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Auntie Mame was on to something when she told her orphaned nephew Patrick to “Haul out the Holly, Put up the Tree Before My Spirit Falls Again” (I just sang this in a concert with my community chorus.  I’ve been hauling out the holly since August, so I’m done.) 

Plants play a big part in our holiday traditions this time of year.  When it comes to decorating for the holidays, needless to say I prefer a simpler, natural approach. The best inspiration — and materials — come from nature and sometimes from your very own landscape. It is no secret that the celebration of Christmas was timed by early church leaders to coincide with other ancient winter festivals that predate Christianity.  It made it easy to get everyone to join in the celebrations. 

Saturnalia was a Roman festival that celebrated Saturn, the deity of agriculture, and the harvest. Yule was a 12-day midwinter festival celebrated by Germanic pagans into which Pope Julius I inserted Christmas in AD 336 (there was no celebration of Christmas until then), giving rise to the 12-day Christmastide festival (or Yuletide, depending on where you are). The celebration of the winter solstice is also of pagan origin, a celebration of the darkest, or shortest, day of the year (in the Northern Hemisphere) and the rebirth of the sun.

Many of these festivals included the bringing of evergreens and natural elements indoors to celebrate the connection between humans and nature. They were a sign that life would return after the cold, harsh winter. And also a way to bring a little cheer indoors to brighten up the home during the cold, dreary winter. 

If you look at many of the more traditional elements of Christmas, you’ll notice their connection with nature, and specifically plants — evergreens, holly, amaryllis, poinsettias and more. Of course, the star of the show is the Christmas tree. Christmas trees as we know them didn’t debut until the 16th century in Germany.

A prickly holiday tradition

I’ve written about several holiday plant traditions over the years, each a different way to look at a plant that or plants that bring tradition and cheer into the home.  Perhaps the most popular of these articles is about one of the most recent additions to the holiday plant family – cacti.  The interest in this holiday plant is surging thanks to the interest in houseplants at the moment.  This is making an issue I pointed out several years ago in the article A Cactus by Any Other Name: A Case of Mistaken Holiday Cactus Identity about the mis-naming of cacti even more important.  There’s a Christmas cactus, but there’s also a Thanksgiving cactus (and an Easter one, too). 

Thanks to years of marketing and selling Thanksgiving cacti as Christmas cacti (since they bloom early enough for the Christmas shopping season and actual Christmas cactus bloom closer to Christmas), many people confuse the two.  This isn’t a big issue for the casual holiday plant shopper, but for those that “collect” the plants it can be an issue trying to sort out which is which.  For example, someone recently posted in a local plant group that they were looking for a Christmas cactus.  Half the responses were telling them where they could buy a Thanksgiving (more commonly called Zygo cactus after the zygomorphous shape of the flower) cactus and the other half were asking if they meant that they wanted a true Christmas cactus or a Thanksgiving cactus. 

As romantic as poop on a stick

As holiday plant traditions go, the hanging of mistletoe has to be both the most interesting and weirdest.  As tradition goes, if two people meet beneath the mistletoe they’re supposed to kiss.  How did kissing become associated with a toxic plant that parasitizes trees and whose name translated from the Anglo-Saxon means “poop on a stick” from the fact that the seeds are spread by bird droppings? 

As I shared in The Myth, the Legend, the Parasite: Romance, Lore, and Science beneath the Mistletoe, Norse legend has it that the goddess Frigga (Frig, or Fricka) made everything on earth promise to not hurt her beloved son Baldr, but she forgot to ask mistletoe since it was so small and innocent looking.  Baldur’s brother, the trickster Loki (you know, the one in all the movies these days) made a spear of mistletoe and that ended up killing Baldr. The white berries of mistletoe represent Frigga’s tears, and she decreed that the plant should represent love and that no harm should befall anyone standing below it.  And there you have it.

The flower that’s really just a bunch of overachieving leaves

Most recently, I shared a bit of history and lore around poinsettias, that common but prissy holiday plant that most people can’t keep alive more than a few weeks.  I mean, the original Aztec name for the plant, cuetlaxochitl (ket-la-sho-she), can be translated as meaning “flower that withers” or “mortal flower that perishes like all that is pure”, so the reputation fits. The actual flowers of the plant are the ugly little greenish-yellow bits in the center, which are highlighted by leaves, called bracts, that change colors to bright red (and more colors now) as the flowers develop.

