Written by Justin Tornatore, ISA Certified Arborist | TIMBERx
Abstract
In 2024, Tucson hit 100°F or hotter on 112 days, the most since the National Weather Service began keeping records in 1895, according to KGUN 9 [1]. Even desert-adapted plants struggled. A Desert Botanical Garden curator told KJZZ that the Phoenix garden went from losing an average of 10 to 12 plants to "more like 50, 56" during the extreme summers of 2023 and 2024 [2]. University of Arizona Cooperative Extension describes how heat and drought force trees to close their stomata, burn through stored energy, and in severe cases suffer cell damage, with symptoms that may appear only in later years [3]. This article explains the physiology of heat stress, how to tell leaf scorch from sunscald and salt burn, and what the extension research recommends for protecting and recovering trees during an Arizona summer.
Background: What Heat Does Inside a Tree
Leaves are covered in tiny pores called stomata. When they're open, carbon dioxide comes in for photosynthesis and water vapor goes out, which is how a tree "sweats" to stay cool. When water runs short, most trees close their stomata to avoid dehydration. UA calls this drought avoidance [3]. The catch is that with stomata closed, photosynthesis stops and the tree starts living off its stored carbohydrates [3].
It's a lot like a person who stops eating to save water. They can get by for a while on reserves, but every day without food draws the tank down. Hot nights make it worse. UA notes that warm nights increase respiration rates and deplete stored energy faster [3]. At the extreme, high temperatures alone, even without drought, can damage the photosynthetic system directly [3].
Isolation matters too. UA notes that plants growing alone, like many trees in urban yards, are more exposed to extreme temperatures than plants growing in groups [3].
What the Research Shows
Symptoms of Heat and Drought Stress
UA lists the typical signs as wilting, drying and browning leaves, leaf drop, branch dieback, sunscald on branches and trunks, reduced or no new growth, and potentially plant death [3]. In trees, reduced growth or death "may also occur in subsequent years" [3]. After the 2024 heat, Tucson Botanical Gardens' director of horticulture warned that some damage might not show until the next year [1].
Leaf Scorch vs. Salt Burn vs. Sunscald
These three problems get confused constantly, but they have different patterns:
- Sun scorch on leaves: On citrus, UA describes large "bleached out" or yellow patches on individual leaves, mostly on the south and west sides of the tree, during late spring through early fall [5].
- Salt burn: Tip or marginal leaf burn on leaves on all sides of the tree, caused by salt buildup from irrigation and fertilizer [5].
- Sunscald (bark injury): Discolored reddish or brownish bark that can progress to sunken, split, or peeling bark, usually on the south or west side of the trunk. Young, thin-barked trees are most susceptible [4]. UA's Yavapai County guidance ties classic sunscald to large temperature swings, warm days followed by freezing nights, and notes that reflective walls and light-colored surfaces add heat and light [4]. In the low desert, summer sunburn of newly exposed bark is the other major cause. UA warns that pruning in heat can expose previously shaded branches to sunscald [3], and ISA notes that topping suddenly exposes bark to high light and heat, which can lead to cankers, bark splitting, and branch death [7].
Watering Is the Number-One Defense
UA's guidance is clear: "Sufficient soil moisture is the most important factor to maintain healthy plants in the southwest during drought and heat" [3]. Water should wet the root zone to a depth of two to three feet, and it should be applied away from the trunk at and beyond the dripline, where active roots are [3]. During the hottest part of summer, trees may need irrigation every four to seven days, depending on soil, species, and size [3]. Overwatering is also harmful. Soil needs to dry between irrigations so roots get air [3].
What Not to Do in the Heat
UA advises avoiding or minimizing pruning during drought and excessive heat, never shearing plants in hot, dry weather, and minimizing or skipping fertilizer, which increases root-zone salinity [3].
Field Observations
A lot of summer heat damage is self-inflicted. A citrus gets skirted in early summer and its trunk sunburns. A mesquite gets heavily thinned in July and its upper limbs bake. A drip schedule set in April never gets bumped up for June. Trees in rock-mulched yards hemmed in by block walls and west-facing stucco take an extra beating from reflected heat.
As I wrote in my irrigation article, summer 2024 showed that a fixed irrigation schedule isn't enough. You have to check what your trees actually need.
