Evergreens are a diverse group of species known for maintaining their foliage across all seasons. Unlike deciduous plants, which shed all their leaves annually in response to cold or dry conditions, evergreens only lose a small portion of their leaves throughout the year. This strategy allows them to retain a green canopy and begin energy production whenever conditions permit. Understanding their unique biology is key to successfully incorporating them into any landscape.
How Evergreens Retain Their Foliage
Evergreens remain vibrant through harsh weather due to several biological and structural adaptations. A primary adaptation is the presence of a thick, waxy coating, known as a cuticle, that covers the leaf surface. This cuticle is composed of a fatty substance called cutin, which reduces water loss through transpiration.
This water conservation mechanism is important during winter, a period of physiological drought when the ground is frozen and roots cannot absorb moisture. Many evergreens also have specialized leaves, such as the slender needles of pines or the small, rolled-up leaves of rhododendrons, which minimize surface area exposed to drying winds and sun. Their dark-colored foliage contains chlorophyll year-round, allowing them to initiate photosynthesis at a low rate even on warmer winter days. To prevent damage from bright winter sun when temperatures are low, the plant uses specialized compounds called xanthophylls to dissipate excess light energy as heat.
The Main Divisions of Evergreen Plants
Evergreens are broadly categorized into two groups based on their leaf structure. The first group is the needled and scale evergreens, which includes most familiar conifers. Needled evergreens feature foliage that is either long and slender, like the needles of pines and spruces, or small and flat, such as the scale-like leaves found on junipers and arborvitae (Thuja).
Pine species are easily identified because their needles grow in bundles, typically grouping two to five needles together. Spruces and firs have needles that are attached individually to the branch. Scale evergreens, like the Hinoki cypress, have tiny, overlapping leaves that create a dense, fan-like texture. This foliage type is adapted to shedding snow and minimizing moisture loss, making these plants resilient in colder climates.
The second group consists of broadleaf evergreens, which possess flat, often large leaves that resemble those of deciduous plants, but they retain their foliage for more than one growing season. Examples include the glossy leaves of hollies (Ilex), the leathery foliage of rhododendrons, and the compact leaves of boxwood (Buxus). Many broadleaf varieties offer ornamental interest, such as the showy flowers of camellias or the colorful berries produced by female holly plants. They provide a rich texture and varied form that is valued in landscape design.
Choosing and Caring for Year-Round Plants
Successful growth begins with selecting a species rated for your specific hardiness zone, which defines the minimum winter temperature a plant can tolerate. After confirming hardiness, consider the plant’s light requirements, as most evergreens prefer full sun to partial shade, and ensure the location has well-draining soil. Evergreens are susceptible to root rot if they sit in standing water. Many varieties, particularly rhododendrons and most conifers, thrive best in slightly acidic soil with a pH between 6.0 and 7.0.
A common issue, known as winter desiccation or winter burn, occurs when the leaves lose water faster than the frozen roots can replace it. To prevent this, deep watering in the late fall is recommended before the ground freezes. If winter temperatures rise above 40°F during dry periods, providing supplemental water at the tree’s drip line is beneficial, but only when the ground is not frozen.
Pruning techniques depend on the evergreen type because conifers, such as pines and spruces, do not regenerate new growth from old, leafless wood. For these needled varieties, light thinning and shaping should focus on cutting back to a side branch or a bud, removing no more than one-third of the growth. Broadleaf evergreens like hollies and boxwood are more tolerant of heavy pruning and can be sheared or cut back severely to maintain a formal shape. Pruning should aim to remove dead or damaged branches and improve air circulation within the canopy.