How Heat and Humidity Impact Outdoor Plants

Few things are as rewarding as watching a plant flourish and grow under your care. If you live in an area with high heat or humidity, take that into account when selecting and caring for your plants to give them their best chance at success.

Hardiness Zones

Plants are surprisingly resilient. Most types can be grown almost anywhere if given proper care. But it takes a lot less work to make them grow in an environment where they’re naturally inclined to thrive.

Most plants are sold with a tag showing what “hardiness zone” they’re a good fit for. Defined by the USDA to reflect average annual winter lows, hardiness zones will help you make informed decisions when selecting plants. But low temperatures are only part of the picture. Many gardeners also need to consider humidity or potential heat waves.

The Hot Seat

Generally speaking, plants native to cooler climates struggle with high temperatures, and need more care during heat waves. If your plants are suffering in heat, watch their leaves. Wilted leaves are a defense mechanism. When shrinking and drooping, less of the leaf is exposed to the sun, keeping water within the plant and lowering its temperature. The leaves should perk up after sunset or an early morning watering. If not, it’s time to provide additional care.

Start by noting the sun’s path. Move potted plants into the shade and adjust their position as needed during the course of the day. For ground-based plants, shade netting can have the same effect.

Consider the time of day when watering your garden. Water droplets act as tiny lenses, amplifying the sun’s rays and potentially damaging leaves. Water your plants early or later in the day when the sun is less intense, and water the soil rather than onto the plant itself. (Soaker hoses are especially useful for this.)

It’s Not just the Heat, It’s the Humidity

Humidity presents a separate set of challenges, and plants that can’t deal with higher moisture levels can struggle to process food and ward off disease.

A plant’s root system is a little like a straw, drawing water up through the roots, spreading moisture and nutrients, then releasing processed moisture into the air through a process called transpiration. In high humidity, transpiration is less effective. If a plant can’t transpire, it’s like someone has pinched the middle of the root-system straw. Plants that have adapted to humid environments have alternate ways to release vapor, such as more stomata (microscopic holes in their leaves).

One way to identify a given plant’s adaptation to humidity is to look for leaves that channel water into a pointed end. These “drip tips” shed excess water from the leaf surface, while leaves that are cupped hold water for longer periods. The former are more likely to do well in high humidity, while the latter may thrive in dry environments but struggle with mildew and blight fungus in humid conditions.

Lastly, don’t forget to water yourself as well! Be sure to hydrate, use sunscreen and don’t work in the garden during the hottest parts of the day.