It’s still too cold here in the Pacific Northwest to see much happening outside, so it seems a perfect time to write about something you can’t see anyway. That “something” is the movement of water and dissolved substances through two pathways: the xylem and the phloem. And before you roll your eyes and go watch TikTok videos, keep in mind that learning about these transport systems is critical to understanding how plants work and caring for them appropriately. To prevent brain overload, we’ll focus on how xylem works this month and tackle phloem next month.
Functional xylem is composed of dead, lignified cells connected into a series of tubes that move water one way – from the roots to the leaves. You can think about xylem like a giant straw sucking water out of the soil and moving it into the atmosphere. You’ll find dissolved substances in xylem water, such as soil minerals and root-stored compounds including growth regulators and sugars. Since this is a one-way highway, everything in the xylem ends up at the end of the straw, which is primarily the leaves. Most of the water dissipates into the atmosphere through the stomata (a process called evapotranspiration) and the dissolved substances are left behind.
I mentioned that sugars can be found in the xylem, which will confuse gardeners who correctly associate sugars moving through the phloem. That’s generally true except during late winter when some trees, most famously maples, will produce a sugary sap in the xylem. While the exact mechanism of sap production remains unclear, we know that the sugars are coming from storage sites in the trunk and require a freeze-thaw cycle to enter the one-way xylem highway.
people see this process as the plant “wasting” water, it is the only way that
soil minerals can reach the leaves. In the summer, evapotranspiration lowers
leaf temperature through evaporative cooling. Thus, doing anything to interfere
with xylem function (like using antitranspirants) will have a long-term,
negative effect on plant health. Likewise, anything in the soil that’s taken up
by roots may end up in the leaves – for better or worse.
Gardeners need to think about this last caveat carefully. Plant species are highly variable in their abilities to regulate what goes into the xylem and what is left behind in the root tissue. Regulation is controlled by a barrier called the Casparian strip, which is a ring of living cells that require water (and its contents) to pass through their membranes to enter the xylem. You can think of the Casparian strip as a customs office at a country’s borders: some things are allowed in, and others are forbidden. Depending on how selective this border crossing is, soil contaminants can be left behind in the roots or carried through the plant. This is why it is so very, very, important to have your vegetable garden soils tested for heavy metals and other contaminants, and to take precautions if contaminants are found.
If you have fruit trees in your garden, then you may already
be aware of the importance of winter cold for the development of blooms and
fruit. Some fruit varieties like apples, cherries, peaches, and blueberries all
need a certain number of hours of temperatures below 45 F to prepare the trees
to bloom in the spring. Surprisingly, other trees you might not think of as
fruit trees also need a period of cold conditions to produce a good crop,
including pecans and olives. Many other garden plants also require cold periods
to provide the best blooms, such as lilac and hydrangea. I miss seeing lilacs
here in the Southeast because we just don’t get cold enough to meet their chill
requirements, although new varieties that can flourish in warmer conditions are
Why do some plants need winter cold?
The cold conditions over winter cause the plants to go into
dormancy. That helps protect the plants from harsh conditions over the winter.
Once the plants experience the number of chill hours required for that plant,
they are ready to begin the blooming process once the temperature warms up. If
the fruit trees do not get enough hours of cold over the winter, they do not
bloom well in the spring and blossoms may be delayed or bloom at irregular
times or not bloom at all. Leaf emergence may also be affected. The potential yield
of fruit is reduced due to the lack of a strong uniform bloom.
Scientists count the number of hours a tree or orchard is below 45 F and measure the accumulation of those hours as “chill hours.” Some publications call them “chilling hours” instead. They are usually accumulated starting around October 1 and go through the winter into the spring bloom period when bud break occurs. An alternate method for calculation only counts the hours between 45 and 32 F. Chill hours are different from cold hardiness, which is the lowest temperature that a tree or plant can tolerate without dying. Recent research has allowed scientists to develop more sophisticated methods that dynamically calculate “chill portions”—these calculators take into account variations in temperature over time and can reset the calculation if cold conditions return after a warm spell.
Picking the right fruit variety for your garden
Most types of fruit other than citrus and each variety has a preferred minimum number of chill hours for that variety to set a good fruit crop. If you live in the Southeast, you may be able to plant varieties of blueberries that require only a few hundred chill hours, where in the north you may be able to plant a more cold-hardy variety that requires over 1,000 chill hours for the best bloom. Because of the cold outbreak we had this December, some of our blueberry varieties are already blooming this year in the warmer weather we have had in January.
If you are planning to plant a fruit tree in your garden, you will want to pick a variety that matches the expected number of chill hours for your area. If you plant a variety that expects a lot of chill hours but your location gets only half that amount, you will not get very good yields from your trees since the plants won’t break dormancy correctly. But if you pick a variety that requires a low number of chill hours for your area, then the tree will be ready to bloom as soon as the chill hour requirement is met, which could put them at risk for a frost if they bloom too soon. So picking the right variety for your area is crucial! Most nurseries can provide the recommended number of chill hours for the fruit variety you pick.
