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Heat & Drought Stress – Will Your Trees Survive?

Please Water Your Drought-stressed Trees

HEAT & DROUGHT STRESS can negatively impact your landscape plants, particularly your trees. The photo above is from an actual irrigated lawn following several heatwaves. Despite receiving water from an irrigation system, the greenest areas of the lawn were those which were “protected” from sun exposure by shade trees most of the day. The rest of the lawn, on the other hand, was drying out. For many homeowners, a “weed-free” green lawn is a priority. They will invest in an in-ground irrigation system with the primary success of their lawn in mind (and their trees only as an afterthought). Perhaps, their reasoning is that ‘trees are big enough plants and can take care of themselves!‘ Unfortunately, when it comes to their existence in our urban settings, that is furthest from the truth!

If you have an irrigation system and your lawn looks like this…

Drought-stressed lawn despite an irrigation system.

Just think about how much your trees are suffering!

Did you know that turfgrass, with or without an irrigation system, can survive a period of prolonged high heat and drought by going into dormancy? Many types of turfgrass can do this with little residual side effects other than a browned appearance, thinning, and yielding space to more tolerant weeds. Once climate conditions become more moderate with sufficient water turfgrass will be restored and produce green shoots once more.

For additional information on recommended grasses for Midwest Lawns and drought recovery, see:

On the other hand, the same restorative response does NOT hold true with trees!

The Red Maple (Acer rubrum) below died in 2017 following 2016’s severe drought conditions. As you can see, the plants surrounding the maple appear healthy and it itself had produced foliage the current season. Sadly, despite being in an irrigated area, it did not survive long into the season when the summer’s heat got the better of it. (Note: It had grown successfully for years along with other maples in a business park, but it was the only tree in the row maples that did not survive. Girdling roots (which restricted some of its ability to transport sufficient amounts of water from its roots) likely contributed significantly to its rapid decline.)

"Sudden death" of Red Maple

Although it is true that harsh conditions will initiate a “shutdown” process, such as the closing of leaf stomates (to prevent water vapors from escaping, thus reducing moisture loss), leaf abscission (leaf drop), cladoptosis (shedding off of limbs), and reduction of root biomass, the hydration needs of trees nonetheless are far more complex than turfgrass. The volume of water needed to sustain trees plays a critical part in their survival. The longer a tree must endure high heat and drought without proper hydration, the more residual damage will occur to its biological system.

And so, even if certain plants appear to have escaped the “Grim Reaper” this year, this does not mean they weren’t affected by the harsh climate conditions. Decline and even death in some trees can occur either suddenly, slowly, or on a “biological” delay. Symptoms of decline, such as stunted growth and dieback, may not become noticeable until the following year and symptoms may increase over the next several years.

Sudden death” is a term we use to describe the situation of a tree that looks fine one day and then suddenly dies (within hours, days, or a few short weeks) following a significant thermal climate event, such as a heatwave, and is characterized by wilted & browned leaves that are still firmly attached to their twigs. It is also not uncommon for a tree planted among its own species to seemingly die at random and for no apparent reason.

 

The weaker or sickly an unmanaged tree, the more likely the tree will succumb to heat & drought stress. As mentioned earlier, trees with girdling roots will have a very tough time keeping up with their hydration needs because the girdling is restricting the flow of nutrients and life-saving waters drawn from their root system. (See “Girdling Roots – The Silent Tree Killer”)

WATER IS THE BEST MEDICINE

In the Indianapolis (Indiana) area, 2018’s unseasonably hot conditions began early in the spring and has become a recurrent theme ever since (as of mid-July). Many Hoosiers have joked that we never had a spring this year, but jumped straight from winter into summer! Lindsey Purcell, the Urban Forest Specialist at Purdue University in West Lafayette, IN, humorously coined the phrase “Sprinter” to describe how winter melded almost seamlessly into spring and went straight into summer! A few years ago, Lindsey wrote an excellent publication on the subject of drought. which we highly recommend you download, read, and keep handy. The PDF of this publication is available here: “Drought? Don’t Forget the Trees!”

