Plant hardiness is greatly affected by cold spots or heat islands within a garden. It is possible to utilize existing and create new microclimates to encourage plants from different climates to survive outside of their normal hardiness zones. A major element to microclimates is temperature, which can allow for planting a range of different plants with varying needs.

Marginally hardy plants that can survive a short exposure to cold may not tolerate extended periods of low temperatures. Hardscape and adjacent structures or features can absorb and radiate heat, creating warm microclimates that can protect plants during the winter.

Areas with northern and/or eastern exposure tend to be colder than locations with southern and/or western orientation. Shaded areas of the garden tend to be cooler than areas with full sun. High elevation areas, such as alpine mountain tops, experience shorter warm seasons and accumulate more snow. Areas with low spots, such as valleys, depressions, and swales tend to have colder temperatures and may trap humidity. Many non-heat tolerant plants, such as alpines, prefer these cooler microclimates.

Daily freezing and thawing cycles can form ice crystals that damage their roots and may even heave plants from the ground. Mulching an area or adding protective cloth during the winter can insulate plants and allow marginally hardy specimens to withstand colder zones.

Planting on sunny hilltops and mounds tends to provide greater temperature averages as well as increase drainage. Plants in close proximity to rocks, buildings, walls, or paving, tend to experience warmer temperatures and less harsh winters.

Seasonal requirements for soil moisture can vary widely among plant species. Plants native to areas of high rainfall will often underperform if not given adequate moisture before entering dormancy. Relative humidity can be increased by utilizing windbreaks that block drying winds, which can limit cold damage by reducing moisture loss from leaves, branches, and buds. Microclimates in close proximity to water sources, such as stream banks, ponds, and water features, can also experience higher levels of moisture.

Low water adapted plants may succumb to rot if they experience too much moisture during their natural dormancy periods. For example, summer watering certain plants from areas that typically have dry summers can result in vulnerability to soil pathogens that are normally resisted by the plants in active growth periods. Species native to warm, arid conditions can adjust to limited durations of inclement weather if they are not too wet, particularly in winter. Marginally hardy plants may sustain injury or succumb to rot if they are not grown in a high drainage soil amended with gravel or sharp sand. Choosing an elevated spot with bright light, and open wind channels can also help to create a drier area.

The amount of sunlight or shade a plant receives can affect how well the plant performs. Large trees and structures often cast shadows that alter the amount of sunlight in a microclimate. Areas shaded from sunlight tend to be cooler than landscapes that offer full sun. The duration of sunlight is also marked by seasonal solstices. Some plants that are typically grow in full sun in a cooler climate may be grown in warmer climates when given some shade. Plants can be injured if exposed to full sun during the winter, due to rapid changes in the plant’s temperature. For example, evergreen plants can get winter burn when the ground is frozen and the plant cannot replenish its moisture. Plants cultivated by porches and under roof overhangs also tend to have more protection from the sun, wind, and heavy frost.

Urban impacts such as pollution, air quality, and water quality can affect an area’s suitability for sensitive plants. Some air quality concerns such as winter inversions, smoke haze and poor air quality can block sunshine and negatively affect plant photosynthesis and respiration in highly sensitive plants. Soil near roadsides and in low lying spots may accumulate pollutant runoff, such as road salt. This can create a salty microclimate that may be more favorable to costal or salt-loving plants. City and metro areas often create urban heat islands, large microclimates with higher temperatures caused by dense, heat-absorbing infrastructure.

To thrive, plants need to be placed where they will receive the proper conditions. Understanding the unique microclimates each garden contains is key to finding the perfect plants for a specific space. Awareness of the differences in temperature, water, air, and/or light will lead to greater insight into site specific gardening. Recognizing and embracing the existing microclimates in any given landscape will help to create success. Creating microclimates can help push the boundaries of what is otherwise unimaginable by altering topography, soil composition, and adding or utilizing existing structures and hardscapes. Microclimates, like many other aspects of natural spaces, are a unique phenomenon that makes gardening within different environments a wonderful, diverse, and (often) ambitious pastime.