In the future, as temperatures get warmer under the influence
of greenhouse warming, most areas will see a decrease in the average
accumulation of chill hours over time. We are seeing this at my weather network
stations in Georgia and in many other states as well. If you plant something
that has a long lifetime, you may want to plant varieties that require fewer
chill hours than your current climatological average to make sure they will
thrive in a warmer climate in the future.
Where can you find information about chill hours for your
For average numbers of chill hours for the United States, check out the map below. Keep in mind that your local microclimate will affect the number of chill hours your garden will receive.
Here are some sources of current chill hours for
If you are interested in this topic or have other questions
about gardens and gardening, we encourage you to explore our archive of blog
posts to see if you can find the answer to your questions. Here are a few of
the previous stories about winter chill you might find interesting:
Companion plants! Great, what a good idea. When you first hear the term and think about the concept it sounds great but there is a lot to not like about it. The term “companion plants” implies that these plants are partners and they “enjoy” each other’s company. The term is an anthropomorphism or overlaying of human qualities on non-human organisms. A more appropriate term may be plant associates or plant associations, a term taken from plant ecology, which has more basis for use.
Plants naturally grow together in groups which are called plant communities. These plants evolved under certain climate, soil, and environmental conditions that allow them to live together in the same place. Coastal sage scrub, oak woodland, and juniper pinyon woodland are some common plant communities where I reside in Ventura County. All of the plants growing in these communities receive winter rainfall and summer drought (Mediterranean climate) to which they are adapted to grow in. Plants growing here either resist drought through specific plant adaptations such as reflective leaf surfaces (white sage), abundant trichomes (sycamore), microphylly (buckwheat), succulent water storage (agaves, yucca and other lily family bulb forming plants), and C-4 metabolism (grasses). Some plants avoid drought by growing in the rainy season, setting seed and then remaining dormant during hot dry weather. Plants can grow in this climate because they have the adaptations to do so.
Plants compete for resources and while doing so may provide a place for other plants to grow. Trees have an advantage over grasses because they can grow above, catching the sun and shade the grasses out. But shade may also provide a place for shade adapted plants to grow. Plants surviving challenges in a specific environment may end up growing together. Woody plants also provide perching birds a place to defecate and spread seeds. This is why unexpected things may grow under other plants. Shade may even be necessary for development of proper form. We have noticed in studying western hackberry (Celtis reticulata) that the tree has no apical control and will not develop into a tree shape when grown in full sun. When grown in shade apical control is present and the plant grows a straight trunk. Birds commonly eat hackberry fruit and likely disseminate it under the canopies of other plants. I don’t think the hackberry minds growing as a blob but its “companion” plants cause it to change form due to changes in light intensity.
Some plants live very closely with others. Mistletoe is a great example. Leafy mistletoe is a hemiparasite deriving its energy from sunlight of its own leaves but utilizing water and photosynthate from its host. Similarly there are free living plants such as Indian paint brush (a member of the Orobanchaceae) that are also hemiparasitic using their roots to extract benefit from neighboring plants. Holoparasites are true parasites deriving all their nourishment from their hosts, e.g., Dodder (Cuscuta spp.). Dwarf mistletoe is also holoparasitic as it largely lacks chlorophyll. These plants are always found on or near their hosts but it is hard to call them true “companions.” The plants clearly associate with each other and in some cases are detrimental as one of the plants stands to gain nothing from the interaction.
One popular example of “companion planting” is The Three Sisters (TTS) polyculture of corn, squash and beans. This agricultural system is said to be synergistic. Corn provides support for beans and shades the squash, and beans provide nitrogen fixed from the air for the other two members of the system. The system was “practiced” by indigenous Americans all across the continent. Soils, rainfall and climate are quite diverse across the United States, and I am sure that TTS agriculture had mixed success. It is an interesting thought that the human diet can be satisfied by these crops and likely the combination was more about ensuring sustained calories and nutrients for those who grew them. In one published study there was no increase in production when comparing TTS to mono-cultures of the component parts, nor was N increased in soil. This makes sense since it’s not available until the plant dies giving up its nitrogen to the next crop which is the basis of legume cover cropping. Continued use of the TTS system is a zero sum game as corn and squash will rapidly use all the nitrogen from the previous year’s legume crop.
Mutualism is the concept that interactions between two organisms benefits both. There are many examples of plants that have a mutual relationship with insects, birds, fungi and bacteria. I found no examples of plants that have mutual relationships with other plants, e.g., “companion plants”, common to the scientific literature. I thought this was unusual so I called a friend who is a plant ecologist and asked her the question. At first she was enthusiastic and pointed to non-plant-plant relationships. As I redirected her to only plant-plant interactions we could not identify anything. My suspicion is I have missed something important or we will discover one day that there are plants evolved to help one another but for now, it evades me.
There is no doubt that one plant can help another but it’s incidental and not a sign of a mutual relationship. Most plants evolved to grow in communities because the growing conditions are suited to all. Knowledgeable gardeners and landscape architects will group plants that grow well together. This is only common sense. Understanding how plants grow in nature informs gardeners about adaptations and this in turn elevates the practice of horticulture.
