The Garden Professors™

With drought conditions or lower than average precipitation becoming more widespread across the country, it’s time to revisit the principles of xeriscape gardening. Let’s take a look at the “classic” principles and we’ll update them, Garden Professor style.
Note: If you’re growing food crops to supply your table not all of these principles will apply. Some will, e.g mulching, and some won’t. This blog post is focused on ornamental landscaping.

SO WHERE DID IT ALL BEGIN?

As an “official” landscaping technique xeriscaping seems to have begun in the early 80’s. Denver Water, the largest and oldest public water utility in Denver, Colorado, coined the term xeriscape in 1981 by combining “landscape” with the Greek prefix xero-, meaning ‘dry’. The utility then began to formally define the main principles of xeriscaping for members of the Denver community interested in modifying gardening practices to save water. The results were the Seven Principles of Xeriscaping, listed below.

THE SEVEN PRINCIPLES OF XERISCAPING
1. Sound landscape planning and design.
2. Limitation of turf/lawn to appropriate, functional areas.
3. Use of water efficient plants.
4. Efficient irrigation.
5. Soil amendments.
6. Use of mulches.
7. Appropriate landscape maintenance.

Let’s review them and apply some up-to-date gardening information.

1. “Sound landscape planning and design” – the ideal starting point for all gardens, “Right Plant, Right Place.” This principle earns a GP thumbs-up.

2. “Limitation of turf/lawn to appropriate, functional areas” – turf has a place in the landscape but perhaps not everywhere or in every landscape. “Right Plant, Right Place” (hmm, that sounds familiar). Another GP thumbs-up.

3. “Use of water efficient plants” – it may be stating the obvious but you want water efficient plants that work in your grow zone or micro-climate. Do some homework and choose plants that will be happy in your region. We’ll give this one a GP “OK” with a few points lost for being vague.

4. “Efficient irrigation” – this one has always been a puzzler. Perhaps it was included for folks who can’t break the habit of watering their gardens. The goal of xeriscaping is to have a landscape that does well on the average precipitation of an area. Granted in times of drought some plants may need a good drink now and then and new plants may need help getting established. But for the most part watering should be at a minimum and at the correct time, seasonally as well as weekly or monthly. Don’t forget to include any natural slope and drainage in your efficient irrigation plan. And “efficient” includes a correctly working automated system if you use one. This gets a GP “OK” as well.

5. “Soil amendments” – We now know that amending the soil is not a recommended practice. It interferes with drainage, causes soil subsidence and is not conducive to root growth. Plants need to be planted in native soil, whatever it may be. This one gets a big “F” for Fail and shall be removed from our list.

6. “Use of mulches” – if you’ve been following the Garden Professors blog you’ll know this is a winner. You also know that, ideally, we recommend using arborist chips but we also know that not everyone has access to them. Mulch choice also depends on the landscape site, plant choice and, in many instances, local codes. An organic mulch (but not bark) is usually the best bet, but there are times when an inorganic rock mulch is desirable. Do your homework and choose the best mulch for your situation. Mulch!
This xeriscape principal gets the GP Seal of Approval.

7. “Appropriate landscape maintenance” – too often xeriscapes are advertised as “maintenance free”; this is false. Like all landscapes and gardens xeriscapes are an artificial environment and require maintenance to thrive. Established xeriscapes will, hopefully, need less maintenance but they do need care. This can include dealing with weeds, regular inspection and maintenance of an irrigation system, and regular plant husbandry items such as pruning and clean up. This gets a GP thumbs-up.

So, based on the above discussion, here are The Garden Professor’s Principals of Xeriscape, Revised Version

THE SIX PRINCIPLES OF XERISCAPING
1. Sound landscape planning and design.
2. Limitation of turf/lawn to appropriate, functional areas.
3. Use of water efficient plants.
4. Efficient irrigation.
5. Use of mulches.
6. Appropriate landscape maintenance.

