The Garden Professors™

Fall is for planting they say when folks talk about shade trees. But is it? When is the best time to plant a tree? In this blog I will cover tree planting times and other particulars, the drawbacks and good points of these decisions.

So is fall the best season to plant a tree? Of course like so many questions it depends on many factors. Where you live (latitude) is a big part of this equation. I reside in Southern California and Southern Arizona. Both mild climes by any standard, but the Arizona property is at a higher elevation (4500ft) and gets cold sooner than the Southern California location. In Alaska, for another example, the planting seasons are much shorter or narrower as the onset of cold weather can be sooner in the calendar.

I think it is important to consider things from a “tree perspective”; or when is it best to plant from a tree’s perspective. Planting is not only the act of installing the tree correctly (see other GP blogs posts for correct planting technique) but it is also an acclimatization process. It is a good idea to purchase your tree beforehand and give it time to get used to the temperatures, light levels and water in the new site. Most fall planted trees are in containers or B&B which requires we do root inspections in order to not plant a tree that has root defects. These root inspections include removing all the old growing medium and root washing (search the GP blog for root washing). Locally sources fall planted trees will automatically be acclimated to reduced light and cooler temperatures. In fact if you plant a deciduous tree it may already be preparing to drop leaves. Fall planted trees still need root ball moisture to establish and thus will need some irrigation, but fall is also a time of reduced water use. One benefit to fall planting is that the trees will grow some roots over the winter and be ready for a big growth push in the spring. They will be partially established and take full advantage of longer days, moist soil and warming temperatures.

What about winter planting? I have a colleague that described in great detail his ambition to move to California and seek academic employment after not getting a $.50/hour raise at his landscaping job during a long stint of chipping ice in Minnesota to plant conifers in frozen ground. My colleague just retired from a nice career in Cooperative Extension, but that winter planting helped him make the move. You can plant trees in frozen soil but winter kill is a thing and the success rate of such efforts is less than for fall planting. In Southern California and other areas of the southwest and southern USA, winter planting is preferred for fruit trees because you have great access to bare-root stock (only in Winter actually) and we don’t contend with frozen soils. If it ends up being a super wet winter (see the previous blog by Pam for insight on that) it can be a problem when newly planted trees sit in saturated soils for weeks on end.

Spring planting is a thing because Arbor Day is in spring and everyone wants to plant trees on Arbor Day right? Spring planting is sometimes limited by availability, bare root stock is usually sold out or moved into containers. I don’t like buying left over stock because the leftovers are often not the best. And, trees may be in a new growth or flowering phase and their root systems are activating.

This leaves summer. In Southern California shade trees are planted all year. Fruit trees planted in summer will be the left overs from winter and again I don’t like left overs, so I generally don’t plant fruit trees then. Subtropicals establish well in warm weather so mangoes, avocados and citrus are easily planted in summer if irrigation is assured.

So the harsher the climate, the more restrictive the planting dates, but Fall is still best. In mild climates of southern states you can plant when you want in most cases. But avoiding months with frost is usually helpful as nursery stock often has tender growth. In almost all cases follow your plantings with a generous ring of arborist chips, avoid planting directly in turfgrass and irrigate your tree like it is still in the nursery for the first few weeks until it roots into the native soil. Do not amend the backfill and PLEASE remove the nursery stake at planting. Provide whatever the tree needs to stand upright with loose ties to poles outside the rootzone. Plant trees where they have room to grow and access to sunlight for most of the day. Plant HO!

Earlier this spring, I posted an article about seasonal climate forecasting and noted that we expected to see the development of an El Niño after three years of La Niña conditions ended in March 2023. And sure enough, an El Niño was declared in August 2023 and has been strengthening ever since. It has a 71% chance of becoming a strong event by December or January before starting to weaken, as they usually do in spring or early summer. In today’s post, I will remind you all what El Niño is and how it is expected to affect our climate this (Northern Hemisphere) winter and spring. That will affect how our gardens survive the colder conditions and prepare for next year’s growing season.

Refresher: What is El Niño?