Martinez, R.T. 2008. An evaluations of the productivity of the native American ‘Three Sisters’ agriculture system in northern Wisconsin. M.S. Thesis. University of Wisconsin-Stevens Point, College of Natural Resources.
Marsh, E. 2023. The Three Sisters of Indigenous American Agriculture. National Agricultural Library (USDA). https://www.nal.usda.gov/collections/stories/three-sisters
As we close out 2022, I thought I would spend a few minutes
reviewing the weather and climate of the past year, both the average conditions
and some of the extremes we saw. While this is skewed towards the United
States, I did include some events happening in other parts of the world for our
What were the average climate conditions in 2022?
Since the year is not quite over, I can’t provide a final average for temperature or precipitation for the complete 365 days, but there are some websites that allow me to look at all but the last few days. The images below are from the High Plains Regional Climate Center for January 1 through December 28. They show the temperature departure from normal and the percent of normal precipitation for the continental United States. (You can see the global temperature statistics for January through November 2022 at the National Centers for Environmental Information.) In most parts of the U.S., the temperature was warmer than the 1991-2020 normal; the exception was the north central part of the country, where temperatures were colder than normal. This pattern is consistent with the La Niña that we have been experiencing for most of the year, although individual months did vary.
Precipitation was more variable, as the map of precipitation percent of normal below shows. The driest areas in California and the Central Plains are consistent with the extensive droughts that covered those parts of the country throughout the year. The eastern Coastal Plain also shows overall drier than normal conditions for the year as a whole but the timing of wet summer and dry fall caused a lot of problems for the farmers there. The wettest areas were New Mexico and Arizona (due to a vigorous monsoon), the Southern Appalachians and Mississippi, and the Florida Peninsula due to the heavy rain associated with Hurricane Ian.
What extremes did we see in 2022?
The averages show the overall conditions that occurred this year, but don’t begin to capture the extremes in temperature and precipitation that occurred. These extremes get washed out in the averaging process but are far more likely to cause serious impacts than deviations from normal conditions over the whole year. These extremes caused 15 billion-dollar disasters in the U. S. alone as of October 11, and I certainly expect that extreme events since then, including last week’s cold outbreak and snowstorm, will add to that number.
In 2022, we experienced a number of heat waves with record-setting temperatures, including unusual warmth in South America, Europe, Asia, and Australia, where their highest temperature ever recorded (50.7 C or 123.3 F) was tied in January, their peak summer month. The United Kingdom experienced their hottest year ever, including temperatures in excess of 40 C for the first time. Much of Asia was also very hot in 2022. In the United States, the Pacific Northwest saw heat waves in both August and October, with the Southwest experiencing blistering heat in September and the Northeast in August. By comparison, cold outbreaks occurred both in January and early February and in December, with an Arctic outbreak spreading southeast from Alaska down to southern Florida, bringing extensive freezing conditions that caused significant damage to citrus in Florida and Georgia, bursting water pipes, and a lake effect blizzard in Buffalo NY that eclipsed their previous record-setting snow event set just a month earlier.
Precipitation was just as variable, with floods and droughts occurring around the world this year. Some of the more notable flood events include the rainfall in Pakistan in August that put a third of their country underwater, the floods that destroyed the northern entry to Yellowstone Park in June, and the local flash floods that occurred in eastern Kentucky in summer and fall, and the southwest monsoon that began in June with precipitation 200-800% of normal, easing drought in that area. At the very end of the year, as I am writing this, an atmospheric river event on the West Coast is bringing heavy rain to areas of California that have been plagued by drought all year. That may provide some relief from the dry conditions going into 2023.
Many other areas of the world experienced significant droughts
in 2022. They include an extreme drought that occurred over most of Europe,
causing damage to many crops and limiting navigation on local rivers. This was
also true in the United States, where the long-lasting drought in the central
United States led to record low levels on the Mississippi River, stopping barge
traffic that usually transports grain from the Midwest down to the Gulf of
Mexico. Drought covered over half of the United States for many months in 2022,
although it waxed and waned in some areas with the movement of rain-producing
How about the tropics?
While La Niña usually means that the Atlantic tropical season is active, this year was oddly quiet between early July and the end of August, with no storms observed during this period for only the third time since 1950. But once the season resumed, we saw Hurricane Fiona (affecting Puerto Rico and Nova Scotia, although it stayed offshore for the continental eastern U. S.) and Hurricane Ian in September. Ian caused tremendous death and destruction to southwestern Florida as it crossed over the peninsula, dropping feet of rain before it moved into the Atlantic Ocean and then recurved west into South Carolina as a weakened storm. In November, Hurricane Nicole brought devastating coastal flooding to areas that were previously affected by Ian, although it caused less damage than Ian did. Damage from tropical systems was not confined to the tropics, however, as the remains of Typhoon Merbok hit the west coast of Alaska in September, causing significant coastal damage with its incredibly strong winds.
What does the past year teach gardeners?
