Heat Resistant Plants

By Richard Flowers, ACCNP-Green Thumb Nursery-Ventura

Did you know that some plants are specially adapted to thrive in heat? They have evolved and developed ways to minimize water loss and stay cooler. It is remarkable how certain plants have adapted to survive in harsh environments. As longer, hotter summers become more common, it is no surprise that more people are interested in plants that can withstand heat.

What happens to a plant when the thermometer stays above 100 degrees for many days? Photosynthesis is one of the most incredible chemical processes on Earth, allowing plants to use sunlight to produce food from water and carbon dioxide. However, at temperatures over 100 degrees, photosynthesis will not work correctly. Plants that endure high heat can become stunted, weakened, and more vulnerable to pests and diseases, even when water is available. This kind of response is typical. Many plants have developed impressive survival mechanisms to help fight or lessen this effect. They have natural systems that respond to heat stress. For example, they can cool themselves by pumping water out through the leaves, creating a swamp cooler effect. They can also produce “heat-shock” proteins, which help reduce problems caused by overheating. How amazing is that?

All these strategies require resources that could be used for other plant needs, such as growth, flowering, and fruiting. In gardens and landscapes I have seen, heat-tolerant plants often look tired, droopy, and weak during a heat wave. Even the most resilient shrubs and flowers can appear sad when the heat is intense. Usually, they enter a dormant phase to conserve energy during the hottest parts of the season. During this time, they may stop blooming, and green leaves might curl or shrivel. If that happens, do not worry—keep them watered and fertilized (but avoid overdoing it). The plants should recover once the temperature drops. 

Not only do internal systems and chemical processes inside the plant influence its survival, but also other fascinating physical and structural features on the plant’s exterior are evident. These heat-tolerant plants display specific characteristics such as small, thick, waxy leaves to help retain moisture during hot, dry summers. Plants with smaller, hairy, waxy, or succulent leaves lose less moisture through transpiration. Some may have thorns or spines that help minimize water loss. Species that endure heat often have lighter-colored leaves: gray, gray-green, silver, or olive green. These plants reflect sunlight. In addition to leaf color, they typically have a waxy, leathery, fuzzy, hairy, or succulent texture. These features help reduce water intake, store water in leaf tissue, retain moisture, and provide shade. Many of these plants adapt their stem and leaf shapes to withstand high temperatures. When it gets hot, they adopt a more open, less dense growth form, which keeps their leaves more upright to conserve moisture and stay cooler, significantly reducing sun radiation. The leaf stalks stretch to allow more wind to pass through, helping dissipate heat. Other plants have developed thick, fibrous roots that can store water for later use. Heat-tolerant plants often have deep root systems that access water and nutrients from deeper soil layers where moisture is more plentiful, as long as water is available. How incredible is that?

The key to establishing a deep root system for plants in the landscape is to understand the concept that roots always follow water. If the water level in the soil is shallow, the roots will be shallow, and likewise, if the water level in the soil is deeper, the roots will be deeper. Be sure to focus on the latter. To achieve this, it is best to water plants deeply, slowly, and over an extended period (several hours) with a slow drip or trickle of water. This allows the water to seep evenly into the deeper parts of the soil around the plant, unlike a watering practice that involves shallow watering every other day for 10 minutes. Water less often for longer periods, rather than more frequently for shorter periods. All plants, including heat-tolerant ones that are young or newly planted, initially have shallow root systems and are therefore more vulnerable to drying conditions in the top few inches of soil. These plants need more consistent watering.

When selecting plant material for the garden, it is important to choose ones that are well-suited to the climate. Many heat-tolerant plants are native to specific regions around the world. All originate from areas that experience hot, dry summers with long heat waves and mild winters. These plants come from the Mediterranean region. Mediterranean climates also face frequent long drought periods. Such climates are found in only five places on the planet:

1. California (extending into southern Oregon and northern Baja California, Mexico)

2. The Mediterranean region itself (Southern Europe, Western Asia, and Northern Africa)

1. Subtropical South or West Coasts of:

• Chile
• South Africa
• Australia

All of these regions support uniquely adapted groups of plants. Much of the Mediterranean is arid, with high evaporation rates and little summer rainfall. Many plants native to the desert southwest are gaining popularity due to climate change. A small part of the Mediterranean climate—specifically the immediate coastal zone, not inland areas—is classified as temperate (coastal semi-desert). These areas experience warm, humid summers with thunderstorms and mild winters. Large, woody, evergreen shrubs dominate the plant communities in these regions.

