Washington State University Extension

Garden Tips

Hot Weather RSS feed


GARDEN TIPS – Written by Marianne C. Ophardt
WSU Extension Faculty
for the Tri-City Herald, Kennewick, WA Published June 19, 2016

Last week I covered the top two factors contributing to the browning and dieback of many area needled evergreens. While it is no surprise that heat and drought stress could cause problems, some of you might wonder why we are just becoming aware of the severity of the situation.

It is easy to tell when a tomato or squash plant is suffering from drought or heat stress. They wilt. This is not the case with trees, especially conifers. Browning needles and excessive needle-drop are telltale signs of stress, but a conifer may not exhibit browning until some time after the damage from stress has occurred. Why is that? Conifer needles have a very waxy coating that slows their drying out, delaying browning, diagnosis, and preemptive action.

I will also point to the cumulative stress from three successive years of extraordinarily hot summers and minimal winter precipitation. Many of these conifers have been declining over that time, but the additional stress year after year has pushed them past the tipping point.

So what about trees that are turning brown despite their owners watering correctly, keeping the top 18 inches of soil moist and mulching to keep the roots cooler? Restricted or girdling roots, the result of improper planting, are often involved in helping push trees over the edge.

Container grown plants are frequently pot-bound with circling roots or very dense, matted root masses. If these roots are not properly cut and loosened at planting time, the roots will not move out into the surrounding soil where they have access to moisture and nutrients. Eventually the roots will girdle or Achoke@ the plant, preventing the uptake of water, and killing the tree. The signs will be that same as signs of drought and heat stress.

A similar problem occurs with balled and burlapped trees that are grown in the field, dug, and their root ball wrapped in treated burlap. In our dry climate, the burlap does not decay quickly enough to allow for good root growth out of the original root ball. University horticulturists, landscape professionals, and The American National Standards Institute (ANSI) all indicate that the burlap should be removed before planting.

Also, the soil in the root ball is commonly very different from the native soil where the tree is being planted. This can lead to difficulty in keeping the soil in the root ball and surrounding soil moist, but not excessively wet.

Soil compaction restricts root growth and can contribute to a tree=s demise. This is because compacted soil has less air available for the roots and roots need air to function. In addition, soil compaction physically impedes root growth. In older landscapes, soil becomes compacted over time from traffic and even sprinkler irrigation. In newer landscapes, the soil may be highly compacted due to the use of heavy construction equipment during building.

Where trees are growing in lawns, periodic aeration can help relieve some of the compaction. However, if the compaction is severe, a certified arborist may recommend more extreme measures. When planting a new landscape, the soil should be properly prepared with tilling before planting.

This week and last week I have mostly focused on the browning and dieback of conifers in our area, but we are also seeing dieback on some local deciduous trees and shrubs. Depending on the plant=s situation, this decline may be attributed to the same factors causing the browning on conifers. I sure hope this summer is cooler. How about you?


GARDEN TIPS – Written by Marianne C. Ophardt
WSU Extension Faculty
for the Tri-City Herald, Kennewick, WA Published June 12, 2016

I noted earlier this year that our last three summers have been abnormally hot with record breaking heat. Even before I started getting calls about dying arborvitae, pines, cedars, and other conifers, I predicted that the stress caused by extreme heat and inadequate watering practices was going to lead to the decline of many area trees. We can not do anything about the weather, but we can try to avoid the same problem in the future by reviewing the contributing factors.

Before we do that, let’s talk about the needles on conifers. While we call trees with needles “evergreens,” they do shed or drop their oldest needles ever year. Evidence of this is the bed of needles underneath pine trees. This is not a problem for healthy trees that are producing a new set of needles each year. These new needles persist on a tree for a year or more. Depending on the species, pines will hold their needles for two or more years and spruces for five years or more. However, if a conifer is stressed or its health is compromised by insects or disease, the production of needles will be reduced or stop altogether. This stress can also lead to the browning and shedding of more than just the oldest needles. As a result, a tree will have fewer and fewer green needles, a sign of its decline.