Most of the United States as well as the rest of the world
experienced a warmer climate in 2022, so gardeners will continue to need to
choose plants that are appropriate for their warming climate zones. But they
will also need to prepare for extreme conditions; devastation by individual
storms as well as climate variability will continue to affect home gardens through
water stress caused by drought and extreme heat as well as damage caused by
floods, high winds, and freezing temperatures. Building a resilient garden that
can withstand these extremes will allow your garden to thrive through whatever
conditions the atmosphere throws at it.
Thank you for another great year!
Finally, I want to end this year by thanking you all again for your loyal readership and your thoughtful questions and comments on many topics. I encourage you to share your 2022 garden challenges (weather or otherwise) in the comments along with your plans for how you plan to address them in 2023 and beyond. I look forward to reading them! We will see you again in the New Year.
Given the growth in home gardening, the fervor around houseplants,
and the interest in hydroponics and other growing methods, it makes sense that
the interest in home greenhouses is also building. Sure, home greenhouses have been a “thing” for
a long, long while – from well-to-do folks with conservatories on their estates
to the more common and basic home greenhouse in the last few decades. But shifting interests, and more/cheaper
options have made home greenhouses more accessible to the masses. The number of calls that I (and other
extension folks) get about greenhouses and other controlled climate production
methods is increasing. And even
retailers like Walmart and Wayfair have greenhouse options on their websites (I
know, because my social media and email is filled with ads about them). So let’s explore some of the benefits and
pitfalls as well as some of the things to keep in mind if you are considering
(or already have) a home greenhouse.
Now, the timing of my thoughts on this couldn’t come at a better time (or really, I just write these blogs on topics that are current in my life). I just bought a new house and one of the benefits of this new house, aside from less peeling paint, fewer creaky floors, and (hopefully) fewer plumbing problems is that it came with a greenhouse! A real, honest-to-goodness greenhouse. Not a fancy one – its just a heated high tunnel, but it’s a real greenhouse. Apparently, the previous homeowner was enough of a plant geek to not only have a greenhouse, but plant it like a conservatory as well. So it is full of palm trees, citrus trees, and tropical plants all planted in the ground. Aside from everything being planted so close you have to bushwhack like you’re in the jungle it is quite lovely. So my big winter project will be controlling those plants and making room for stuff I want to grow. But I digress…
A few considerations for home greenhouses
There are a lot of things we could discuss about managing a
greenhouse, so I won’t go in depth on a lot of things (maybe more articles to
come?). But there are definitely several
things you should consider before buying or installing a home greenhouse. Here’s just a few of them:
Purposes and uses
First up is to think about how you’re going to use the
greenhouse. Will it be for tropical
plants (houseplants), for starting seeds, growing produce, or something else? Having the purpose of your greenhouse in mind
can help you choose things like size, materials, and more. For example, if you’re just planning on
starting seeds in the spring for your home garden you might get away with a
small greenhouse that’s just a few feet by a few feet. If you are going to grow fruits and vegetables,
keep large tropical plants, or grow on a larger scale you’ll need something
bigger. Keep in mind that the bigger the
greenhouse, the more it will cost both in terms of materials or installation
and in terms of climate control.
Climate Control & Associated Costs
Whether you live in a cold climate or hot climate, you’ll need to have some sort of climate control in your greenhouse. In cold climates, heat is the major factor. In warm climates and during hot spells, cooling and air flow can be major issues. While we think most plants like heat, there’s definitely a temperature sweet spot for plant growth and it can definitely be too hot. You’ll want to be able to control your temps to keep your plants growing best. Many of the kits available from box stores/etc. don’t include a heat source or fans, so you’ll have to either find alternatives or install your own. Amazingly, the greenhouse I inherited is heated with a space heater. This is a cheap, no-frills option but can also be risky. Shorts and fire hazards are risks, of course. But so is failure of the unit and also power outages. I’m not putting my fancy/expensive plants in the greenhouse because I could imagine a power failure during a Nebraska snowstorm with below zero temps killing everything in the greenhouse. I keep an eye on the temperatures in the greenhouse with a smart sensor (left by the previous owner) that sends temperature information and alerts to my phone, but that can only do so much when you don’t have a method to keep the temperatures up.
Of course, the other hazard will come when I see the
electric bill for keeping the greenhouse warm.
Even on cold days the temperatures usually warm up on their own with
even a little bit of sun, but keeping the plants warm at night is the
problem. I have the heater set to keep
the greenhouse just warm enough so that the plants aren’t damaged killed (between
45 and 50), so I’m not paying to keep it super warm all winter long.
Of course, using fans to cool or control air flow will be
another expense for most greenhouses, as will water. Greenhouses are typically pretty high humidity,
but if you have a lot of air flow you’ll need to water more often. Amazingly, my greenhouse doesn’t have vents
or fans, but it is small enough that it doesn’t really need them. If it were a big greenhouse, it would require
some airflow to keep it much cooler in the summer. Right now, an open door (covered with netting)
One way that some folks are reducing energy costs in cold
areas is by building geothermal greenhouses.
These are becoming oddly common in Nebraska. These are a much bigger undertaking than just
popping up a kit from a box store in the back yard. You have to dig down deep enough for the ground
to aid in temperature control, have to install an underground system to intake
and deliver air (to help with temperature control), and usually build a solid block
or concrete wall to absorb solar heat amongst other considerations.