If you are a new reader of this blog, you may be wondering what El Niño is. El Niño and its companion, La Niña, are two opposite phases of an oscillation in atmospheric and oceanic weather patterns linked to the water temperature in the Eastern Pacific Ocean (EPO). When the ocean there is warmer than usual as it is now, rising air over the warm water creates thunderstorms which can affect the movement of global air currents that bring stormy weather to parts of the earth while leaving other areas high and dry. When the ocean there is cooler than normal in the La Niña phase of the pattern those currents shift, changing the expected weather pattern to something quite different resulting in a different pattern of temperature and precipitation than in El Niño . The maps below show how the climate changes globally in an El Niño in the December-February and June-August periods, corresponding to Northern and Southern Hemisphere winters, respectively. The El Niño pattern is linked to a variety of unusual weather phenomena around the globe. The variations in temperature and rainfall are what affect how our gardens grow or rest in preparation for the next growing season.

What is the current state of El Niño and how is it changing?

Right now, ocean temperatures in the EPO are from 1 to 3 degrees C ( 2-5 degrees F) warmer than normal all the way from the west coast of South America all the way west to the International Date Line. This is a large area of very warm conditions that are being heated even more by the trend towards warmer temperatures due to increases in greenhouse gases in the atmosphere. The heated water provides a lot of water vapor to the atmosphere that helps fuel thunderstorms and tropical systems. Normally in an El Niño year the number of tropical cyclones in the Eastern Pacific is larger than the number in the Atlantic because of that pool of warm water. This year the Atlantic has near record-setting sea surface temperatures which are helping to produce one named tropical storm after another (today we are have “Philippe” and “Rina” active in the Atlantic but only through the name “Kenneth” in the Eastern Pacific). Global warming has affected our climate patterns to such an extent that what used to be established El Niño and La Niña patterns are less likely than in previous decades, although there have always been variations from one event to the next.

What will happen from N. H. winter through spring?

According to the predictions of how the current El Niño will evolve, we can expect the current pool of warm water and the associated global weather patterns to last through at least the April-June period. Since El Niño seldom lasts for longer than a year we are likely to go back into neutral conditions after that and neither El Niño nor La Niña will dominate. This means that over the next few months we can expect the southern part of the United States to be cooler and wetter than normal since in El Niño years the jet stream is positioned over that part of North America. As storms are pushed through the region rainy and cloudy conditions keep daytime temperatures cool as precipitation in the form of rain or sometimes snow or ice falls. In northern parts of the United States extending north into Canada, warm and dry conditions are likely to lead to a lack of snow cover and shorter ice coverage on what are usually frozen lakes. Warmer than normal conditions are also likely to occur in most of Southeast Asia stretching from India to Japan. Drier than normal conditions are likely in the Western Pacific Ocean and in southern Africa as well as South America, leading to the possibility of droughts in those areas.

What does this all mean for our gardens the next few months?

In the parts of the world that are under the jet stream, cooler and wetter than normal conditions should lead to high levels of humidity and increases in soil moisture over the winter since evaporation will be low in the cold winter months. That means gardens in those areas should be fairly wet going into spring. That means a spring or summer drought there will be less likely than after a La Niña winter, but it could be muddy working in your garden areas next planting season. Since El Niño is already strong and getting stronger this wet winter pattern may start early this year so don’t dawdle in preparing your fall garden for winter since it might be hard to work in those wet conditions. Soil temperatures may stay cool later in the spring, delaying planting of seeds and vegetables or flowers that require warm ground to germinate and grow.

If you are in northern parts of the United States and up into Canada, you can expect warmer and drier conditions than usual. That could mean a lack of snow cover and loss of some plants that need insulation provided by the snow to survive the winter. Even though temperatures will be overall warmer than in non-El Niño years, there are still going to be cold outbreaks that can cause damage to plants that are over-wintering. The lack of precipitation could also lead to dry soil conditions in spring that could require increased irrigation or hinder the growth of seeds or new seedlings you might plant. The lack of soil moisture could also contribute to the development of drought later in the growing season.

Of course, even though El Niño and La Niña are the most reliable predictors for climate several months ahead, there are always other factors that can affect climate patterns too. There is no guarantee we will see these exact patterns this winter and there are sure to be some surprises that we don’t expect.

I’ve been doing horticulture myth-busting for almost 25 years now – and what I’ve learned is that myths are zombies. Not only do myths not stay dead, but new zombie myths are also continually created. One of the newest bright-n-shiny distractions is electroculture. It’s EVERYWHERE.

What is electroculture, you might ask? Well, Jaccard (1939) described it as “the stimulation of growth in plants by means of electricity passed into the atmosphere surrounding them or into the soil in which they are growing.”