Drought: We are in a region with very limited precipitation, making adequate irrigation essential to growing healthy trees. Many homeowners assume that their trees are getting enough water, but they do not check the soil moisture in the tree’s root zone to make sure.

The water absorbing roots of most landscape trees are within the top 12 to 18 inches of soil. It is this zone that should be kept moist with deep watering. Watering trees just with the lawn for 15 to 20 minutes once or twice a day may not be enough to moisten the soil to the needed depth. It of course depends on how much water is being applied, weather conditions, and the type of soil. The only way you will know if the soil is moist enough is to use a trowel to dig down and check.

It is also important to moisten the soil where the water absorbing roots are located. They are not close to the trunk or under the canopy of mature trees. They are located at and beyond the canopy or outermost spread of the branches called the “drip-line.” It is to this area that water should be applied.

Heat Stress: We can not offer much relief from the heat to established trees, but when planting new trees we should consider planting trees that are well adapted to hot summer weather. If you plant a species that is not well suited to our climate, select the site carefully to provide it with protection from sun, heat, and wind. On younger and smaller trees, mulch the root zone with three or four inches of wood chips or bark to keep the roots cooler and to maintain soil moisture.

Oh my, I have come to the end and I have not had the chance to talk about soil compaction and other factors contributing to local conifers turning brown. More next week…


GARDEN TIPS – Written by Marianne C. Ophardt
WSU Extension Faculty
for the Tri-City Herald, Kennewick, WA Written December 27, 2015

Now that we have almost closed the book on 2015, let us reflect a bit on the extraordinary weather of this past year and years past. It is no surprise that the summer of 2015 was the hottest on record in the region with a total annual precipitation of 3.72 inches. That is not much considering that 9.16 inches of moisture were lost through evapotranspiration (ET) during the month of August. Plus, some areas experienced limited irrigation water.

This extraordinary year was compounded by the fact that until 2015, 2014 was the hottest on record with 3.44 inches of precipitation, a total ET of 9.02 inches in August, and an unusually mild December. Also consider that 2013 was the 2nd hottest summer on record until 2015 with a total annual precipitation of 2.79 inches. Plus, the summer of 2013 was the 6th year of summer weather with near or above average temperatures.

That is a great deal of summer heat. Heat stresses plants, especially ones not well adapted to our hot summer climate. Heat stress is compounded by drought stress that results from limited irrigation water or improper watering practices. Drought stress may also be inflicted on a tree due to a lack of an adequate root system, physical injury to the trunk, restricted or girdling roots, compacted soil, or other factors that affect the tree’s ability to absorb or transport water to the top of the tree.

Stress on trees makes them more vulnerable to attack by certain insects, especially boring insects. As a result of the several years of summer heat stress that local trees have endured, we are seeing an increasing number of wood borers attacking local ornamental trees.

Even if a tree looks fairly healthy to you and me, it may become fodder for borers because it is stressed. Stressed trees are more vulnerable to attack by borers because stress causes them to produce volatile chemicals, such as terpenes, that attract boring insects.

In addition, some borers (such as bark beetles) will emit an aggregation pheromone (a chemical compound that elicits insect behavior) once it locates a stressed tree. This insect pheromone lets other bark beetles in the area know of a stressed tree’s presence. One bark beetle might not significantly harm a tree, but a bunch of them feeding spells trouble.

Once the volatile chemicals emitted by the tree and the aggregation pheromone gets a gang of bark beetles to the tree, they still have to get under the bark. As they start to eat their way into the tree, the sap pressure in a healthy tree will often drown or push them back out. However, in a stressed tree the sap flow is lower and they can successfully eat their way in.

The quote of “the best defense is a good offense” may or may not be true in sports, but it is very true when it comes to protecting your trees from boring insects. Keeping a tree healthy is not just its best defense against borers, in many cases it is the only defense.