Looking over these principles we see no reason why they can’t be applied in every region and in every landscape. Learning to garden with what you have and where you are is the hallmark of a wise gardener.
Garden smarter, not harder.

In the past week, two new major climate reports have been released. One is the latest (6^th) report from the Intergovernmental Panel on Climate Change (IPCC) and the other is the State of the Climate 2020 report. Of the two, the IPCC report has garnered a lot more press, but both are compilations of work by hundreds of scientists looking at recent weather and climate patterns and how they are affecting us here on earth. The IPCC report also provides projections of what the future climate might be like, using a number of assumptions about how the earth behaves, which can be difficult, and how humans respond, which is arguably even tougher to determine. In this post, there is no way that I can cover both sets of reports in meaningful detail and I won’t address how we need to address the rapidly changing climate here, but I do want to try to pull out some things that you can use as gardeners now. {Note, the pictures are ones I have taken myself on recent trips to use as eye candy!}

What do the new reports tell us?

The State of the Climate 2020 report, published jointly by NOAA and the American Meteorological Society, focuses on global climate events that happened in 2020. You can read some of the notable findings from the report at my blog. The report also discusses many of the “big” climate events of 2020 and puts them into historical context, including how frequently these extreme events occur and how the changing climate is making them more likely.

The United Nations’ IPCC 6^th Assessment Report presents similar information but also makes more explicit the cause of the warming, which scientists have known for well over 100 years has been caused primarily by human emissions of greenhouse gases in the atmosphere that trap heat near the earth’s surface. The IPCC report makes it clear that the rapid pace of the warming will cause severe changes to the earth’s climate that will be difficult for humans and ecosystems to adapt to.

What do the conclusions of these reports mean for gardeners?

Here are some of the changes that we will have to adapt to in the future:

• Rising temperatures across the globe—Temperatures are rising across nearly all the globe, both on land and in the oceans. Warmer temperatures mean warmer winters, hotter summers, and longer growing seasons. They also mean more increases in both evaporation from water surfaces and more evapotranspiration from plants, resulting in increases in water stress. That means you may need to water more often or use other techniques like mulch to preserve soil moisture. You may also need to switch to more heat-tolerant species as the USDA plant hardiness zones shift north (in the Northern Hemisphere). It may become harder to work in the middle of the day when it is the hottest.
• Rising temperature leads to rising humidity levels, at least where there is a source of water vapor nearby. The higher humidity is contributing to higher night-time temperatures, which puts stress on animals living outdoors (pets, livestock, and wildlife) and also stresses some plant species. It can also lead to more clouds, which reduce direct sunlight and cool the air but also reduce solar radiation available for plants, slowing their development. You may have to manage your gardens for more diseases that are related to the high humidity levels.
• Some areas like the northern US may see more rain, while others like the Southwest become increasingly dry. Year-to-year variability in precipitation is also likely to increase, with both more floods and more droughts. In both cases, water management of your gardens will become increasingly important, with the heavy rain events causing more erosion and the potential for loss of plants and trees from too much water and not enough air in the soil, and the longer dry spells making gardens more dependent on either drought-tolerant species or more frequent irrigation. You may have to put in rain gardens to help slow the movement of water through your gardens in heavy rain.
• With the rising temperatures, frost and snow will become less likely but will still occur (there will still be winter!). This will allow you plant earlier than in previous decades but will still make the plants vulnerable to late-season frosts.
• Increases in carbon dioxide may provide some fertilization of some plants, but only if there is enough water available for growth. Since some weedy species are more efficient at using carbon dioxide than other plants, you may need to deal with more weeds and invasive species in the future than you do now.
• Strong storms like hurricanes and derechos may occur more often and be more damaging than the ones we are already seeing now. The research in this area is less definitive than that for rising temperatures because there are many different factors that go into storm development, but scientists generally agree that the number of hurricanes seems to be climbing upward and that the seasons are getting longer. In addition, the storms appear to be moving slower, and that is likely to lead to more rain from the storms over a specific area and more likelihood of rapid storm development. If you live in an area that is prone to strong thunderstorms or tropical cyclones, you may see them more often and the season may start earlier in the year. Rains and winds are likely to increase, leading to more tree damage and flattened plants.