There was a surge in research in the late 1800s through the early 1900s, partially due to earlier observations which tied electrical storms to improved plant growth. (Further research determined that lightning fixes atmospheric nitrogen into a solid form (nitrate), which dissolves in raindrops and enters the soil system. This was undoubtedly responsible for the reported improvement in plant growth after electrical storms.)

Scientific interest in electroculture tapered off with advances in plant physiology and the development of commercial fertilizers. Furthermore, the few scientific publications that came from early studies showed no consistent benefit from electroculture:

• “Favourable results in increased growth and yield have been obtained from time to time, but they are uncertain and largely dependent on weather conditions.”
• “Plants on poor soil are little influenced, since electricity…does not provide either nutrients or energy.”
• “A current of 10 milliamps inhibited growth in 5 plots, but in 10 others yields increased.”
• “Electroculture experiments produced no differences in the treated trees in growth or yield.”

In the 21^st century, a desire to use fewer chemical fertilizers has spurred renewed interest in electroculture. There are vast numbers of websites proclaiming improved plant growth from sticking copper wires in the ground. None of these are backed with any reliable evidence, but proponents argue that new research (since 2000) supports this practice.

I did a search of the scientific literature through AGRICOLA, CABI, and Web of Science/BIOSIS. There were zero publications after 1968. However, Google Scholar lists several. Google Scholar searches for publications in any form on the internet that have been authored by scientists. Here’s an example of what you will find:

• A 2021 conference report for the IEEE 13th International Conference on Humanoid, Nanotechnology, Information Technology, Communication and Control, Environment, and Management (HNICEM).”
• A 2021 Research Centre Lab report from the Late Ramesh Warpudkar ACS College Sonpeth, India.

These are not peer-reviewed, scientific journal publications. Even earlier ones from the 1980s are in irrelevant journals such as Journal of Biological Physics. Plant scientists publish in plant science-related journals. Researchers outside the field of plant sciences really have no clue how plants function, nor do they have the expertise to design experiments which consider the variables that affect research in applied plant sciences.

Yet, proponents of permaculture push this concept without evidence, using anecdotes as equivalents to scientific data. Conspiracy theories abound, with accusations that chemical companies have forced scientists to cease electroculture research.

Proponents of electroculture “rely on theories of geobiology and use light devices, antennas, or magnets intended to act on the cosmo-telluric electromagnetic fields of the universe, as in dowsing (Wikipedia).”

Worldwide, scientists consider electroculture to be a pseudoscience, particularly because it does not propose any plausible scientific mechanism to explain how electricity would stimulate plant growth.

If and when this changes – when recognized plant science experts publish positive results that are confirmed by other plant researchers – those results will be in bona fide plant science journals and be worth discussing. Right now, don’t waste your time with another horticulture myth that refuses to die.

If you are interested in honing your BS detector, please take advantage of this peer-reviewed Extension Bulletin.
My thanks to Sylvia Hacker for finding great vintage photos to help illustrate this post.

Last week there was much ballyhooing over a new article on the benefits of native plants in supporting insect populations. I’ve posted on the fallacy of native plant superiority before, pointing out that landscape biodiversity not plant provenance, is most important for supporting all types of beneficial wildlife. Despite robust, published evidence to the contrary, more people and governing bodies believe that native plants are the magic bullet for urban landscapes. (Never mind the fact that there are no plants native to urban environments.)

Using CRAP analysis to assess information:

• C = credibility. Are the authors experts in the field of interest?
• R = relevance. Is the information applicable to the field of interest (in this case, management of plants in urban landscapes)?
• A = accuracy. Is the information grounded in current, relevant science?
• P = purpose. What is the underlying reason that the information is being shared?

This most recent paper warrants a careful dissection as it has gone viral online. For me, the first red flag is that there are no plant or soil scientists on this team. The first two authors, who were responsible for developing the main ideas and designing methodologies, are both ecologists by training. The other authors are involved in insect collection and identification as well as ecological modeling. Not having plant and soil scientists on the team to ensure science-based practices are followed during landscape modification is a serious oversight.

The methods section regarding the study site is astonishingly vague, given this is essentially a landscape plant installation and management project (i.e., applied plant science, not ecology). A well-designed experimental project would include control plots, replication of treatments, site analyses (including soil type and texture as well as soil testing for nutrients and organic matter), and detailed explanations of how the site was prepared, how plants were selected, prepared, and installed, and what site management occurred post-installation.