There is an array of plants that can be useful and endure the inland heat, thriving well in the landscape. These heat-tolerant plants are a wonderful addition to any garden, especially in hot climates. By selecting the right varieties and providing proper care, you can enjoy the beauty of these resilient flowers throughout the summer and beyond. These plants attract butterflies, hummingbirds, and other pollinators. Plants with succulent leaves, like ice plants, agaves, aloes, and cacti, absorb moisture in their fleshy stems, thorns, and leaves, which helps them resist heat and drought. Many California natives are adapted to high temperatures. Native sages (Salvia), Cleveland Sage, along with all its naturally occurring hybrids and selections, are excellent choices. These sages can bloom from mid-spring to early fall with whorls of lavender-blue flower spikes. Bees, butterflies, and hummingbirds all find them attractive. A favorite sage of mine is White sage (Salvia apiana); the smell is wonderful. Other California native plants that can handle the heat with little water include Manzanita and Ceanothus (California Lilac). Like all California natives and Mediterranean plants, it is best to plant them in the fall after the summer heat. Once established, they will endure hot summers in the following years. The list isn’t limited to California natives; many other species are also well-adapted to and tolerant of heat. These include Butterfly Bush, Germander, Texas Sage, Rudbeckia, Coreopsis, Plumbago, Gaillardia, Oregano, and Verbena. These Mediterranean plants thrive well in a suitable garden. 

Many heat-tolerant plants provide beautiful seasonal (annual) color, including the Marigold, a summer annual known for its vibrant orange and yellow shades, and its ability to thrive in hot, dry conditions. A favorite flower of mine, the Zinnia, is another summer annual that comes in a wide range of colors and is highly heat-tolerant. The Petunia is a summer annual and is a versatile flower that can withstand high temperatures and bloom profusely throughout the summer. They are available in various colors and patterns. Lantanas are heat-tolerant perennials that produce clusters of colorful flowers. They can bloom about half the year, especially near the coast. They are often used in hanging baskets or as ground cover. Portulaca is a summer annual and is also known as moss rose, which is a low-growing flower that thrives in hot and dry conditions.

A large shrub or tree called the Olive is a classic Mediterranean native that has become very popular over the years. The Olive can be either a fruiting or a fruitless type, growing up to 35 feet tall and 20 to 30 feet wide. Olives thrive in hot, dry summers and will adapt to shallow, stony, or poor soil. Wilson Olive is appreciated for its fruitless qualities as well, and several fruiting varieties of olives are available. This picturesque tree looks handsome as a multi-trunk or spectacular in a traditional tree form.

Rosemary is a well-known herb used for cooking, but it can also serve as an excellent landscape shrub that attracts bees and adds a lovely aroma. Rosemary comes in two varieties: an upright type that can grow up to 8 feet tall and 5 feet wide, making it perfect as a medium-sized background shrub or a large filler—pruning it lower can make it easier to manage. The other type of rosemary is a ground cover that grows 3 to 4 feet tall, but it can also be pruned lower. Additionally, it is useful for erosion control or even as a low clipped hedge. 

Grevilleas are native to Australia, and hummingbirds love them. Grevilleas tolerate low water and poor soil as long as it is well-drained. Some types of Grevilleas are low shrubs, while others can grow into large trees or shrubs, with many varieties in between. 

Another native Australian plant is the Acacia. These plants are durable, tough, and easy to care for. Some Acacias are large shrubs or small trees, while others are wispy and low-growing. 

The Crepe Myrtle (Lagerstromia) might be the only plant that seems out of place in this discussion because it is not native to the Mediterranean. However, this Chinese tree has become increasingly popular due to its heat tolerance, low water needs, manageable size, summer blooms in various colors, and interesting bark. The Crepe Myrtle sheds all its leaves in winter, grows to an average height of 20 feet, and produces flowers that can be red, white, lavender, or pink, depending on the variety. Some Crepe Myrtles are multi-trunked, while others may have a more traditional tree shape. Currently, many interior areas are in full bloom with this plant. Dwarf varieties are sometimes available. Crepe Myrtle thrives in areas with intense heat and lots of sun, but is not suited for the immediate coastal zone. 

The following are some tips to keep in mind for heat-tolerant plants to thrive in the garden:

1. Most heat-tolerant plants love lots of sun; however, sometimes they appreciate some partial shade, especially if they are to be planted in interior areas where the temperature exceeds 100 degrees most of the summer.

2. All new plants, even drought-tolerant ones, need water while establishing their root systems and during periods of extreme drought. New plantings require a small amount of pampering to develop into vigorous, drought-tolerant, mature plants.

3. Try amending the soil with organic material so the plants have a good foundation to grow in.

4. Remember to water wisely, especially during a dry spell — in other words, water deeply and less frequently to encourage deeper root growth.

5. Apply a layer of organic mulch around the base of the plants to help retain moisture, suppress weeds, and regulate soil temperature.

6. Often overlooked but essential is to deadhead the spent flowers to encourage continuous blooming and prevent the plants from diverting energy into seed production because it is tougher on the plants when they are producing seed, especially during the heat.

7. Fertilize with a slow-release fertilizer to provide essential nutrients for healthy growth and to make the plant stronger.

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