The flower and the bright red represented a gift from the gods for the sacrifices that the Aztecs would make to them. You can read a bit about the history, and how to save them and get them to rebloom from year to year (if you dare) in my festive little ditty Poinsettias: from ditch weed to holiday super star (history, lore, and how to get those d@!% things to rebloom next year).  Of course, there’s been recent discussion on how the story of the poinsettia is similar to the pagan traditions associated with Christmas – a plant tradition of the Aztecs was changed as a way to fit with the Christmas story and work as a conversion tool. It is good to know a bit about the original tradition of the plant to understand its importance throughout history. 

Haul out the holly….and the magnolia….and the rhododendron…..

Holiday plant traditions don’t stop at those finicky plants we grow in greenhouses- the original traditions of Solstice and Yule consisted of bringing in all different types of plants for the season. Sometimes you have to look no farther than your yard to find natural elements fit for a holiday celebration. If you have evergreen trees or shrubs in your landscape, they can easily be incorporated into holiday arrangements indoors or out.

While you can readily purchase live Christmas greenery at many retailers and markets (except for this year, with all the supply issues going on), what you can harvest from your own yard or nearby woods (where permissible) will be much fresher and last much longer. Remember that harvesting branches from your own trees and shrubs is actually pruning them, so you can kill two birds with one stone.

Both white and Virginia pine make excellent decorating choices as they are long-lived and retain their needles well. Junipers, cedars and firs all have interesting foliage and wonderful evergreen scents. Junipers can sometimes be found with berries and cedars sometimes have tiny cones. Spruce is a great choice for wreaths, as it is bristly and holds its shape.

Decorating doesn’t have to be left to the needled evergreens, though. Holly is a classic choice, but mountain laurel, rhododendron, boxwood and magnolia and many others can provide beauty and elegance in holiday displays.

You don’t even need to stick with green things in your decorating, either. Attractive bare branches, plumes from ornamental grasses and other “brown” things can add contrast. Other natural items, such as fruits, nuts and pinecones can also be incorporated for color and interest.  Just take a look around your yard or neighborhood to find those natural elements to bring indoors.

To keep your greenery fresh, there are some basic tips that you can follow:

• Use sharp pruners to make cuts, and keep the ends of branches in water until ready for use.
• Soak evergreens in water overnight to absorb moisture and extend shelf life.
• Keep arrangements in a cool location until use.
• Keep evergreens out of sunlight to reduce moisture loss.
• Use an anti-transpirant spray to keep in moisture.
• Don’t decorate too early — live plants have a limited lifespan.
• If purchasing plant materials, select healthy, green plants and flexible needles/leaves.

Very large and old trees don’t often attain size and splendor free of defect and disease. A mature tree may have accumulated some damage, injury or disease along its path to grandeur. It can contain hazardous branch attachments that have not failed yet, large sections of dead wood or it may bear the burden of an assemblage of cables and other devices installed over the years to sustain it. Very old mature trees have endured perhaps multiple human lifetimes of exposure to storms, droughts, pests and, perhaps worst, early pruning practices. Trees gain age and majesty when they don’t fall down or fail multiple large branches. To do this trees defend their wood biochemically from invading fungi. Trees also cover over decayed wood with new wood that is fairly impervious to the old infection. High vigor trees with adequate resources (water, soil, sunlight) should be able to contain decay that would shorten their lives otherwise.

Dr. Alex Shigo (2003) devised a chart showing the amount of pruning over a tree’s life time. Young trees have all living cells and most of the wood is sapwood, therefore the trimmings in young trees are of living tissues. As trees grow older and shade themselves heartwood develops in the canopy. It’s not living, doesn’t conduct water and minerals but does provide support to branches and strength to the canopy. Heartwood is also more resistant to decay than sapwood. The pruning of a mature tree removes a combination of living and dead cells. An over mature or ancient tree has much more dead cell tissue and only deadwood should be removed with an attempt made to preserve all stems with leaves.

Large trees have large branches and the temptation is to remove large branches to achieve pruning objectives. This would get more done and we would then have to prune less, right? But this is a flawed strategy and the results are the opposite of what the tree owners are trying to achieve. As trees get large, pruning cuts should be smaller and occur frequently. Guidance can be given to branch growth by many small cuts rather than large ones. This approach directs growth without causing rampant regrowth of dormant or latent buds. Latent buds have a purpose, they rescue trees from death should there be a catastrophe. They are preformed along large branches and will sprout when exposed to sunlight or when the terminal buds are removed. Large topping or heading cuts typically activate latent buds and the tree subsequently grows back too many branches in one place. These require selection and careful removal over time and can involve quite a bit of repair pruning to fix the tangle cause by massive heading cuts. Aging trees develop an extensive heartwood that is easily exposed to decay from large pruning wounds so large cuts should be avoided.