Here is the section on how this “experiment” was designed:

“In mid- April 2016, 80% of the site was substantially transformed through weeding, the addition of new topsoil, soil decompaction and fertilisation, organic mulching and the addition of 12 indigenous plant species…Selected plant species met the criteria of being locally indigenous to the City of Melbourne and represented a range of growth forms— including graminoids, lilioids, forbs and trees— requiring no ongoing management such as watering and fertilisation.”

The purpose of the methods section is to provide detailed explanations on how the study was conducted so that it could be replicated by other scientists elsewhere in the world. There is no way to replicate this study properly, as the methods are vague and very possibly not based on applied plant and soil sciences:

1. “Addition of new topsoil” As we’ve pointed out in this blog numerous times over the past 14 years, you don’t add new topsoil to landscapes.
2. “Soil decompaction” What is this? Does it involve tllling, which would directly affect the health of the two existing trees?
3. “Fertilization” What is the fertilizer? When was it applied in the process? At what concentration and based on what data was it applied? You need to know baseline levels of nutrients before you can rationally add any fertilizer.
4. “Organic mulching” What material? Compost? Cardboard? Bark? How deeply was it applied?
5. “Addition of…plant species” Were these bare-rooted or simply popped out of the pot and dropped in the new topsoil?
6. “Requiring no ongoing management such as watering” News flash: newly installed plants REQUIRE irrigation during the establishment period regardless of their nativity. And this site now contains substantially more plants than before, meaning increased competition for water and other resources.

The problems with this nonscience-based approach to landscape plant management can be see by comparing the two spotted gum trees that were on site before this project began. Corymbia maculata is a threatened native species in Australia and the continued health of these trees should have been paramount before any site work was initiated.

Unfortunately, these sorts of projects, conducted by teams with no soil or plant scientists and published in journals that are not specific to urban plant and soil sciences, are neither well-designed nor useful. The mindset of many researchers outside the applied plant and soil sciences is that there’s no real science to preparing soil, installing plants, and maintaining the site. This current paper does not even meet the standard of being experimental: it is merely a report on what happens to insect populations when a landscape is altered. There is no basis for comparisons. Any conclusions drawn are anecdotal.

It’s bad science.

Since my last post, the news has been full of one weather disaster after another. Wildfires in Maui. The remains of Hurricane Hilary moving north into California and other parts of the western USA with moisture even streaming as far east as Wisconsin. Record-breaking heat and humidity across most of the continental USA and severe weather outbreaks in the Midwest and Northeast. This does not even include the typhoons, floods, droughts, and heat waves occurring in other parts of the world at the same time. And today I am watching the development of a tropical system in the northwestern Caribbean that is likely to become Tropical Storm or Hurricane Idalia (pronounced ee-DAL-ya) by early next week, bringing rain and strong winds to parts of Florida and southeastern Georgia.

Each of these weather events can impact our gardens and properties. Not all the impacts are bad since tropical rainfall is an important source of summer moisture in many areas. In the Southeast as much as 40% of our summer precipitation comes from tropical systems and if we don’t get that rain, we can go into a drought quickly in our hot summertime temperatures. The rain from the remains of Hurricane Hilary helped provide some needed rain to help increase reservoir levels in the desert Southwest USA, which desperately needs the water. But if we get too much very strong rain or winds the damage to our homes and yards can be severe. This week I want to discuss some ways you can prepare your gardens and landscaping for the severe weather that will inevitably occur in your area at some point in the future (and that future may be nearer than you think).

Awareness is key to proper preparation

To properly prepare your gardens and homes for severe weather, you need to know what kinds of weather to expect and how it will impact your plants and buildings. The types of weather you are likely to experience will drive how you prepare. If you live in the Pacific Northwest, you are not too likely to experience hurricanes, but you certainly can experience extreme rain storms in winter and wildfires in summer especially if you have a heat wave like you did last year. If you live in the Southeast then hurricanes and tropical storms are much more likely but damage from straight-line winds can be just as important, as I found out in late July when strong thunderstorms knocked down so many trees in my neighborhood that we lost power for 44 hours. A friend of mine lost the roof of her house to two pine trees that toppled over in the strong winds. We can even experience ice and snow storms here in the South from time to time; this means you should not just prepare for the most common disasters but for any extreme event that could occur there. Of course, getting ready for the most common natural hazards is the best way to save your homes and gardens because those are the most likely to occur where you live. But you should also think about rare events like floods even if you are not in a flood plain because the consequences of an event are so severe.