Practical Recommendations
- Water deep, wide, and on time. Wet the soil to two to three feet at the dripline and beyond. Check depth with a soil probe or smooth metal rod pushed in after watering [3][8].
- Adjust with the weather. Plants use three to five times as much water in hot, dry, windy seasons as they do in winter [8]. Revisit the controller as temperatures climb.
- Mulch the root zone. A 3- to 4-inch layer of organic mulch reduces evaporation and watering frequency. Keep it away from the trunk [8]. UA's sunscald guidance recommends coarse organic mulch to minimize reflected light and heat [4].
- Leach salts once or twice a year by watering about three times longer than normal [8], especially if you see marginal leaf burn on all sides of the tree [5].
- Leave the shade on the tree. Skip heavy summer pruning [3]. On young trees, avoid stripping lower branches that shade the trunk [4]. For citrus, don't skirt for looks, and don't prune from May through mid-October [6].
- Protect exposed bark. Paint exposed citrus trunks and limbs with white latex paint diluted 50% with water, or whitewash, and never use oil-based paint [6]. Light-colored tree paint or wraps can also protect young thin-barked trees [4].
- Shade the vulnerable. Pima County guidance recommends 30–40% shade cloth for vulnerable plants during extreme heat, held a few feet above the plants [9].
- Skip the fertilizer until it cools off [3].
- Be patient with recovery. Damage can show up a season or more later [1][3], so give a stressed tree time before you write it off.
Key Takeaways
- Heat stress drains a tree's energy reserves, and hot nights speed up the drain [3].
- Leaf scorch on the sunny side, salt burn on all sides, and sunscald on the bark each point to different fixes [4][5].
- Deep, correctly placed irrigation is the most important protection [3].
- Don't prune, shear, or fertilize in extreme heat, and paint or shade exposed bark [3][6].
Stay cool out there, friends, and keep your trees cool too.
Talk to an Arborist
Seeing scorch or bark damage this summer? Text Justin at (520) 451-8790 (texting is preferred over calling) or email Justin@TimberxTrees.com. TIMBERx is a veteran-owned tree care company led by Justin Tornatore, ISA Certified Arborist, serving Tucson, Oro Valley, Marana, Vail, Sahuarita, Green Valley, and Benson.
References
- Fish, R. (2024). Even desert plants have been feeling the record heat. KGUN 9. https://www.kgun9.com/news/community-inspired-journalism/midtown-news/even-desert-plants-have-been-feeling-the-record-heat
- Goodman, A. (2024). How the extreme heat from the past 2 summers is affecting Arizona plants. KJZZ. https://www.kjzz.org/2024-09-25/how-the-extreme-heat-from-the-past-2-summers-are-affecting-arizona-plants
- Schuch, U. (2021). Drought and Extreme Heat: Plant Responses and Landscape Maintenance Practices (AZ1876). University of Arizona Cooperative Extension. https://extension.arizona.edu/publication/drought-and-extreme-heat-plant-responses-and-landscape-maintenance-practices
- Schalau, J. (adapted 2022). Sunscald (Backyard Gardener #25). University of Arizona Cooperative Extension, Yavapai County. https://extension.arizona.edu/sites/extension.arizona.edu/files/attachment/Sunscald.pdf
- Begeman, J., & Wright, G. (2009). Diagnosing Home Citrus Problems (AZ1492). University of Arizona Cooperative Extension. https://extension.arizona.edu/sites/default/files/2024-08/az1492.pdf
- Wright, G., & Kelly, J. (2008). Pruning Citrus (AZ1455). University of Arizona Cooperative Extension. https://extension.arizona.edu/publication/pruning-citrus
- International Society of Arboriculture. (2021). Why Topping Hurts Trees. TreesAreGood.org. https://www.treesaregood.org/Portals/0/siteContent/pdf/Web%20TAG%20brochures/TAG_CBweb_Why%20Topping%20Hurts_v2.pdf
- Call, R. E., & Daily, C. (2017). Watering Trees and Shrubs (AZ1298). University of Arizona Cooperative Extension. https://www.extension.arizona.edu/publication/watering-trees-and-shrubs
- Larson, P., Vance, L., & North, D. (2025). July Monthly Gardening Guide for Pima County. University of Arizona Cooperative Extension. https://www.extension.arizona.edu/publication/july-monthly-gardening-guide-pima-county