Larger trees develop more sapwood which needs energy for respiration relative to the amount of leaves in the canopy. The first duty of the leaves is to provide energy to keep all the cells in the young phloem and sapwood metabolically active. A typical tree only utilizes a few growth rings of sapwood for water transport and energy storage. As these rings age they narrow and there is less tissue from which to store and retrieve energy. Despite the subtle balance between canopy and energy utilization trees continue to grow throughout their lives at more or less the same rate. As trees age the amount of non-functional wood increases, this may be heartwood or deadwood in the canopy. At the end of their lives trees have nearly no living sapwood and the energy of the tree is mostly depleted. Over pruning causes further energy depletion. Removing leaves and buds that give rise to leaves from old trees reduces the amount of energy stored. This forces the tree to utilize sapwood reserves to recover from the pruning losses of the photosynthetic part of the tree. Severe pruning in old trees ultimately depletes them of stored sugars and leaves them without energy to make defensive compounds. This can exacerbate conditions by allowing disease or decay organisms to enter the sapwood.

What if your large tree was never structurally pruned and has branch faults or you have concerns for failure? You should always consult a certified arborist with training in Tree Risk Assessment. The health of the tree is important but the safety of those living around it is more important. Sometimes targeted pruning can be used to shift the balance of growth in a tree and strengthen branch attachments. This can take years of careful pruning with small or modest cuts. When large branches must be removed they should be left as long as feasible and the cut should be circular not an oval shape to decrease exposed surface area. Large pruning wounds should be limited to the fewest number required to achieve the pruning goal. Pruning paints and wound dressings are not recommended.

Large trees accumulate deadwood during natural senescence of inner branches or due to injury, disease or insect attack. Common practice has suggested removing deadwood from old trees but as arborists become more sensitive to urban ecology issues we realize that deadwood is prime habitat for cavity nesting birds and other life. Deadwood serves a purpose and, if it does not pose a threat to those living under or around the tree, it should be retained especially if evidence of utilization is present.

Caring for large trees is tedious because there is a lot at stake. Old trees are valuable and provide many benefits to their owners. Always seek expert opinions and be willing to pay for them, preferably from the arborist who does not do the pruning. Then act on those opinions with care and consideration and always closely supervise any pruning being done to your trees.
Use the following pointers from Gillman for mature tree pruning strategies:
-Evaluate your tree for health, strong branch structure and evidence of decay
-Remove dead branches as needed but retain some for wildlife if needed
-Perform structural pruning when feasible
-Shorten or remove branches with cracks swellings or deformities
-Remove parasites such as mistletoe or canker infested branches
-Retain as much live tissue as possible while achieving pruning objectives
-Keep cuts small (2inches) or moderate (4″) if the tree is a good decay compartmentalizer

References:
Gillman, E.F. 2012. An Illustrated Guide to Pruning 3rd ed.
Delman Cengage Learning, Clifton Park, NY.
Manion, P.D. 1991. Tree Disease Concepts. 2nd ed. Prentice-Hall, Englewoood Cliffs, N.J.
Shigo, A.L. 2003. Modern Arboriculture A systems approach to the care of trees and their associates. Shigo and Trees Associates Durham, NH. 3rd ed.

Welcome to the first People and Plants GP blog post. These posts, which will be on a random basis, will take a behind the scenes look at the namesake people behind many plant names. This first one will be about Andrew Jackson Downing.

Andrew Jackson Downing was born October 30/31, 1815 in Newburgh, New York to Samuel Downing and Eunice (née Bridge) Downing. His father was originally a wheelwright but later became a nurseryman. After he finished his schooling Andrew worked in his father’s nursery and was soon bitten by “the plant bug”. I’m sure many of us can identify with that “affliction”.
Over time he became more interested in landscape gardening, architecture and the relationship between them. He began writing articles about botany and landscaping but soon realized he needed to know more about both topics and so began a course of self-education. By the 1830’s he was producing pieces for newspapers and hort journals and in 1841 his first book, *A Treatise on the Theory and Practice of Landscape Gardening, Adapted to North America, met with great success. It was the first book of its kind published in the United States.