Location, location, location
A place to put your greenhouse is also an important consideration. Whether you are an apartment dweller popping
a small greenhouse structure on a balcony (it is possible) or someone installing
a walk-in greenhouse on a larger property location is important. Some greenhouse structures are lean-to or
attach to the house, meaning that you get the added benefit of the heat holding
properties of your house siding. I’ve
even seen some greenhouses used as sun rooms – which basically is a modern day
solarium. If you attach the greenhouse to your house, you’ll also want to have
a safer heat source than just a space heater.
Orientation of a greenhouse can be important for those that are
either elongated (like mine) or those that are on the side of a house in terms
of sun exposure and temperature control. Square or roughly square freestanding
greenhouses don’t have to be as planned out.
For greenhouses on the side of the house, you’ll want to place it on the
side of the house, or closest to the side of the house, that faces the sun. For the northern hemisphere this would be the
south facing wall. For the southern
hemisphere this would be on the north wall. However, in warmer climates where
over heating may be an issue you might want to place it elsewhere, like on an east
facing wall to get morning sun but protect it from the hot afternoon sun. This is, of course, dependent on light levels
available to plants, as you’ll want to maximize light exposure in winter.
For elongated greenhouses, a common orientation is to have
the greenhouse oriented north to south so that the sun passes over and both
sides receive the same amount of light.
An east-west orientation will mean that one side will receive more light.
In windy areas like Nebraska, orientation is also important to preserve the
structure. My greenhouse has a curved or
hooped top (sometimes called Quonset style), which allows wind to blow over the
structure. If the flat end of my
greenhouse was facing into the wind (west), a strong gust of wind could damage
the structure. So location and structure
play a part in the ability of a greenhouse to weather the storm.
We’ve covered purpose, climate control, and location in
setting up a home greenhouse. There’s
plenty more to talk about when building and managing a home greenhouse, so if
interest is high enough perhaps I’ll talk more about the topic in future
installments. What say you? Is there enough interest to keep talking
about home greenhouses and following the adventures of my home greenhouse
I have a very interesting research project on the effects of urban pressure on Coast Live Oak (CLO). CLO is a California native oak and I am interested in seeing if urban cultural conditions prevent the development of mycorrhizal fungi on their roots. My study is blocked, that means that all the treatments occur in a block and the blocks are repeated for replication. Blocking allows the statistics to account for variability in field locations. Its a good thing too, since one of the blocks has never done well. One tree died, two are severely chlorotic etc. This was not just the effect of the urban pressure treatments, but way more severe than any other trees growing in other blocks. It turns out there was a reason… I had unwittingly planted my sapling oaks in an area of our research farm where buried landscape fabric was installed.
So most trees that were covered by the landscape fabric were chlorotic. One died and one grew normally. The one growing normally had extended roots over the top of the fabric and then grown into soil beyond the fabric. Note in the picture above a lack of roots despite adequate moisture.
How does landscape fabric hurt trees? Let me describe the mechanisms… First and foremost soil coverings reduce the ability of soil to diffuse gases, both into and out of soil. As we know from other blogs on this subject in the archive Dr. Linda Chalker Scott and colleagues conducted research on gas diffusion rates under different kinds of landscape or soil coverings. It is important to understand that gases go both ways. For roots to remain healthy, they must convert sugar to energy through the process of respiration. During chemical respiration oxygen is combined with glucose and converted into energy (for cell growth) and carbon dioxide is produced. Carbon dioxide must diffuse out of soil and oxygen diffuse into soil for this reaction to occur.
Many of our blogs have touted the benefits of coarse, fresh, arborist chips for woody plants. One of the supreme benefits is the increase in rooting under these mulches. Unlike landscape fabrics, wood chip mulches eventually modify soil actually promoting gas exchange into deeper levels. Also, landscape fabrics prevent soil arthropods and other organisms from transporting organic matter to lower levels. Think of plastics and fabrics as a suffocating blanket over root systems, they deprive roots of moisture and gas exchange and prevent soil modification and organic matter movement.
While thick, coarse organic mulches actually enhance establishment and rooting of landscape plants without limiting gas exchange they can not overcome the impact of landscape fabrics. A common practice is to lay down fabrics and then apply mulch over the fabric. This often results in a “tatty” look years later when the mulch decomposes and the fabric shows through. Landscape fabrics and weed barriers are landscape pollutants. We should be limiting the use of petroleum products in landscapes because they do not break down easily and they have a bad impact on all forms of life.