We no longer have the long-term residual insecticides that were once used to protect trees from borers. Insecticide applications as a spray to the trunk are generally not a good “offense” and even the systemic insecticides applied to the soil or injected into the trunk are only effective against a few types of borers.

Hopefully in 2016 we will not experience another summer of extraordinary heat and limited irrigation water. If we do, make every effort to keep your trees healthy and “unstressed” to protect them from attack by borers.


GARDEN TIPS – written by Marianne C. Ophardt
WSU Extension Faculty
for the Tri-City Herald, Kennewick, WA Written Septembe 13, 2015

Two years of record heat is just too much. I am tired of seeing samples of crispy brown plants and being asked the cause of the problem. Quite simply, it has been hot and difficult for plants to survive, especially plants that are not well adapted to our climate.

Several weeks ago I took a look at some local Norway and Colorado spruces that were turning brown and dying. While not always the best clue to the origins of a plant, these spruces’ common names do give us a hint that they may not be well suited well to the hot, dry summer weather of the mid-Columbia. Norway spruce is native to northern and central Europe and prefers a cool, moist, humid climate. Colorado spruce, native to the central and southern Rocky Mountains of the US, is also best adapted to a cool, humid climate.

In their publication on “Spruce Problems” the University of Illinois points out that spruces as a group “prefer locations with acidic, well drained soils” and that “spruces are not well adapted to hot dry locations and often suffer when planted in the warmer regions of the US.” In this publication they also discuss heat injury of spruces, indicating that high temperatures can cause heat damage, especially when the high temperatures are preceded by cool weather. Heat damage on spruces is expressed as browning and dropping of new needles, leaving dead branch tips. After two or three years of this, a spruce is pretty much dead.

Just because spruces are not well suited to our climate does not mean that you can not find healthy spruces growing locally. However, our climate does stress them and make them more vulnerable to attack by a number of pests, like spruce mite, bud scale, and needle miner. I did find spruce bud scale on one of the browning Norway spruces, but the infestation was not severe enough to kill the tree. At least we can be thankful that our dry climate keeps fungal diseases that infect spruce from being a problem here.

Flowering dogwood is another tree that suffers when faced with hot, dry, windy summer weather. In its native habitat, it is an under-story tree that grows in the filtered shade of other forest trees. It grows best in a moist, well-drained, slightly acidic soil that is high in organic matter.

Dogwoods become stressed when planted in full sun and subjected to high temperatures, reflected heat and light from pavement and structures, wind, compacted soil, and poor watering practices. This stress shows up as curled leaves and crispy brown leaf edges. Nevertheless, many of these trees still put on a beautiful display of flowers each spring. To help dogwoods better cope with mid-Columbia conditions, plant them where they will have filtered shade and will not be subjected to a southern-western exposure or drying winds.

Dogwoods and spruce have very shallow root systems, making them more subject to drought and heat stress. It is advisable to keep the roots cooler and moist by mulching the roots with wood chip or shredded bark mulch. Apply a 3 to 4 inch layer of mulch all the way out to the tree’s dripline and beyond. Keep the soil moderately moist, but avoid excessive soil moisture. Be sure to water the spruces during the winter if the weather stays dry and mild.

I hope next summer is a little cooler, don’t you?


GARDEN TIPS – written by Marianne C. Ophardt
WSU Extension Faculty
for the Tri-City Herald, Kennewick, WA Written July 30, 2015


We had a brief respite from the hot weather this past week, but now it is back. I am hoping that my garden tomatoes will have set more fruit during the cooler weather. I have been able to start harvesting a few ripe tomatoes, but alas many of these tomatoes are cracked.