Downing continued to write on the importance of linking landscaping and architecture and was well thought of in the industry. What really put him on the public gardening radar was a book he and his brother Charles wrote. The 1845 book, The Fruits and Fruit Trees of America, was the most complete one of its kind to date and led to Downing’s becoming the editor of a new periodical, the The Horticulturist and Journal of Rural Art and Rural Taste. The journal came to be his strongest influence on society and operated under the premises of horticulture, botany, entomology, pomology, rural architecture, landscape gardening, and ideas dedicated to public welfare, specifically public parks and what we now call “open space”. As an example he argued for a New York City park, which in time became Central Park, in this journal. It was also in this publication that Downing repeatedly pushed for state agricultural schools.
He held the position of editor until his death in 1852.

Downing was greatly influenced by the British “picturesque” landscape theories of the era. By the time he published the second edition of his Treatise he was a firm believer in the advantages of the picturesque landscape movement. He embraced the use of local landscape features especially “the raw materials of wood, water, and surface” and, when possible, the blending in with the local terrain instead of creating an artificial, out-of-place one. He became the mid 19th century champion of natural style landscapes and helped steer American gardeners away from the more formal, geometric lines that had dominated the 18th and early 19th centuries.

As an editor, Downing repeatedly addressed the importance of urban and suburban public gardens and parks. He felt such spaces would aid in the fostering of moral and civic virtues in the American public. In the October 1848 Horticulturist editorial “A Talk about Public Parks and Gardens,” Downing wrote that public parks would play an important role “in elevating the national character.” He also believed interacting with nature had a healing effect on mankind, wanted all people to be able to experience nature and felt city parks were vital in helping maintain a healthy community.
Downing had a wide audience through his books and editorials in the Horticulturist but his influence went beyond the readership of his publications. Private and public gardens and city parks that proliferated through out the USA during the mid 1800’s owed their existence to his ideas.

Downing did much more than this blog post will discuss so let’s move on to the plant.

The genus **Downingia is named after Andrew Jackson Downing. It contains 13 annual plants which are native to the western USA and Chile. Commonly known as calico flowers, they are noted for forming large displays of small colorful blooms around seasonal or ephemeral pools of water. Interestingly each pool will usually only host one or two species of Downingia even in areas where multiple species exist.
It’s a member of the bellflower family and the plants are 3-10″ tall.  Flower size varies but each flower has five corolla lobes or petals. The two upper petals are smaller than the three lower ones. Coloration is mainly blue/purple/pink with a little white, yellow, and black.
Downingia must cross-pollinate. To prevent accidental self-pollination, the flowers mature in stages, they go from male to female. This change is time coordinated across each patch of flowers. How this is done is still unknown.
The species is pollinated by native solitary bees.

*https://openlibrary.org/works/OL1471543W/A_treatise_on_the_theory_and_practice_of_landscape_gardening?edition=treatiseontheory01down

**https://www.wildflower.org/plants/search.php?start=0&pagecount=10

Linda’s post last week about “drought-resistant” plants made me think about drought and how different types of drought affect gardeners in different ways. In her article, she defined drought as “an unusual lack of rainfall”. This is one of four different kinds of drought that climatologists talk about, and I thought it might be interesting for you to hear about how the four (or maybe five) types of drought differ and how they affect gardeners in diverse ways. A great source of drought information across the U.S. is https://www.drought.gov/.

Meteorological drought

The first type of drought, the one Linda described last week, is what climatologists consider a meteorological drought. A meteorological drought is related to how much rain you get compared to usual conditions at your location. I like to think of it as “too many days of nice weather in a row”, since in these dry conditions, the sun is shining and it is a great time to garden, play golf, or do construction. Of course, if you don’t get rain for a long time, you start to see impacts on plants, water bodies, and wells, but meteorological drought is usually not identified in terms of impacts, just on the amount of precipitation measured over weeks, months or years. Meteorological droughts look different depending on where you are. It is possible to have drought even in a desert if rain does not fall over an unusually long time. Droughts in the Pacific Northwest might look quite different since the frequency and amount of rain looks a lot different there. In the Southeast, drought can be hard to identify by looks since even when rain does not fall for a long time, things tend to stay relatively green because in our worst droughts we still get 35 inches of rain a year. Most gardeners can cope with meteorological drought by watering their plants at appropriate intervals and reducing impacts of the dry conditions by mulching to help keep moisture in the soil.