Cahill, A., L. Chalker-Scott and K. Ewing. 2005. Wood-chip mulch improves plant survival and establishment at no-maintenance restoration site (Washington). Ecological Restoration 23:212-213. https://www.researchgate.net/publication/303445066_Wood-chip_mulch_improves_plant_survival_and_establishment_at_no-maintenance_restoration_site
Chalker-Scott, L. and A. Downer. 2022. Garden Myth-Busting for Extension Educators: The Science Behind the Use of Arborist Wood Chips as Landscape Mulches. Journal of the NACAA 15(2). https://www.nacaa.com/file.ashx?id=6c7d4542-7481-4f0a-9508-d8263a437348
Shahzad, K., A.I.Bary, D.P. Collins, L. Chalker-Scott, M. Abid, H.Y. Sintim and M. Flury. 2019. Carbon dioxide and oxygen exchange at the soil-atmosphere boundary as affected by various mulch materials. Soil & Tillage Research 194. https://doi.org/10.1016/j.still.2019.104335
If you follow current weather news, you have likely read the astounding story of the recent lake effect snowfall in Buffalo, New York, and other areas downwind of the Great Lakes, where over 6 feet of snow fell in just a day or two in some locations. My mom, who still lives in Grand Rapids, Michigan where I grew up, reported that in her city some areas got up to 30 inches during the same time frame. So this month I want to discuss lake effect snows and how heavy snows can affect your trees and gardens.
As the cold dry air crosses the warm water, copious amounts of water vapor evaporate into the air mass and once that warmer, moister air blows onshore again clouds drop huge amounts of snow in the areas downwind of the lakes. The snow usually falls in heavy bands that drop snow in areas that are highly dependent on the direction of the wind. Often the bands are just a few miles wide but if you drive through one your visibility can drop to near zero in just a short distance. When I lived in Valparaiso, IN, near the south end of Lake Michigan, winds blew straight from the north for much of the month of December 2000 dropping 32.0 inches of snow when Decembers there usually get just a few inches, thanks to the lake effect snow that occurred. (I moved to Georgia the next month, although it was not because of the snow—mostly.) As winter progresses and the lakes get colder with more ice cover, lake effect snow is reduced because of the decrease in available water vapor so fall and early winter are the prime times of year for the heaviest lake effect snow.
In this case, weather
forecasters were well aware of the potential for record-breaking snow
because of their knowledge of the lake temperatures plus the computer-generated
forecasts of wind direction and persistence over time. Winter storm warnings
and maps of predicted snowfall were produced well ahead of time. Even so, the
amount of snow that was produced from this historic event is still amazing.
Source: Carolyn Thompson /
Why did Buffalo experience such extreme snowfall amounts?
Buffalo is known for its incredible snowfalls due to its position downwind of Lake Erie, a long and shallow lake that is usually warm well into fall. The long distance of the wind blowing over the lake (called the “fetch”) allows the air to pick up tremendous amounts of water that becomes snow as it hits the land NE of the lake; the exact location of heaviest snow depends on the direction of the wind over the lake (see my poorly drawn map annotated on a screen capture of the Earth Nullschool streamline map for the day of the heaviest snowfall below). In this month’s case, the lake had temperatures well above the long-term average, and the wind across the lake was very consistent over a few days, allowing the snow to pile up dramatically. In some locations snow was falling at the rate of several inches an hour and the extended period of snowfall allowed it to build up to over six feet in some locations in just a day or two, while other areas not along the direct path of the wind received much less. The area of heaviest snowfall shifted as the winds changed direction over time.
The result of this weather event was the nearly complete shutdown of Buffalo and other areas affected by the heavy snow. Even a city that experiences as much annual snow as Buffalo does can be stopped in its tracks for a while by the sheer volume of snow that has to be removed. The weight of the snow also caused problems for a number of building roofs and caused some power outages as well. Even the professional football game between the Buffalo Bills and the Detroit Lions had to be moved from Buffalo to Detroit because of the impossibility of clearing out the open-air stadium and the roads around it for fans to get there safely (or at all).
Does climate change affect lake effect snow?
A warming climate does have some impact on the conditions that make lake effect snowfalls most likely. The lakes are generally staying warmer later into the fall, so when cold continental air does develop over Canada and move across the lakes there is more potential for large amounts of water vapor to be evaporated, increasing the chance of heavy snow. It is likely that there may be some reduction in the production of the coldest, driest air in polar regions, but it will still occur often enough for lake effect snow to continue to be a climate factor downwind of the lakes. It is more difficult to say how or if the weather patterns that determine the direction of wind flow will change as the climate gets warmer.
Lake effect snow is often very wet and heavy which makes damage to trees and power lines more likely. An average snowfall may have about one inch of water equivalent in ten inches of snow, but in a lake effect snow it is often more like six inches of snow to one inch of water equivalent which means it is very dense stuff to shovel. Wet snow may weigh up to four times as much as newly fallen regular snow per square foot. No wonder most heart attacks from clearing snow occur when this very wet and heavy snow has fallen.
The weight of this much snow can easily collapse the roofs of buildings. It can also do a lot of damage to tree limbs and shrubs, especially when the wet snow sticks and freezes to either needles or leaves adding to the weight on the limbs. Trees and bush varieties that are brittle or have poor branching structure are especially vulnerable to damage from heavy snow. Snow on the ground can help insulate the plants from very cold weather, but the moisture that is left after the snow melts can cause saturated soils that can negatively impact roots. Salt added to help melt the snow from paved surfaces can also harm plants and the deep snow cover in some lake effect storms can also provide cover for voles and other critters that like to nibble on bark.