The repetitive heatwaves have pushed even healthy plants to their limits! Ornamental landscape plants with even the slightest ailments are having a more difficult time coping with the heat! You may have already noticed that some trees have already lost many of their leaves. Some of these trees are suffering the results from nonlethal diseases (e.g. Apple Scab on Flowering Crabapples (see photo below), Powdery Mildew on Sycamores, Anthracnose on White Ash trees, Tubakia Leaf Spot on Oaks, etc.) or that may have suffered injury from insect feeding (e.g. Japanese Beetles on Lindens, Eastern Tent Caterpillars on Crabapples, Aphids on River Birches, White Pine Weevils on Spruce trees, etc.). These ailment examples (and there are many others) combined with unseasonable high heat will make sick trees appear especially “ugly” (i.e. defoliated and/or “extra-crispy” looking) by early summer season onward. This was especially true for Indianapolis trees when temperatures lingered above 85°F into the 90’s over several days. Not too surprisingly, a few of these ill trees have not survived.

Heat stress exacerbates Apple Scab infected Crabapple tree's leaf drop. Tree eventually dies.

Apple Scab (Venturia inaequalis) is a fungal leaf spot disease (see first inset) which causes premature leaf abscission in Malus spp. trees. Leaf loss is more profound and severe when it is exposed to prolonged periods of intense heat and drought conditions. Eventually, the crabapple succumbs and dies (second inset).

Watering according to a plants needs, however, is still the best medicine. You don’t have to water every day, but if you do water please make sure it’s a good, deep, soak to encourage deeper growing roots. Shallow watering usually only encourages shallow, less drought-tolerant, roots and so during periods of high heat and drought, shallow roots are the first to dry up and may eventually die. What is your contingency plan? Don’t wait until the heat is upon you. Have a plan in place and ready. Your trees are counting on you! (See our article “Watering Trees – How Much?)

BE PREPARED

How can you protect your landscape from heat and drought? Well, one way is to simply pay close attention to drought trends, keep updated to the latest weather reports and forecasts, and by looking up historical data. It’s nearly impossible to predict the weather with 100% accuracy (because sometimes relying on weather forecasts can be a game of roulette), but by combining resources you’ll discover clues that can prepare you in managing the needs of your plants so that they receive adequate supplies of life-sustaining water during the hot & dry seasons.

To check soil moisture, use a screwdriver (over 8″ long) to poke into the soil. If the soil is moist, it should pass easily through it, but if the soil is too dry and you can’t push the screwdriver into it at least 6″ deep, then it may be time to water (or maybe, you have some very rocky soil!). A handy tool to have is a Soil Moisture Meter (SMM). An SMM is a probing device that you insert about 6 or more inches into the soil. Its display will give you a general idea of whether your soil ranges from dry to moist to wet. An example of what a simple SMM looks like can be found here on Amazon’s website.*

SHOULD I FERTILIZE MY TREES?

Performing some plant health care (PHC) treatments in temperatures above 85°F may put a plant at greater risk of phototoxicity“leaf burn” and additional plant stress. Poor timing, choice of chemicals, application method, and rates used are some of the main factors that may influence a less than desirable result. PHC treatments might include applying fertilizers (usually to address nutrient deficiencies) during high temperatures, managing diseases & controlling insect infestations late after infection/infestation has already caused significant plant injury, etc. In the case of fertilizers applied to an already stressed plant, there is a risk that these plants could run out of energy while trying to metabolize the nutrients fed to them. Additionally, these mineral “salts” can cause a drying effect (a.k.a. reverse osmosis) and deprive the plant of vital moisture. So use caution because applying supplemental fertilizers on stressed trees may do far more harm than good! It is imperative as a best management practice to read product labels in their entirety and to follow their directions exactly to minimize plant injuries, to maximize results, and in certain cases, to stay in compliance with the Law. (Related articles: “Leaf Scorch” and “Why Does Over-Fertilization Kill Plants?”)

*Please Note: The mention of trade names or commercial products in this article is solely for the purpose of providing subject matter-related examples and do not represent, imply, nor constitute an endorsement of one brand over another. Arbor Rangers, LLC or its associates do not warrant, nor guarantee results by use of any specific name brand product in addressing your specific needs. This article may contain pesticide, equipment, and/or product type recommendations that are subject to change at any time. The recommendations are provided only as a guide.

