Scorching summer heat can be tough on gardens. Many plants wilt or stop blooming when temperatures soar. But some trees actually thrive in hot, sunny conditions. If you live in a climate with steaming summers consider adding one of these full sun trees to your landscape.
Red Maple
With brilliant fall color ranging from bright yellow to blazing red, the red maple (Acer rubrum) is one of the best small trees for full sun. It has a rounded oval shape and grows fast, up to 2 to 3 feet per year when young. Eventually reaching 40 to 60 feet tall and almost as wide, red maple casts cooling shade over yards. This versatile American native tolerates drought, pollution, poor drainage, and compacted soil. Plant red maple in zones 3-9.
White Oak
A symbol of strength, the white oak (Quercus alba) becomes a massive, spreading shade tree at maturity. It grows well in full sun and adapts to most soil types, including clay and acidic soil. The dark green leaves provide dappled shade beneath the 50 to 100 foot canopy. White oak is hardy in zones 3-9. Its acorns attract wildlife, its lumber is highly valued, and it’s resistant to drought and pollution.
River Birch
River birch (Betula nigra) is a fast-growing deciduous tree that thrives in hot climates Native to the eastern US., this multi-trunked birch grows 40 to 70 feet tall. The peeling, paper-like bark provides year-round interest. River birch needs regular watering when young but becomes quite drought tolerant. Plant it in full sun or light shade. Grow river birch in zones 4-9.
Ginkgo
With unique fan-shaped leaves, the ginkgo (Ginkgo biloba) is one of the oldest species of trees on earth. It thrives in heat and resist pests, pollution, salt, and urban conditions. Male ginkgo trees are preferred because females produce messy, smelly fruit. Ginkgo grows slowly, eventually reaching 60 to 100 feet high. Its golden yellow foliage is spectacular in fall. Grow ginkgo in zones 3-9.
American Hornbeam
Also called musclewood, American hornbeam (Carpinus caroliniana) has smooth gray bark and sinewy trunks, hence its common name. This small understory tree grows 20 to 30 feet tall and wide. American hornbeam thrives in full sun or partial shade and adapts to most soil types. Yellowish fall color and slightly irregular shape give it artistic appeal. American hornbeam grows well in zones 3-9.
Sweetbay Magnolia
A Southern magnolia cousin, sweetbay magnolia (Magnolia virginiana) brings graceful beauty to gardens. This small flowering evergreen tree reaches just 20 to 30 feet tall. Fragrant white blooms appear in late spring and sporadically all summer. Sweetbay magnolia grows in zones 5-9. It needs regular watering but tolerates intermittent mild drought. Plant it in full sun or partial shade.
Serviceberry
Serviceberry (Amelanchier) offers multi-season interest with spring blooms, tasty summer berries, and reliable fall color. These understory trees grow 15 to 30 feet high. Varieties like Autumn Brilliance and Princess Diana serviceberry thrive in full sun and zones 4-9. Their delicate white flowers and oval, lobed leaves add grace to landscapes. Plant serviceberry as a specimen plant or in groupings.
When designing an outdoor oasis that withstands summer swelters, use trees strategically for cooling shade and comfort. Place them to shade patios, porches, driveways, and air conditioning units. Groupings of both evergreen and deciduous trees create shelter from the afternoon sun. Even smaller sun-loving trees make excellent focal points while helping filter intense sunlight.
With smart tree selection and placement, you can enjoy outdoor living despite rising temperatures. But provide plenty of water for new plantings, especially during their first two years. Mulch around trees to conserve soil moisture. Once established, many hardy, drought-resistant trees will thrive with minimal care through blazing summers.
Piedmont Master Gardeners • Sharing knowledge, Empowering communities PMG News
- By Cathy Caldwell
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- November 2021-Vol.7, No.11
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The original idea behind this article was to list the types of trees that will be able to handle the heat and drought of the future so that people who are planting trees this fall could use the information to help them choose which trees to buy. I soon discovered that this was not a simple task.
It’s not easy to figure out how well a species can handle drought, and it can be different even within the same species depending on how it has adapted to its environment. Perhaps this explains the conflicting information in lists of drought-tolerant plants. As for heat or thermal tolerance, there doesn’t seem to have been much research on the subject until recently. Lastly, new research suggests that trees that have been able to handle drought in the past might not be able to handle the higher temperatures and droughts we can expect in the next few decades—unless we cut down on the greenhouse gases we are putting into the air. After reading this, I became more interested in the subject and looked at all the lists and labels for drought and heat tolerance with a fresh mind.