Will we be able to see these changes over the next few years?

Year-to-year variations in climate will continue to plague gardeners, since whatever happened last year is unlikely to occur again this year. The climate naturally varies over time and space as well as exhibits these long-term changes. That means it can be hard to see the creeping trends in temperature and precipitation in the noise of yearly climate swings. If you are only worried about next year’s garden, what is happening in 50 years may not be of much interest. But if you care about your children’s gardens and their future on a warmer earth, than it is something these two reports make clear we have to think about and do something about.

Personal note: This week I was also invited to participate as an author on another upcoming large climate report, this one the 5^th National Climate Assessment (NCA) that focuses on changing climate in the United States. I will be one of a number of authors contributing to the chapter on the Southeast US. If you are interested in what the content of that report includes, you can view the 4^th National Climate Assessment, released in November 2018. There are chapters for each section of the country, but also chapters that deal with economic sectors like water and agriculture. The 5^th NCA will update the information in the previous version as well as add additional information based on scientific studies completed since then.

References:

The State of the Climate report in a peer-reviewed series published annually as a special supplement to the Bulletin of the American Meteorological Society. The journal makes the full report openly available online, here. NCEI’s high-level overview report is also available online, here.

Sixth Assessment Report, Climate Change 2021: The Physical Science Basis is now out.  The report addresses the most up-to-date physical understanding of the climate system and climate change, bringing together the latest advances in climate science, and combining multiple lines of evidence from paleoclimate, observations, process understanding, and global and regional climate simulations. Get more information including links to the press release and some videos here.

In this blog I continue to examine maladies caused by environmental conditions in the absence of a disease agent or insect.

Salinity
Salt in soils or water is simply the presence of too many soluble ions in the soil-water solution. This tends to happen in dry climates where evaporation rates exceed precipitation rates. In these climates salts accumulate in soil when surface waters pick up minerals from soil that is high in precipitated salts. In wetter climates water leaches salts from soil so surface waters (rivers and lakes) have fewer dissolved salts. Also, in dry climates irrigation is often a must and irrigation sources usually have high amounts of dissolved salts. In high salt environments plants must use energy to increase their own salt balance at the root interface to make uptake of fresh water through their membranes possible. This energy is thus not available for growth. Salt affected plants are often smaller, even stunted depending on salinity levels and are more susceptible to root pathogens as their roots are more likely to be “leaky” giving pathogens chemical signals of their susceptibility.  Salt damaged leaves often show “edge” necrosis or burning on the oldest leaves.

Salt affected soils should not be allowed to dry out as roots will be damaged. Leaching to dissolve salts and move them below the root zone is one approach to prevent further symptoms.

Soil compaction
Soil compaction is the increase in soil bulk density beyond a point where roots function and grow. Bulk density is a measure of soil compactness and is calculated as the weight of dry soil per volume. Optimal and harmful bulk densities vary by soil texture. Sands have higher bulk densities than loams which are higher than clays in their growing range. Normal bulk density for a sand will be a compacted value for a clay. Values above 1.1, 1.4 and 1.6 g/cm3 can be restrictive for roots growing clays, loams and sands respectively. Compacted soils of any texture restrict plant growth. Stunting, poor growth and nutrient deficiencies due to loss of root function are common.

Extremes of light
Light is necessary for photosynthesis but it is also a radiation source that can include damaging light energy when it reaches tissues that are not accustomed to it. This happens frequently on over-pruned or damaged trees, where the canopy is suddenly reduced and stem tissues receive intense sunlight. On thin or green barked trees this can cause sun scald. Apples are particularly sensitive and will develop large cankers on upper branch surfaces if too much light is allowed into the canopy during summer. Canopy loss compounds light injury because the tree is not cooling itself as efficiently with fewer leaves. Infrared energy (heat) builds up on branch surfaces and can kill underlying stem cambium layers.