For me, lake effect snow in my Michigan winter when I was growing up was the ultimate fluffy Christmas snow, with big fat flakes drifting down like a picture postcard. But when the flakes come down fast and heavy the holiday snow becomes a problem that can affect travelers, home owners, and gardeners too. I hope that as you travel over the holidays this winter, the snow that you see, whether you stay or go, is a delight and not an obstacle to spending time with your friends and family.
resorting to clickbait tactics to get your attention. Here’s another – “Warning!
Graphic photos follow!”
Uncorrected roots in containers or landscapes will create chronic water stress problems for trees.
If you have failing
trees on your own property or on property you manage, you need to do one simple
thing before you assume that pests or disease are responsible: you need to
determine whether the root system is healthy and functional.
Now, I know you
can’t see underground, but you can draw some informed conclusions based on
whether you can see the root flare. To find the root flare, pull away any mulch
or groundcover that’s obstructing your view. Once you can see soil, you should
be able to see the root flare. If your tree looks like a utility pole (meaning
you can’t see the flare), then it’s been planted incorrectly. This single
mistake will have myriad consequences:
If it’s buried too deeply, it’s likely the tree was planted without removing the materials surrounding the roots. Bare–rooting woody plants before planting is crucial to their survival.
Roots that are buried too deeply will not have sufficient oxygen to establish a fine root system for water and nutrient uptake, much less develop any structural roots.
Moreover, without removing the materials around the root, the roots are less likely to establish into the surrounding native soil. Neither are you able to remove poor structural roots. Check out this post for more information.
Oxygen-stressed roots will die, compounding the reduced water uptake problem.
As the crown experiences chronic water stress, it will experience dieback while opportunistic pests and disease take advantage of a tree unable to chemically defend itself.
Opportunistic pests and diseases are not the cause of tree failure – they are simply indicators of an environmental problem. Proper diagnosis is discussed here and here.
Bare-rooting plants allows you to correct defective structural roots before planting.
You should be able to confirm lack of root establishment by performing the wiggle test (that’s the secret hack). This will allow you to see whether the soil around the roots moves. If it does, that means the roots are not established. If the tree has been in the ground for more than 6 months, it’s probably not going to establish. The sooner you can dig up and correctly replant a relatively newly planted tree the better your chances that will recover and establish.
There’s a new1 blog on the social media block—The Big Blog of Gardening (BBoG). Already it’s a heavy hitter in the gardening social media world. The question is: How may foul balls are hit?
My wife came to me recently saying “Hey! Did you know that your friend Linda Chalker-Scott changed her institution?” “What?” I said. “Yeah, she moved to University of Washington. It says right here on this MSNBC article.” “It’s from the Big Blog of Gardening? What’s that?” Turns out that the BBoG is hooked into national media and gets consistent play on the home pages of those who go to MSNBC. This is because the BBoG is “now part of the Microsoft Start Program” that places content on the MSN homepage whenever a user logs in.
The originator of the BBoG is not a scientist and in the “About” section of his web page states that he started gardening as a child (like many other of us that had school gardening programs around the country). I also started gardening as a child, volunteered at Descanso Gardens in La Canada, but in my own case I followed up my childhood experiences with a dual major in Botany and Horticulture, an MS in plant pathology, a PhD in plant pathology, and a 30-year career with Cooperative Extension advising and researching in landscape horticulture. These are the typical qualifications for the blog writers at the Garden Professors web pages. Unlike writers for the BBoG, we are the folks who actually conduct research on horticulture and gardening subjects that other people quote and cite.
For us scientists, one of the pitfalls of the BBoG is it’s not a science-based blog. In a blog on pruning, the title proposes to inform how to prune any landscape plant. When you read that article, it just directs you to a link to Amazon.com to purchase a publication of the American Horticultural Society (which is not a science-based organization even though it sounds very much like the American Society for Horticultural Science – the oldest horticultural science society in the US). Rather than cite current research or address the blog title’s topic, the article leads to a product you have to buy to get your information. BBoG posts are full of of links to products available online or to “paid for links” for non-scientific and misleading garden books or other resources.
Broad strokes are used in titles of the blog. For example: “Staking a tree is almost never the right thing to do”. In some ideal world where nurseries grow trees w/o stakes and landscapes don’t require protection from damaging elements this may be true, but this is not the world we live in. Trees grown in nurseries are often staked to facilitate production and shipping. Staking can be used as a protection process on trees that may suffer impact from moving vehicles (these stakes have no attachment to the tree but serve as protective bollards). Titles in some other posts are merely attention getting or serve to promote products – not to reflect accurate horticultural science.
The BBoG often cites Dr. Linda Chalker-Scott, quotes her published work, provides links to her white papers, but doesn’t actually email or otherwise contact the original author. The problem is that often there are peer-reviewed sources by the same author containing this newer information (e.g., WSU Extension publications). BBoG often seems to miss the actual scientific or peer reviewed work but focuses on popular sources such as Fine Gardening or, even worse, Consumer Reports. Consumer Reports is not a legitimate resource for science-based information. (link to Jeff Gilman’s blog post)
Beware the comments at the end of articles on the BBoG as there can be pseudo-scientific information there (using gypsum to create drainage in soils) that goes unrefuted by the article authors. It is important that site administrators approve comments before they are listed, or at least address the misconceptions in a response.