The traditional method of identifying drought tolerance has focused on features like leaf size and crown characteristics. Once expert summed it up this way: “An ideal tree for a drought-resistant landscape is a native, early to mid-successional, upland hardwood species with a multi-layered canopy, small and/or deeply lobed leaves, and a conical to cylindrical crown shape.” University of Georgia School of Forestry (2012). But newer approaches focus on climate adaptations and the hidden hydrology of tree functions.
Some scientists have used a tree species’ “climate envelope” to figure out how well it will do in a climate that changes. For instance, a study from 2009 looked at the climate range of each type of tree growing in Philadelphia at the time and compared it to what the climate would be like in the future. Based on “annual mean temperature, minimum temperature of the coldest month, maximum temperature of the warmest month, annual precipitation, precipitation in the warmest quarter, and precipitation in the coldest quarter,” a tree’s climate envelope is the climate niche it lives in. ” Also factored in was the predicted increase in diseases and pests due to climate change. In the end, the study found that Philadelphia’s climate would become “less optimal” for 10 species but “more optimal” for two species (American holly and sweetgum).
The climate envelope approach has been subject to criticism: “There is little evidence that the climate envelope of a plant species directly relates to the drought and thermal tolerance of that species, at least not at the resolution required to identify or rank species vulnerability.” Hanley et al, Sci.Total Environ. 2021 In addition, some scientists have argued that while “the mechanisms of how trees die are becoming clear, the environmental conditions under which trees can persist under very high temperatures associated with heat waves have not been documented,” Overwhelming Heat Waves: Climate Envelope Development for Pinus edulis Seedlings.
A lot of research is being done on the parts of a tree that allow it to survive drought or heat. One of the first things I learned is that trees don’t just sit there when there is a drought; most of them change how they work to save water. As I explored this phenomena, I began to slog through sentences like this one:
Having no idea what that meant, I decided I better learn a few basics about tree hydrology. Thankfully, I came upon an interview with a leading researcher in the field, Dr. Craig Brodersen, and soon I had better understanding of tree functioning. You may have heard of xylem. It refers to the “hollow, pipe-like vessels that carry water and nutrients from the roots to the shoots and leaves.” The xylem sap is under negative pressure, which means that as water evaporates from the leaves, it pulls water up the trunk. During drought, many plants choose to close their stomata, which are the tiny holes on the surface of their leaves. This stops water from escaping into the air. Dr. Brodersen explains the process as follows:
Drought tends to exacerbate the effect of heat stress. Plants typically handle heat through the cooling of leaves and tissues via transpiration. But since most trees’ response to drought stress is to close stomata to reduce water loss, the heat stress can be much more severe because transpirational cooling is reduced. Teskey et al, Plant, Cell & Environment. In addition, the whole tree hydraulic system can be damaged when heat is accompanied by drought:
“During exceptionally warm conditions, if a particular tree’s soil becomes really dry, bubbles form in these tubes. When that happens to a particular xylem tube, it is unusable forever. If most or all of a tree’s xylem gets emptied out — or cavitated — the tree dies.” “How Plants Adapt to Climate Change,”news.harvard.edu.
Sadly, drought and heat have contributed to an increase in tree deaths worldwide. I think of myself as a well-informed person, but this was news to me. In our own country, the West has suffered the most, with California losing almost 150 million trees during the drought that began in 2011, Smithsonian Magazine (July 2019), and in Colorado, tree mortality has increased dramatically. University of Colorado Boulder News (July 2021).
Even though the research was sad and showed how many questions still need to be answered, I wasn’t going to let it stop me from making a tree list that was both mostly correct and useful. When we talk about being accurate, it was clear that the experts had different ideas about which trees can handle drought better. I started by making a list of trees that have been said to be tolerant of drought by different groups. Then I focused on trees that would do well in our area and hardiness zone. There are some things that aren’t on the list, and it could change, but most people seem to agree with it.
Drought Tolerant Trees (* denotes heat tolerant also)
*American Holly (Ilex opaca)
The black gum tree, or Nyssa sylvatica, is also called the black tupelo. It can grow in both very wet and mostly dry conditions.