Low light levels also harm plant productivity. All trees tend to lose interior branches as normal growth increases canopy density and light levels decrease in the innermost canopy. Inner branches store less and less energy and essentially die due to light starvation. The same thing can happen to entire trees if they are overgrown by vines, other trees or shaded by buildings. While canopy thinning will preserve inner branches, it is not absolutely necessary as branch dieback is a natural process in most trees.

Effects of Herbicides  Sometimes herbicides cause damage to non target plants.  This happens when herbicides are applied unknowingly, such as residues in composts, drift from off-site applications, or by choosing the wrong herbicide to use in a garden setting.  Herbicides affect plants in different ways: some only affect tissues they contact, others are systemic, and some affect seeds as pre-emergent herbicides and have activity in soil over time.  Diagnosing herbicide damage often requires sleuthing and inquiry of what has happened in the past and what materials your neighbors may be applying.  As with any pesticide, herbicides should be applied according to label instructions. 

References

Costello, L., Perry, EJ, Matheny, NP, Henry, MJ, and PM Geisel. 2003. Abiotic Disorders of landscape plants a diagnostic guide. ANR publication 3420 University of California, Communication Services, Oakland CA.

Manion, P. 1981. Tree Disease Concepts. Prentice-Hall Inc., 399pp.

Do you have a favorite kind of weather that you love to experience? For me, it’s the first warm evening of spring, when the air is just warm enough and the wind just strong enough for the air to feel as though it is dripping off my fingers. On nights like that, I can tell that the air really is a fluid, the topic of this week’s blog.

What causes atmospheric weather patterns?

You may have noticed this year that the weather patterns across both the United States and Europe have been very persistent. That has led to the occurrence of record-setting high temperatures in some locations like the Pacific Northwest and the central US and southern Europe as well as day after day of rain in the Southeastern US. Both of these weather patterns have caused no end of grief for gardeners and farmers, since weather is seldom stuck on day after day of “perfect” conditions (even if you could define what those are).

To understand how weather patterns get stuck, it helps to know how the air moves through the atmosphere. Wind is driven by differences in heating between two areas. That contrast leads to differences in density and pressure between the areas, and the air flows from higher pressure to lower pressure to try to equalize the amount of air between those areas. The large-scale weather patterns across the globe are caused by differences in the sun heating the spherical earth at the equator and at the poles at different angles due to latitude. There are also smaller-scale wind circulations due to differences in heating between land and water (oceans or lakes) that cause the same movement of air molecules. The earth is not uniform, with continents in some areas and oceans in others and has mountain ranges that also divert the flow of air. The earth is also rotating, and that makes the wind appear to be turning towards the right in the Northern Hemisphere (left in the Southern Hemisphere), which we call the “Coriolis force.” Friction can also slow down the wind and cause it to change direction near the earth’s surface, which adds to the calculation.

Atmospheric waves

The net result of all these forces acting on the air are a series of atmospheric waves of high and low pressure around the earth that control where the weather goes (these are known as Rossby waves after the great meteorologist Carl-Gustav Rossby). A great website to view these waves is at https://earth.nullschool.net/, with a dynamic view of the air flowing around the earth. You can move the earth around with your mouse by clicking and dragging it, and you can change the size by pinching and dragging on your touchscreen if you don’t have a mouse with a wheel. If you click on “earth” on the bottom left of their map, it pulls up a menu that allows you to pick different heights in the atmosphere (1000 mb is closest to the surface, 500 mb is about halfway up in the atmosphere, and 250 mb (shown in this figure from July 29, 2021) is roughly the height that jets fly.