When BBoG stories hit the mainstream media (like MSN) the blog owner does not always mention the original sources of their stories, or the scientists who developed the information: they take credit and reap the rewards of increased eyeballs on their posts and clicks on their advertising links. Wouldn’t it be nice if members of the media could dig a little deeper and find the science-based gardening sites and give them some well-deserved publicity?
Here in the Southeast we were surprised last week by a much earlier than usual freeze, putting an end to many gardens full of tender plants, although the cold was not deep enough to kill more cold-hardy species. In many parts of the region the frost came earlier than the 10% probability of frost indicating that early freezing conditions like this will come in fewer than one in ten years. Of course many of you in more northern interior parts of the United States have already seen your first frost this year, but here we never seem to be ready for it. In fact in parts of southwest Georgia last year’s first fall frost did not turn out to be until well into January, which caused a lot of problems for gardeners and farmers who had to deal with pests and diseases that easily overwintered the mild conditions.
Frost versus freeze
One of the questions I often get this time of year is what is the difference between frost and freeze? The National Weather Service (NWS) puts out both frost and freeze warnings but has different criteria for each. For a frost warning, the predicted temperature may not even get down to 32 F (0 C), but may hover in the mid 30’s. For a freeze warning the predicted temperature is expected to get down to 32 F or below and for a hard or killing freeze it usually gets down to 28 F or lower. Once the area has gotten down to 28 F or lower, the NWS usually stops issuing additional freeze warnings since at that point all but the most cold-hardy plants have completed their growing season and are either dead or dormant.
How does frost form if the temperature does not get down to
To understand frost formation and when warnings are issued it helps to know both how frost forms and how temperatures vary near the ground. Frost crystals form on surfaces that get down to freezing and have something on the surface that is conducive to seeding crystal formation. This can happen even when the air temperature is above freezing in conditions of light wind and clear skies that allow surfaces to cool to freezing temperatures by emitting heat radiation out to space at night when there is no incoming solar radiation. Conditions for this can occur with temperatures anywhere in the 30s with a reasonable amount of water vapor in the air and as long as the surface (a metal car body, an asphalt roof, or a blade of grass) can cool to the freezing point. At that point, anywhere on that surface that has an appropriate scratch, particle, or other imperfection can serve as a place for ice crystals to form and start to grow. These are called nucleation sites and allow the initial formation of an ice crystal upon which more ice can grow into delicate but visible frost.
Frost will not form if the humidity is too low because there
is not enough moisture to produce visible crystals. Often frost does not damage
the plants a lot because most of the frozen water is confined to the surface of
the plant and does not affect the interior cell walls, although there may
certainly be some damage where the ice forms. Large formations of ice crystals
can sometimes form on trees or fences if the conditions are right; this is
called hoar frost.
Frost forecasts are also provided with the understanding that the NWS is forecasting temperature values for their thermometer heights of about 2 meters or 6 feet high, since that is how they verify the accuracy of their forecasts. In light winds and clear skies the temperature at the ground level is often colder than the temperature at the thermometer height due to cold air sinking so the ground in your garden may be colder than the forecast would predict. Frost is also more likely to form on elevated surfaces that don’t have contact with the ground, since soil temperature keeps the ground surface warmer in Fall than later in the year due to residual heat from the summer warmth. Blueberry farmers that I work with tell me that you can sometimes see quite a difference in frost damage to their bushes from top to bottom due to the different temperatures that the plant may experience at different heights above the ground. Bridges often have signs that they freeze first for the same reason.
Freeze damage to garden plants
The NWS issues freeze forecasts when the temperature is expected to get down to or below 32 F. The damage that the freeze does to plants depends on how long the temperature drops below freezing and how susceptible the plant is to cold temperatures. If the temperature barely gets down to 32 F for a short period damage is likely to be minimal since the water inside the plant cells did not have sufficient time to freeze. But if it lasts longer the water in the cells freezes and, as you undoubtedly know, ice expands and breaks the cell walls causing irreversible damage to plant leaves and stems that leads to their death. John Porter provided a useful table of how different garden vegetables respond to cold temperatures in his 2020 blog on spring frosts, which underlines why some vegetables like spinach and cauliflower do better as late-season vegetables than tomatoes and melons.
The discerning reader who looks at John’s article will also note some differences between the first frost map he published in his blog and the map above, because they cover different time periods. John used the map for 1980-81 to 2009-2010, since that was the current one at the time of his post. The map here uses the 1990-91 to 2019-2020 period since the normal temperatures have been updated since John’s blog was published. Average frost dates change over time as you can see especially in some areas like eastern Oregon and northern New York State and generally, as the earth gets warmer, the first frost of fall is occurring later in the year than it did in the past (although there are a few exceptions such as parts of northern Georgia).
With winter on the way, we are sure to see many more examples of frosts and freezes in the coming weeks for almost everyone other than those who live in tropical areas. For those of us who enjoy chilly weather, the magic of frosts and freezes is something we look forward to as it paints our dying gardens in icy white.