; it consists of layers of cells that can separate from each other. The cells are sensitive to a plant hormone called auxin that is produced by the leaf and other parts of the plant. When auxin coming from the leaf is produced at a rate consistent with that from the body of the plant, the cells of the abscission layer remain connected; in autumn, or when under stress, the auxin flow from the leaf decreases or stops, triggering cellular elongation within the abscission layer. The elongation of these cells break the connection between the different cell layers, allowing the leaf to break away from the plant. It also forms a layer that seals the break, so the plant does not lose sap.

Some trees, particularly oaks and beeches, exhibit a behavior known as "marcescence" whereby dead leaves are not shed in the fall and remain on the tree until being blown off by the weather.

This is caused by incomplete development of the abscission layer.

It is mainly seen in the seedling and sapling stage, although mature trees may have marcescence of leaves on the lower branches.

A number of deciduous plants remove nitrogen and carbon from the foliage before they are shed and store them in the form of proteins in the vacuoles of parenchyma cells in the roots and the inner bark. In the spring, these proteins are used as a nitrogen source during the growth of new leaves or flowers.^[14]

Plants with deciduous foliage have advantages and disadvantages compared to plants with evergreen foliage.

Since deciduous plants lose their leaves to conserve water or to better survive winter weather conditions, they must regrow new foliage during the next suitable growing season; this uses resources which evergreens do not need to expend.

Evergreens suffer greater water loss during the winter and they also can experience greater predation pressure, especially when small.

Losing leaves in winter may reduce damage from insects; repairing leaves and keeping them functional may be more costly than just losing and regrowing them.^[16] Removing leaves also reduces cavitation which can damage xylem vessels in plants. This then allows deciduous plants to have xylem vessels with larger diameters and therefore a greater rate of transpiration (and hence CO2 uptake as this occurs when stomata are open) during the summer growth period.

The deciduous characteristic has developed repeatedly among woody plants.

Trees include maple, many oaks and nothofagus, elm, aspen, and birch, among others, as well as a number of coniferous genera, such as larch and Metasequoia. Deciduous shrubs include honeysuckle, viburnum, and many others. Most temperate woody vines are also deciduous, including grapes, poison ivy, Virginia creeper, wisteria, etc. The characteristic is useful in plant identification; for instance in parts of Southern California and the American Southeast, deciduous and evergreen oak species may grow side by side.

Periods of leaf fall often coincide with seasons: winter in the case of cool-climate plants or the dry-season in the case of tropical plants,^[17] however there are no deciduous species among tree-like monocotyledonous plants, e.g. palms, yuccas, and dracaenas. The hydrangea hirta is a deciduous woody shrub found in Japan.

Forests where a majority of the trees lose their foliage at the end of the typical growing season are called deciduous forests.

These forests are found in many areas worldwide and have distinctive ecosystems, understory growth, and soil dynamics.^[18]

Two distinctive types of deciduous forest are found growing around the world.

Temperate deciduous forest

Tropical and subtropical deciduous forest biomes have developed in response not to seasonal temperature variations but to seasonal rainfall patterns. During prolonged dry periods the foliage is dropped to conserve water and prevent death from drought. Leaf drop is not seasonally dependent as it is in temperate climates, and can occur any time of year and varies by region of the world. Even within a small local area there can be variations in the timing and duration of leaf drop; different sides of the same mountain and areas that have high water tables or areas along streams and rivers can produce a patchwork of leafy and leafless trees.^[19]