Heat-stressed Arborvitae

The following chart shows the unseasonably high temperatures that popped up and lingered near the end of spring and into summer in the Indianapolis area from May 15, 2018 – July 19. 2018. Included for comparison are historical averages from the National Weather Service and high temperatures recorded from 2017.


2018 OVERVIEW OF UNSEASONABLE HEAT TRENDS (Indianapolis, IN) May 15 – July 19

As you will see from the chart below, this year Indianapolis has experienced unseasonably HOT temperatures (highlighted in yellow) and very little precipitation (highlighted in blue). Compared to last year’s temperatures, which started out unseasonably high but then registered closer to the historical averages (notable exceptions highlighted in orange), this year will definitely have a tremendous (and devastating) effect on landscape plants over the next few years.

DATE

2018-HI°F

Weather Condition

NWS-AVG°F*

Notes

2017-HI°F

5/15

75

overcast

72

83

5/16

74

light rain

72

84

5/17

84

overcast

72

83

5/18

72

light rain

73

85

5/19

79

overcast

73

81

5/20

84

scattered clouds

73

78

5/21

79

thundershowers

74

74

5/22

83

scattered clouds

74

71

5/23

82

passing clouds

74

73

5/24

83

passing clouds

75

64

5/25

88

passing clouds

75

Unseasonable High Temps

64

5/26

88

partly sunny

75

 

78

5/27

90

partly sunny

76

 

79

5/28

94

partly sunny

76

 

77

5/29

88

partly sunny

76

 

82

5/30

79

partly sunny

77

76

5/31

88

passing clouds

77

Unseasonable High Temps

78

6/1

85

partly sunny

77

 

79

6/2

88

partly sunny

77

 

83

6/3

81

passing clouds

78

85

6/4

77

passing clouds

78

87

6/5

86

passing clouds

78

Unseasonable High Temp

87

6/6

79

passing clouds

79

78

6/7

90

scattered clouds

79

Unseasonable Severe Temps

71

6/8

90

light rain

79

 

76

6/9

85

scattered clouds

79

 

83

6/10

70

light rain

80

84

6/11

75

rain

80

87

6/12

77

overcast

80

90

6/13

86

partly sunny

80

Unseasonable High Temp

85

6/14

81

scattered clouds

81

90

6/15

85

scattered clouds

81

 

81

6/16

91

scattered clouds

81

Unseasonable Severe Temps

86

6/17

91

scattered clouds

81

 

86

6/18

91

scattered clouds

81

 

81

6/19

91

thundershowers

82

 

81

6/20

84

partly sunny

82

83

6/21

75

light rain

82

88

6/22

76

partly sunny

82

84

6/23

73

partly sunny

82

77

6/24

84

scattered clouds

82

77

6/25

76

overcast

82

76

6/26

75

thundershowers

83

74

6/27

83

partly sunny

83

74

6/28

85

partly sunny

83

Higher Than Normal Temps

80

6/29

90

scattered clouds

83

 

87

6/30

90

scattered clouds

83

 

85

7/1

93

scattered clouds

84

 

84

7/2

87

partly sunny

84

 

85

7/3

90

partly sunny

84

 

87

7/4

93

scattered clouds

84

 

87

7/5

90

scattered clouds

84

 

85

7/6

82

scattered clouds

84

84

7/7

82

sunny

84

89

7/8

84

passing clouds

84

80

7/9

91

scattered clouds

84

Higher Than Normal Temps

83

7/10

91

partly sunny

84

 

84

7/11

87

scattered clouds

84

 

79

7/12

85

partly sunny

84

 

90

7/13

91

sunny

84

 

84

7/14

93

partly sunny

84

 

83

7/15

88

partly sunny

84

 

81

7/16

88

partly sunny

84

 

85

7/17

88

passing clouds

84

 

87

7/18

84

scattered clouds

84

88

7/19

84

partly sunny

84

88

*National Weather Service Historic Daily Avg Temperature.


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