Once you have identified the natural hazards that affect your area, then you need to think about what kinds of weather conditions are likely to occur in each of those events. In a hurricane or a strong winter storm on the West Coast, heavy rain and high winds are both likely weather conditions your garden will experience. In a heat wave, high evaporation rates and dangerous outdoor working conditions are likely to be the major dangers. Those are the impacts you will need to consider when protecting gardens and gardeners.

Look at your landscape and home to identify potential problems

Once you have determined what hazards are likely to affect your property, you need to do an assessment of where your risks are. Take a walk around your garden and look at the trees and plants you have. Are there dead trees that could fall over or broken limbs that could snap off in high winds from hurricanes or thunderstorms and become wind-borne missiles? Do you have garden decorations like garden gnomes or mirror balls that could also blow into the sides of cars or buildings? Do you live in an area that is prone to frequent flooding? How will you keep that water away from your house and out of your garden plots? Are there a lot of plants close to your home where they could spread wildfire in a drought under gusty winds?

After you have determined the risks take steps to minimize or remove those risks where you can. There are a number of ways that you can storm-proof your house and garden, many of which should be done anyway to maintain your garden’s health.

I was interested to read the story of the lone house in Lahaina, Hawaii, that survived the recent fires that destroyed much of the town because they had a large area around the foundation covered by river rock, which did not burn and helped keep fire from igniting the house (although I am sure there was some luck involved there too). One of my favorite books, The Control of Nature by John McPhee, also describes the increase in debris flows in western landscapes that occur due to fires that burn waxy plants. This creates perfect conditions for rapid land movement downhill after rain events following the fires, often right through people’s yards and houses.

After the storm is over and you are safe, then it is time to assess your garden and house for damage and take care of your plants, lawns, and buildings. Be careful since there may be downed power lines and dangling tree limbs that could be hazardous. You may need professional help to prune or remove trees or clean out contaminated soil after flooding. Once the immediate hazards are taken care of, then the longer-term work of repairing drainage, eliminating the effects of erosion, rebuilding beds, and other work can begin.

Take care of your family and pets too

Of course, all this planning for your garden and property should not take the place of emergency planning for your own family. If extreme weather does occur, you need to have a plan already in place to determine where to shelter in your home if you can and how to evacuate safely if you can’t stay. Every minute saved makes a difference although in the worst cases even that may not be enough time. FEMA has a good website that provides a lot of information on how to plan for both natural hazards and other emergencies like chemical spills. Many states also have excellent resources for dealing with emergencies, such as the Resident’s Handbook To Prepare for Natural Hazards in Georgia, which covers all kinds of severe weather and how to prepare for it in Georgia and beyond.

This past week I was out of town at a conference, and since the week was supposed to be a scorcher I made sure my husband was going to water the container plants daily. And indeed, temperatures were in the 90s, dropping to the mid-60s at night. But the container plants looked great when I got home and I didn’t think much more about it until the next day. My husband called me into the living room, pointing at our massive mophead hydrangea which looked like it had been torched. Leaves and blossoms were wilted and browning. Every single stem was affected. Since our landscape is on an automated sprinkler system, what the heck happened?

This is when caution and objectivity are important. I wasn’t going to go cut the whole thing down, even though it looked terrible. Instead, I made observations of the site (not just the plant):

• The site is on the north side of the house, where plant only receive direct sunlight in the morning and late afternoon during the summer.
• No other plants were affected – not even the smaller hydrangea to the west of the damaged plant.
• The irrigation system had been working normally.

When diagnosing plant problems, it’s also important to consider the history of the plant and the landscape:

• Hydrangea is at least 55 years old.
• No soil disruption or other site disturbance
• No pesticide or fertilizer use
• Mulched with arborist wood chips

Given that no other plants were affected, the problem was with the hydrangea itself. Hydrangeas use a lot of water to support their large, thin leaves and massive flower heads. When the weather suddenly turned hotter and temperatures stayed abnormally high in the evenings, the plant could not recover its water loss overnight. Many flowers and leaves experienced terminal wilt – that means they lost too much water and tissues turned brown. Other flowers and leaves were able to recover as day and night temperatures returned to normal.

What could we have done to prevent this? Had we seen the wilt occurring during the day, we could have turned on the sprinklers manually in that part of the landscape. Hydrangeas are a good indicator of low soil water. In future summers, as we continue to experience hotter and drier conditions, we will keep an eye on our hydrangea and use additional irrigation if necessary.