The image shows areas of strong winds (in red) and weak winds (in blue). The strong winds at 250 mb are called “jet streams”, and they push weather systems around. The strong jet stream over the upper Great Lakes in this image helps explain why the severe weather that moved through Wisconsin on July 29 moved from northwest to southeast. The jet streams outline the atmospheric waves that control the large-scale weather patterns. In the Northern Hemisphere, winds blow clockwise around areas of high pressure (“ridges”) and counterclockwise around low pressure (“troughs”). It is much easier to see on the website than on this static image, although traditional weather maps usually show the wind direction using arrows to help. As the image shows, often a ridge in one half of the continent is accompanied by a trough in the other half just like an ocean wave.

The large-scale weather patterns that are connected to atmospheric waves are constantly shifting as the air in the atmosphere tries to balance out all the forces that are pushing it around. (This 30-second video shows how the waves typically evolve over time.) Often the waves move, usually from west to east in the mid-latitudes, and so the weather at those latitudes also tends to move from west to east. But sometimes the weather patterns get stuck in one spot, and cause day after day of the same weather. We call these “blocking” patterns because they block the natural movement of the waves, locking the weather pattern in place for long periods. That is what we have seen this summer, with a very strong high-pressure center locked over the western U. S. and a trough of low pressure draped over the eastern U. S. for much of the last few months. In summer, high pressure usually causes sinking air, lack of clouds, warm temperatures, light winds, and no rain. In contrast, low pressure leads to rising air that forms clouds and rain and cooler temperatures due to the clouds.

What is happening this summer?

The blocking this summer has been more persistent than usual, with strong high pressure in the west causing a very hot and dry area to form (sometimes called a “heat dome”). Once you get a strong high like that to form, it can get anchored in place by the dry conditions and high temperatures at the surface, making it very hard to move. The length of time that this summer’s high has lasted has contributed to the string of record-setting temperatures and lack of rain and subsequent drought that region has experienced this year. By contrast, in the Southeast, low pressure has led to a stubborn low-pressure trough that has brought rain to the region almost every day, preventing farmers from doing field work or drying hay and increasing the occurrence of fungal diseases on their crops. If you know you are in a blocking pattern, that means you should prepare to experience the same weather for protracted periods of time and adjust your gardening schedule to accommodate the weather you are likely to see for the next week.

How will atmospheric patterns change in the future?

Some but not all atmospheric scientists think that as the earth gets warmer, the temperature difference between the equator and the poles will decrease and that this will lead to atmospheric wave patterns that are loopier, like winding rivers in an almost flat coastal plain. This could lead to both more extreme weather and potentially, more frequent blocking patterns. Gardeners need to prepare by designing their gardens to handle both more extremes of weather, including heat waves and floods, and periods of more persistent weather, which could lead to more frequent and longer droughts. That means making sure that you need to have good drainage for high-intensity rainfall events but also need to use methods like mulching with arborist chips to preserve the moisture in the soil for those times when no rain is in the forecast for extended periods.

Four years ago we moved to the family farm (where I grew up) and we’ve enjoyed restoring the 1 acre landscape around the farmhouse. Given that the residential part of this farm is surrounded by pastureland, there is a continual influx of weed seeds into our managed beds. While our thick applications of arborist wood chips have kept out many weeds, they still pop up where mulch hasn’t been applied yet or is too thin.

One of these weeds is Hypericum perforatum (also known as Klamath weed or St. John’s wort), a species native to Eurasia. The latter common name can confuse gardeners, as there are several ornamental species of Hypericum also called St. John’s wort, but H. perforatum is easily identified by the perforations in the leaf. This invasive species is a problem for our cattle, as Klamath weed causes photosensitivity when it’s consumed and can be toxic in large amounts.

In the last few years H. perforatum colonized our stockpile of native soil waiting to be used in our raised beds. It was a small enough infestation that we could pull it all up, but a closer look revealed that some shiny metallic beetles were already busy feasting on the leaves. Putting on my IPM hat, I first needed to identify these interesting beetles. It didn’t take long to find out they were a Chrysolina species.

Chrysolina hyperici and C. quadrigemina (or St. John’s wort beetles) are also native to Eurasia and are specialist feeders – they only feed on Hypericum species. They were imported as biological control agents several decades ago and have been effective in controlling dense populations of St. John’s wort. C. quadrigemina in particular has been reported to feed on both ornamental and native species of Hypericum but not to the extent of causing significant damage.