Cracked Tomatoes: After a long wait for ripe tomatoes, it is disappointing that many have radial cracks or splits in the skin starting at the stem. This occurs because the flesh of the fruit grows faster than the skin. Temperatures over 90 degrees or temperatures fluctuating widely between day and night can lead to cracking. It also can be attributed to heavy pruning when staking tomatoes. This exposes the fruit to light and increases the temperature of the fruit.

Uneven watering, fluctuating between very dry to very wet soil, can also give rise to tomato cracks. Heavy nitrogen fertilization causes very fast vine growth, also causing more fruit to crack.

Finally, some varieties are more prone to cracking. A number of the heirloom varieties, like Brandywine, are notorious for their tendency to crack. That is one of the reasons why modern hybrids were developed. For example, Burpee developed their hybrid Brandy Boy to have the same great taste of Brandywine without the problems, including cracking, associated with growing this heirloom.

The problem with cracked tomatoes is that the cracks in the skin make it easy for secondary fungi and bacteria to infect the fruit. The appearance of mold, extremely soft fruit, or liquid leaking from the tomato are sure signs of infection. Discard them. You can eat cracked fruit that are not infected, but do so quickly because they will not last long.

Blossom Drop: Astute veggie gardeners have noted the lack of beans on their bean plants and attributed it to the heat. When temperatures climb well above 90 degrees, beans and peas will drop their flowers without setting fruit. The same often happens with tomatoes and peppers too. The reason for “blossom drop” and the lack of fruit involves pollination and the subsequent fertilization of the embryo in the ovary within the flower.

You might think the reason for this is that bees are not active in the garden when temperatures are above 100 degrees, but beans, peas, tomatoes, and peppers are self-pollinating and thus do not depend on bees or other insects to enable pollination and fertilization. However, these self-pollinating flowers do need wind or insects to shake the pollen from a male part of the flower (anther) onto a female part of the flower (stigma).

What about the veggies that do require an assist from bees and other pollinating insects? Fruit production of cucumbers, melons, and squash seems to slow in hot weather too. Aborted or misshapen fruit are because bees are not very active in the hot weather and because pollen does not remain viable for very long.

We can not do anything about the hot weather, but we can reduce plant stress and minimize cracking and blossom drop by keeping the garden soil evenly moist, mulching to cool the soil and conserve moisture, and not applying excessive amounts of nitrogen fertilizer. It’s also a good idea to provide bees with some moisture. Turn a bird bath or large pot saucer into a bee bath by filling it with coarse gravel or river rock and adding water. The stones are used as perches, so the bees will drown in the open water.


GARDEN TIPS – written by Marianne C. Ophardt
WSU Extension Faculty
for the Tri-City Herald, Kennewick, WA Written July 23, 2015


Did you ever wonder why it seems like some weeds grow even faster in hot weather? It is because they do!

Growth of cool-season turfgrasses, like Kentucky bluegrass and fine fescues, slows during the sweltering heat of summer. However, some weeds are able to make the most of the heat and sun because they have a different type of respiration and function better when temperatures are between 85 to 117 degrees. Some of these warm-season weeds are Bermuda grass, crabgrass, foxtail, goosegrass, purslane and prostrate spurge.

I hate crabgrass because it sticks out like a sore thumb with its lighter green leaves and rapid growth, making it very obvious that there are major crabgrass infestations in many local lawns this summer. How could this happen despite applying a crabgrass “preventer” or preemergent herbicide? Even if the herbicide was applied at the recommended time, spring temperatures fluctuated up and down and may have thrown off the timing and resulted in applying the material too late or too early. If applied too early, the material may have lost its effectiveness before the crabgrass germinated.

Failure to control crabgrass could also be due to a lack of good coverage or not using the right rate. Always be sure to calibrate your sprayer or spreader before application. For more even coverage, apply half the amount of herbicide in one direction and then apply the other half in the direction perpendicular to your first pass.