Black Locust (Robinia pseudoacacia)
Catalpa (Catalpa speciosa and Catalpa bignonioides)
*Crape Myrtle (Lagostroemia indica)
Eastern Red Cedar (Juniperus virginiana)
*Golden Raintree (Koelreuteria paniculata)
∗Hackberry (Celtis occidentalis) and Dwarf Hackberry (Celtis tenuifolia)
*Honeylocust (Gleditsia triacanthos or var. inermis (thornless))
Hop Hornbeam (Ostrya virginiana) (somewhat drought tolerant but not flood-tolerant)
Japanese Pagoda tree (Styphnolobium japonicum, Sophora japonica) (not heat tolerant)
∗Kentucky Coffee tree (Gymnocladus dioicus)
What kinds of oaks are there? White oak, Bur oak, Black oak, Northern red oak, Pin oak, Swamp white oak, Chinkapin oak, and Willow oak (though some experts disagree on this one).
Redbud (Cersis canadensis) (moderately drought tolerant)
Sassafras (Sassafras albidem) (could be threatened by new disease, laurel wilt)
Shagbark Hickory (Carya ovata)
Trident Maple (Acer buergeranum) (native to China and Japan)
*Also heat-tolerant
Several lists of drought-tolerant trees from states as different as Minnesota and Alabama keep coming up. Some trees show up more than once. Hackberry, honeylocust, ginkgo, bur oak, Kentucky coffeetree, and golden raintree are some of these “drought winners.” Not all of them are native to the area. Nor are they even familiar specimens in local gardens. But I want to learn more about the Hackberry, Bur Oak, and Kentucky Coffeetree that are already there.
The search for drought and heat tolerant trees could conflict with the goal of planting more natives. Although several natives are among the “drought” winners, I was impressed by the fact that golden raintree — a native of Asia — seems to be a leading contender for a climate change resilience award! Golden raintree has been described as “one of the most drought and heat tolerant trees and grows well all over the United States except where the winter temperature drops below -20°F.” Urban Horticulture Institute/City of Ithaca. But it has also been “reported as becoming weedy in the eastern portion of Virginia,” according to the Va. Cooperative Ext. You may have spotted this weediness yourself along the north side of Ivy Road in the vicinity of Ednam and Farmington. Another non-native that scores well on drought and heat resistance is crape myrtle.
If at all possible, you’ll want to choose natives. We gardeners will probably no longer be planting all of our old favorites — like beech and some maples — at least not unless we are sure we can provide the type of site and amount of water that they will need to survive. Plants are not the only ones that are going to have to adapt! Choosing a tree that is highly adaptable to a variety of situations is another way to give it a head start. Trees that are adaptable are noted as such under the category of Growing Conditions on the Tree Stewards’ Right Tree/Right Place List,
Whatever trees we choose to plant, we gardeners can help to equip them for the challenges of climate change. We can choose sites that provide plenty of room for root growth and avoid siting trees near heat-islands like sidewalks and tarmac. For excellent advice on site selection and its impact, see Site Assessment and Tree Selection for Stress Tolerance. And we can be sure to water our new trees adequately and regularly for the critical first three years. Only after it’s established does a tree begin to exhibit its tolerance for drought or heat. For detailed guidance on watering new trees, review Watering newly planted trees and shrubs/Minn.Ext.
There’s some good news emanating from all the recent research. As one scientist explained, “Genetic variation in the response of processes to heat has received limited study in trees, but variation exists within species and could be exploited to improve heat stress tolerance in economically important species.” Teskey et al, Wiley Online Library Breeding and genetic engineering for heat and drought tolerance is on the horizon. Trees are not the only plants threatened by climate change; similar research is ongoing with respect to crops like corn and soybeans. For example, see “Research aims to increase crop drought tolerance using biotechnology,” Nevada Today/University of Nevada, Reno (9/27/21).
SOURCES:
Featured Photo: Kentucky coffee tree. Photo courtesy of Missouri Botanical Garden PlantFinder
Piedmont Native Plants: A Guide for Landscapes and Gardens (Plant Northern Piedmont Natives Partnership), also available online at Plant Va. Natives.org
“Heat and Drought Tolerant Plants,” Penn State Ext (2017)
“Plants That Tolerate Drought,” Clemson Coop.Ext. (2019)
“Drought-Tolerant Landscapes for Alabama,” Alabama Ext (2019)
Recommended Urban Trees: Site Assessment and Tree Selection for Stress Tolerance,” Cornell University Dept. of Horticulture (2009)
“Drought and Landscape Trees: Effects, Signs, and Watering Guidelines,” Alabama Ext
USDA Plants Database, plants.usda.gov/home
“Are Northeastern U.S. forests vulnerable to extreme drought? DigitalCommons/University of Nebraska (2017)
“Diospyros virginiana,” University of Florida Horticulture (Gilman & Watson 1993)
“Black Locust and Drought,” USDA Southern Research Station (2020) (Although black locust is relatively drought tolerant, drought can slow the rate of its symbiotic nitrogen fixing, impacting other plants that depend on the nitrogen it shares)
“Responses of tree species to heat waves and extreme heat events,” Plant, Cell & Environment. (Teskey et al, July 2014)
“Assessing the Impact of Climate Change on Urban Tree Species Selection: A Case Study in Philadelphia,” Journal of Forestry/ researchgate.net (2009)
“Drought and Tree Mortality: Science Reveals Harsh Future for World’s Forests,” Yale School of the Environment (News/7/19/2018)
Evolutionary relationships between drought-related traits and climate shape large hydraulic safety margins in western North American oaks, Proceedings of the National Academy of Sciences (Mar. 2021).