Both species of the St. John’s wort beetle feed on the leaves, where they also lay thousands of eggs. The larvae that emerge from the eggs are voracious feeders and can defoliate dense stands of St. John’s wort. Like other animals that eat Hypericum perforatum, the larvae become photosensitive and generally feed before sunrise to avoid damage.

Since biological control agents depend on the presence of their host, it’s important to retain a small population of the host. And because this particular beetle is a leaf feeder, one can remove the flowers of the plants to reduce reproduction, but maintain the plants to support the beetle.

Many other introduced, invasive weeds can be controlled using carefully researched microbes and insects. Some of these biocontrol agents may already be found in your area – so it’s important to avoid using insecticides and fungicides, in particular, to conserve these garden assets.

Insects and pathogens cause damage and disease in garden plants, but damage can also occur in absence of pests. We refer to these diseases as abiotic disorders. Plant pathologists consider abiotic disorders diseases because plants develop symptoms that reflect the changes in their physiology over time. Unlike outbreaks caused by insects or pathogens, abiotic disorders do not cause epidemics or as plant pathologists say “epiphytotics” because abiotic disorders do not spread the way insects and pathogens can. Like all diseases, abiotic disorders are a perturbation of plant physiology that show up as different or “not normal” appearance. Symptoms typically define most abiotic disorders since signs (of the actual thing causing the disorder) are not usually visible.

Since abiotic disorders do not require an organism to begin or complete a life history, they can occur at any time and are often of sudden onset. The reverse can also be true, depending on the agent causing disease symptoms which may not show for years in some disorders. Abiotic disorders are often associated with the degree to which a plant is adapted to its environment. Adaptation and establishment in an environment are different. New plantings  (those not yet established) do not tolerate abiotic extremes as well as established plants. Plants poorly adapted to the climate, soils or water of a region may be prone to abiotic conditions while plants adapted to their planting site thrive among the same abiotic factors.

Nutrient Disorders

Plants require mineral nutrients (which arrive in the sap flow after intake by roots) from soil solutions. While carbon, hydrogen and oxygen come from air and water, virtually all the other elements plants need for their growth and physiology come through the root system. Minerals are dissolved in water as ions and are available at various pH levels depending on their solubility characteristics. In general, under alkaline conditions many minerals are held in soil as insoluble precipitates and are unavailable; under acid soil conditions some elements again become insoluble and many leach away from the root zone causing soils to become depleted, especially of metals. Since roots take up minerals as ions (charged molecules) roots must be alive to regulate osmotic potential and the charge balance of ions entering and leaving roots. Anything that compromises root function can lead to inability to take up nutrients and eventually symptoms of nutrient deficiency. Compaction, flooding, root injury, poor soil food web conditions, and pathogens can all impair root function. Plants can show nutrient deficiencies for the following reasons:
• The minerals are missing from the soil
• The pH is not favorable for absorption of the nutrient which is insoluble
• The roots are not able to function and absorb nutrients
• Lack of mycorrhizae in soil or poor conditions for microbial growth

A soils diagnostic lab can help identify soil conditions and nutrient content of your samples, and suggest methods to provide optimum plant nutrition. Several soil samples should be taken all through the areas where affected plants grow, combined and sent to the lab soon after collection. Soil pH is like blood pressure – you can’t tell when it is too high or too low – so you need to have it tested. Knowing the soil reaction (pH) is the first step in investigating nutrient issues. Mulched plantings (with coarse tree trimmings chips) usually have few deficiencies in a wide range of soil conditions because nutrients are slowly but constantly provided and beneficial microbes can assist roots in nutrient uptake.