A healthy dense turf is the best protection against crabgrass. Last summer’s extraordinary heat was tough on lawns and resulted in thinner turf that is now being stressed again by excessive summer heat and in some areas severe drought stress. Keep in mind that the grass in most lawns is comprised of cool-season turfgrasses that actively grow during the cooler months (March, April, May, October, and November) of the growing season. With the extraordinarily warm fall last year and very warm spring this year, many lawns have not been able to fully recover.

When (and if) cool fall weather arrives, fall fertilization at the recommended times of early September and early November will be important for green lawns that have made it successfully through the summer. However, only fertilize if there is adequate water available and the grass is green and growing.

A number of local residents have also been noting the proliferation of prostrate spurge in their lawns and gardens. There are four types of prostrate spurge, with spotted spurge being the most common in this area. These low-growing plants have tiny leaves and form a prostrate mat along the ground. As members of the Euphorbia family they have a milky sap that can cause skin irritation.

Spurge grows best in dry open areas and takes advantage of bare garden soil and dry lawn edges. In the garden or landscape, I recommend pulling or hoeing for spurge control. In lawns, you can kill spurge with an application of a “spurge killer” herbicide containing triclopyr, but you may find it easier just to dig up the plants if you only have a few here and there.
Of course, the weeds are the only green plants in some lawns where there has been restricted watering this summer. In a few weeks we will discuss what, if anything, can be done to bring these lawns back from the brink of doom.


GARDEN TIPS – written by Marianne C. Ophardt
WSU Extension Faculty for the Tri-City Herald, Kennewick, WA
Written July 2, 2015


I was trying to be optimistic and have avoided talking about the affects of all this extremely hot weather on our landscape and garden plants. However, continued record high temperatures in the region and some of the “warmest months” on record have compelled me to talk about the affects of heat on plants.

Perhaps you can recall learning about how plants take the sun’s energy via the process of photosynthesis to create carbohydrates. The carbohydrates provide the energy for plant growth. The rate of photosynthesis increases with increasing temperatures up to a point. Once temperatures reach about 95 degrees, the rate of photosynthesis decreases.

At the same time, higher night temperatures increase a plant’s rate of respiration. Respiration is the process that breaks down carbohydrates to provide the plant with energy. As a result of decreased photosynthesis and increased respiration, the plant has to start using its energy reserves. Sugars and other carbohydrates that would ordinarily be used for plant growth and the development of fruit are used to keep the plant alive. Plant growth slows to a stop along with the production of flowers and fruit.

However, there is more to the story. Plants use the process of transpiration to cool themselves, similar to our bodies producing sweat. Transpiration involves the absorption of water by the roots. The water then moves up through a plant into the leaves where it changes to water vapor and exits through pores, called stomates, in the leaves.

If a plant does not have an adequate root system or there is little available water, transpiration stops and the stomates close. When this happens, a plant has no way to cool itself. As a result, damage can occur to plant tissues in the form of sunburn or sunscald (large brown blotches), especially on a plant’s south and west sides. Other symptoms of heat damage include stalled growth, leaf drop, and even death.

In addition to the extraordinarily hot weather, many area gardeners are faced with a limited supply of irrigation water. Plus, the hot weather arrived so early in the season that some transplanted trees, shrubs, flowers and vegetables may not have had the chance to grow a root system capable of absorbing adequate amounts of water to keep transpiration going.

The problems brought on by high heat are exacerbated when plants are surrounded by materials that absorb heat, such as dark rock mulches or brick walls, or situated next to surfaces that reflect light onto the plant, such as light colored building walls or white rock mulches.