“Relating the climate envelopes of urban tree species to their drought and thermal tolerance,” Science of the Total Environment/PubMed.gov (Jan. 2021)
“Triggers of tree mortality under drought,” Nature (2018)
“Low Vulnerability to Xylem Embolism in Leaves and Stems of North American Oaks,” Plant Physiology 177 (July 2018), pubmed.ncbi.nim.nih.gov/29789436
“Discovery increases likelihood of growing food despite drought: Genes to keep plants green,” UC Riverside News (Univ. of California 2021)
“Drought tolerance in the context of ongoing Eastern US oak decline: Are Quercus species less likely to die from drought than co-occurring non-oak species?” NASA Astrophysics Data System/Harvard.edu, American Geophysical Union, Fall Meeting 2020 (abstract #B064-0018, December 2020)
“Tree height and leaf drought tolerance traits shape growth responses across droughts in a temperate broadleaf forest,” Wiley.com/New Phytologist (Oct. 2020)
“UChicago-led research could yield increased food production, boost drought tolerance,” University of Chicago News (July 2021)
“Northern forest tree populations are physiologically maladapted to drought,” Nature Communications/Nature.com (2018)
“The Dynamics of Embolism Repair in Xylem: In Vivo Visualizations Using High-Resolution Computed Tomography,” Plant Physiology, Vol.154, Issue 3, Nov. 2010.
Many plant species worldwide are armed with thorns, spines, prickles, and other hazardous outgrowths. But why?
The November To-Do List
In the Edible Garden
Explore winter squash and discover how to grow and enjoy this versatile, highly nutritious garden produce
It’s time to close up the vegetable garden for the winter and start doing things that will help it come spring.
In the Ornamental Garden
Are there some resilient enough for climate change?
Recipe of the Month
As American as apple pie
10 BEST All Season Small Trees For Front Yard | Low Maintenance Dwarf Trees for Landscaping
FAQ
What are the most heat tolerant trees?
What makes a tree more heat tolerant?
Some characteristics make trees more heat tolerant than others, such as trees that have good drought tolerance and prefer full sun exposure. When looking for trees that resist drought, look for trees with smaller and waxier leaves. These characteristics help your trees maintain moisture levels.
Are junipers heat tolerant?
Maples, oaks, and junipers are lauded for their overall temperature tolerance (in both directions). Some characteristics make trees more heat tolerant than others, such as trees that have good drought tolerance and prefer full sun exposure. When looking for trees that resist drought, look for trees with smaller and waxier leaves.
What plants can be planted in full sun?
Also called “strawberry tree”, the arbutus is native to the Mediterranean basin. Modest in terms of maintenance, this shrub likes full sun and supports dry grounds. 11. The laurel-tin One of the main advantages of the laurel-tin to be planted in full sun is that it blooms in winter, when nature is asleep, from January to April.
Which pine tree is best for a hot climate?
Hardy for almost all zones, pine trees are some of the best options to try if you’re searching for great heat tolerance. There are various types of pine trees that can bring that long-lasting familiar scent to the landscape. One variety that’s particularly well adapted to hot climates is the Canary Island pine tree.
What is a good small tree for landscaping?
‘Prairifire’ is a standout variety that has dark pink flowers, reddish-purple foliage, and great disease resistance. Another popular small tree for landscaping is ‘Centurion’ which has rose-pink flowers, an upright shape, and great disease resistance. Growing Conditions: Full sun in medium moisture, well-drained soil
Can shrubs withstand full sun?
Bushes that happily bask in full sun in northern zones might haplessly bake in southern ones. Ahead, we’ve collected some stunning shrubs that can withstand heat and full sun, as long as they don’t dry out.