Temperature Extremes

Temperature extremes cause injury and may cause abiotic disease to landscape trees. As the climate continues to warm, extreme hot weather is increasingly likely. In California, we had record all time high temperatures in the last three consecutive years and this year in the Pacific Northwest.   In some cases these temperatures were damaging to native tree species, suggesting that they are no longer adapted to the new normal temperature extremes. Years of record breaking freezing temperatures have declined, although cold temperatures can harm sensitive species if freezing occurs suddenly or for prolonged periods. Sunburn comes when temperatures exceed the ability of bark and leaves to adequately cool the tissue. Burned leaves fall from the tree and bark often splits, cracks and dies. This damage can become an important entry point for fungal pathogens such as Botryosphaeria spp. that cause canker diseases in most landscape trees. Planning for a warmer climate means selecting trees that can tolerate higher peak temperatures in summer while surviving the low temperatures of winter.

Air pollutants

Another air pollutant is sulfur dioxide, which reacts in the atmosphere to form sulfuric acid,  and may reduce the pH of surface waters.  Acid deposition due to SO2 (the precursor) is an eastern US problem and often tied to coal use for electrical generation.  Air pollution damage to plants depends on the specific air pollutant, its concentration, and the sensitivity of the plant species, with ozone the air pollutant in California having the greatest effect on plants.

Air pollutants originate from a variety of both natural and anthropogenic sources.  Some, called primary pollutants, are released directly into the atmosphere.  Others, called secondary pollutants, are formed via atmospheric reactions of precursors.  Some air pollutants are both primary and secondary.  Ozone is a principal air pollutant in California which also affects plants. It is formed in the lower atmosphere when volatile organic compounds (VOC), i.e., short-chain carbon-containing compounds, which are released from a variety of anthropogenic as well as natural sources, react in the presence of sunlight with oxides of nitrogen (NOx) which come from internal combustion engines.

Ozone is toxic to plant cells because it is very reactive and quickly binds to plant tissues causing damage. Note that ozone in the stratosphere is necessary and protective of life.  It is the same molecule but has a different chemistry of formation. In urban areas, such as the Los Angeles Basin, pollutants may be held in the lower atmosphere by topography and meteorology, and ozone levels  may exceed federal standards for air quality, although much progress has been made since the 1960s . Conifers are particularly sensitive to ozone. Needle retention is reduced and the trees thin and appear yellowed.

While all plant tissues are susceptible to abiotic disorders, stems are most resistant, while leaves, shoots and young roots are perhaps most at risk of environmental factors that cause these disorders. Like biotic diseases, plants with abiotic disorders may require time to develop symptoms. There is a progression from slight to severe symptoms depending on the intensity and duration of the environmental factor causing the disorder. Below are some of the most common causes of abiotic disorders.

References

Costello, L., Perry, EJ, Matheny, NP, Henry, MJ, and PM Geisel. 2003. Abiotic Disorders of landscape plants a diagnostic guide. ANR publication 3420 University of California, Communication Services, Oakland CA.

Manion, P. 1981. Tree Disease Concepts. Prentice-Hall Inc., 399pp.

Schumann, G.L. and C.J. D’Arcy. 2010. Essential Plant pathology. 2nd ed. APS Press The American Phytopathological Society. St. Paul, MN. 369pp

How do you plan your work in your garden? One of the things that is most likely to affect what you do is rainfall. But how do you know when and how much rain is likely to fall? One way to get an idea of the possibility of rain is to look at something called “Probability of Precipitation”, or as we call it, “PoP”. How often have you heard someone say that the weatherman (or woman) was wrong because they predicted 30 percent chance of rain and they did not get anything? Or someone else says there was only a 10 percent chance of rain and they got flooded? If you understand how these forecasts are made, it might help you plan your outdoor activities, including your garden work and when you water.

How is “PoP” defined?

According to the National Weather Service (NWS):

PoP = C x A where “C” = the confidence that precipitation will occur somewhere in the forecast area, and where “A” = the percent of the area that will receive measurable precipitation, if it occurs at all. The forecast is what we call a “conditional” forecast—that means it depends on two different things, one of which requires the other to occur. It’s important to keep in mind that these forecasts are made for a particular period of time (often 12 hours) and for a particular area (the forecast zone). The first part of the calculation is whether or not it will rain at all anywhere in the forecast zone during the time that the forecast covers. The second is how much of the forecast zone will be hit by precipitation sometime during the forecast period.