What does this all mean? If hot weather persists, it means we may see sunscald on vegetable plant leaves and fruit, especially if gardeners are not able to keep the soil moist. Woody plant leaves may exhibit sunscald or leaf scorch (browning of leaf edges and tissues between the veins.) It also means plants are stressed. Woody plants will be vulnerable to attack by borers and other insects. Tree and shrub roots may succumb to dessication leading to dieback of branches from the top of the tree downwards now and in future years.
What can be done? If irrigation water is available, keep garden soil and container mixes evenly moist. Deep water trees. Apply a mulch of wood chips or bark in landscape beds if you don’t already have mulch in place. Mulch veggie gardens with compost or newspaper. Where practical, shade recent transplants during the heat of the day. Finally, hope that this weather moderates and cooler temperatures come quickly.


written by
Marianne C. Ophardt
WSU Extension Faculty
for the Tri-City Herald, Kennewick, WA
published- JULY 25, 2014


I have lived in this region for over 30 years, but every summer I have a hard time adjusting to our extreme summer heat. This year I had deluded myself into believing that our blissfully mild early summer would continue. Not a chance!

Scorching heat is not only extremely stressful on us, but also on our plants. Some ill affects of the high temperatures on plants are related to cultural factors, such as watering, but the heat itself can lead to a variety of ‘hot weather woes.’ One reason for this is ‘thermoperiod.’

Thermoperiod refers to the daily temperature change from daytime to nighttime.

Plant growth is generally best when daytime temperatures are about 80 degrees and nighttime temperatures are 15 degrees lower, although this varies with the type of plant. With this 15 degree thermoperiod plants are making and building up carbohydrates via photosynthesis at a higher rate than they are being broken down through respiration.

Higher temperatures lead to higher rates of photosynthesis, up to a point. Respiration also increases with higher temperatures. In extremely hot weather carbohydrates are used up more quickly than they can be replaced. As a result growth slows or stops.

That’s why so many garden plants look stressed when 100 degree weather prevails. Even heat tolerant annuals stop growing and flowering. Spent flowers shrivel and no new ones replace them. If they are not drought stressed, they should bounce back just fine with cooler weather.

The failure of plants to produce fruit despite flowering is a common hot weather complaint of veggie gardeners. Many vegetable crops are dependent on cross-pollination by bees. When temperatures go above 100 degrees, bee activity and cross-pollination slows.

Even with plenty of bee activity, pollination and fruit set may still suffer because extremely hot, dry weather reduces the pollen viability of many crops. Blossom-drop or the failure of flowers to set fruit is a frequent complaint in area gardens, even with plants not dependent on insects for pollination. Blistering weather is often the cause for poor fruit set on beans, tomatoes, peppers, squash, cucumbers, and melons. As a result of hot weather these crops may also develop deformed fruit due to incomplete pollination.

If you have trees in your yard, you may also note another hot weather phenomenon, seemingly healthy trees dropping an alarming number of leaves without warning. Some trees will suddenly drop some of their leaves in mid-summer, typically when hot, dry weather arrives. This is a physiologic adjustment because the tree cannot support all the leaves it developed when the weather was cooler and less stressful.

A tree can lose as many as 10% of its leaves without adversely effecting its overall health. However, it is important to make sure the leaf drop is not the result of other stresses such as lack of adequate water, root problems, or an insect infestation. You can help prevent drought stress on shade trees and harmful excessive leaf drop during hot weather by providing them a weekly deep watering.

Our summers are hot. There is no way to escape it, but we can avoid some of these garden woes by selecting plants, including flowers, vegetables, and trees rated as ‘heat tolerant’ and giving them the best growing conditions possible.

GARDEN NOTE: A bee-friendly garden should not only provide a variety of flowering plants that bees like, but also a source of water. An easy way to provide bees with water is with a shallow bird bath, bowl, or plant saucer that contains water and some bee ‘perches,’ such as corks or partially submerged stones.

Published: 7/25/2014 11:38 AM



Garden Tips, WSU Extension, Benton County, 5600-E West Canal Drive, Kennewick, WA 99336-1387, 509-735-3551, Contact Us

WSU Extension, Franklin County, 1016 North 4th Ave, Pasco, WA 99301-3706, 509-545-3511, Contact Us
© 2017 Washington State University | Accessibility | Policies | Copyright | Log in