How likely is it that precipitation will occur?

The first part of the equation above, “C”, is whether rain will occur or not in the forecast zone during the forecast period. Sometimes that is easy to determine if a big high-pressure center is over the area and no rain is expected anywhere in the region. That means that the first part of the equation is zero, and so the PoP forecast would be zero. If a strong front is moving through your area or a tropical storm is headed your way, the probability of rain somewhere in the area is probably close to 100%. But often, the likelihood of rain is not so clear. What if you are not sure about the timing of the front or the tropical storm? If it moves slower than expected, it might not make it to the forecast area before the clock ends for that forecast period. Or if you are not sure the conditions are going to be right for a rain shower to occur, then you might or not get precipitation, depending on the actual conditions. Then C becomes something between 0 percent and 100 percent, depending on how much you trust the computer models that produce the forecasts.

How much area will be covered by the storms if it does rain?

As I am writing this, it is raining at my house just southeast of Athens, GA (you can see the tiny yellow splotch on the radar map above). Sometimes it is clear that rain will cover the entire forecast area during the forecast period. But most of the time, we think it will rain in parts of the forecast area, but it will be “hit or miss” rainfall from discrete storms, not widespread coverage. The second part of the equation, A, is the forecaster’s estimate of how much of the area will be hit by rain sometime during the forecast period. It’s not as easy as you think, because those storms are moving, and they cover more of the region than you might expect. In my research I have found that often the PoP forecast is too low, and that rain as estimated by radar covers a wider area than you might expect based on the forecast. Fortunately, the NWS does provide radar estimates of precipitation that are calibrated by actual ground-truth rainfall data. The map below shows the 24-hour rainfall ending on the morning of June 27, 2021, including the rain you see in the maps above. For my CoCoRaHS rain gauge, the 0.05 inches I got correspond quite well to the light blue on the map.

Where do you get PoP forecasts?

So where do you get PoP forecasts and how do you use them? Most meteorologists provide PoP forecasts on their broadcasts or written forecasts. I tend to use the similar Precipitation Potential from the NWS hourly forecasts because they go six days out, which allows a longer planning horizon. You can see a simplified example of a forecast graph below. The “Rain” categories of Slight Chance, Chance, Likely, and Occasional correspond to Precipitation Potentials of roughly 5-25%, 25-50%, 50-75%, and 75-100%. The amount of rain in inches is shown superimposed on the bars, so you get an estimate of how wet it will be in that time period. You can find instructions for how to get your own hourly forecasts at my blog.

The hourly graph allows me to plan well ahead of whatever outdoor activity I am doing. The forecasts show hour by hour how likely precipitation is, including the chance of thunderstorms, and an estimate of how much rain will occur. Keep in mind that the forecasts six days out are not likely to be as accurate as the ones for tomorrow, but they are still useful for planning purposes. Using this information tells me when and how much rain to expect.

How do you use PoP forecasts in your planning?

These forecasts are especially useful for determining when to irrigate or apply insecticides or fertilizers that require specific wet or dry conditions to work properly. If you can see rain will be starting soon, you might choose not to water your garden unless the rain amounts are likely to be small or the chance low. Or maybe you want to mow your lawn now before it gets wet. If you are planning to fertilize and if it needs to be watered in, now might be a great time! If you need to apply a treatment that requires wet leaves to be effective, then you might wait until after the rain is over rather than applying now and seeing the chemical wash away in the storm. These precipitation forecasts can help you make the best use of your time by providing targeted, timely information on when rain will occur and how much is likely to fall when it does.

For more information on PoP forecasts, check out ” Do You (Or Your Meteorologist) Understand What 40% Chance of Rain Means?” by Dr. Marshall Shepherd, my colleague at the University of Georgia.