Agricultural drought

I spend a lot of time talking about agricultural drought to the farmers and extension agents I work with, because agricultural drought is always on their mind. Agricultural drought is defined by a negative water balance that can be related to both lack of rainfall and/or high temperatures that increase evaporative water stress on growing plants. It occurs mainly in the growing season because that is when the crops are actively growing and impacts are most noticeable. A 3-week dry spell may not be a problem for most gardeners that water their plots, but if you are a dryland farmer without irrigation, you can lose an entire crop of corn for the year if the dry spell occurs when the corn is pollinating and the silk dries out before the pollen can stick to it. Often agricultural drought can occur even when there are no other impacts to us because it is subtle; most people don’t see the impacts until months later during harvest. If you have limited access to water for irrigation or very sandy soil in your garden, then you are more likely to be affected by agricultural drought since it will be harder to maintain plant health when the soil is dry.

Agricultural droughts are often related to flash droughts. Flash droughts are characterized by very rapid development or intensification over a short time period, and crops are often the first things affected because of their need for frequent watering. Flash droughts are often characterized by a lengthy dry spell coupled with very high temperatures, something that is common when you have a persistent area of high pressure right over your location.

Hydrological drought

Where agricultural drought is related to a shortage of water over time periods as short as a week to a month, hydrological drought is related to a shortage of water over months or years. Climatologists measure hydrological drought as precipitation deficits over periods that range from three months to multiple years. You can see hydrological drought in dropping stream, lake, and reservoir levels and in dropping groundwater levels if the deficit lasts long enough. A hydrological drought can occur even if no agricultural drought is observed when you get rain at frequent intervals but it is less than normal over a long time period, as long as the rainfall is enough to sustain the crops (or if it is winter, when there are not many crops growing).

Hydrological drought tends to affect gardeners’ access to water for irrigation because the long-term water deficits lead communities to enact water conservation measures to protect drinking water supplies. Most local and state governments have tiered conservation measures that get more strict as the water supplies get lower and lower. They may start by merely providing educational materials on water conservation and then progress to even-odd watering by dates or watering during overnight hours only (since there is less loss of water due to evaporation in cooler night-time temperatures). In the worst droughts, they may cut off the use of water for establishing new lawns and gardens (often with an exception for gardens that are used for food production). If a drought lasts for many years or even decades, then it is considered a megadrought, such as the one that is occurring now in the Southwest U.S. Megadroughts are related to long-period shifts in global atmospheric patterns and can lead to the abandonment of cities because of the loss of water to keep their citizens alive over time.

Socio-economic drought

Socio-economic drought is a little different than the other kinds of drought mentioned above. It is drought caused by a lack of water due to overuse, hoarding, or war. An example of a socio-economic drought might be one caused by one country damming a major river in their country to create a reservoir, keeping the river water from flowing downstream to other countries that depend on the water for agriculture or water supply. In the United States, disagreements between who is allowed to use available water often end up in court as cases like the Georgia-Florida “water war” that was recently adjudicated in the U.S. Supreme Court. Locally, disagreements about who is allowed to use the water sometimes result in tiered water pricing, where the more water you use, the higher the price. This affects gardeners who have plots that use a lot of irrigation because of the use of water features, plants with significant water needs, or lack of mulching or other methods of protecting soil moisture.

Recently, a fifth type of drought called ecological drought has been identified, since a lack of rainfall can affect natural ecosystems in ways that are distinct from gardens, farms, or watersheds. I won’t address it further here, but if you are interested in how natural ecosystems are affected by dry conditions, you will no doubt read about ecological drought in publications in the future.

Drought is a naturally occurring part of the climate across the world, and gardeners must understand the nature of drought in their area to recognize how it affects the weather and climate where they live. Linda’s article last week gives some good guidelines for how to make your garden work in your climate.

This month I continue the series on pruning with a look at pruning established landscape trees.  These are trees in the prime of life, growing well, starting to shade the garden beneath them and expanding their canopies. Various reasons can prompt the call for tree care professionals.

What reasons would we have to prune a healthy vigorous mid-aged tree? For those we have to examine what may have happened in the past. The fact that a tree is growing well does not always mean it was “selected” well. After a few years time, that cute little nursery tree is flexing its branches and spreading out and, more worryingly, upward. One of the prime reasons for pruning is to reduce the size or expansion of a tree canopy. There may be impending interference with power transmission or other utility lines. The tree may be blocking a view, it may just be frightening in its shear mass or size and pose a psychological threat to its owner. Size reduction is a frequent object of tree pruning operations.

The tree is too large for my comfort

If you find that you want to reduce size of a tree you should ask yourself is it possible and is it sustainable? Ultimately, do I have the right tree for this spot?
Size reduction pruning is a battle with tree genetics. The tree wants to achieve a designated height and the tree owner wants to limit that height. Terminal or leaders can be pruned back to a lateral branch to reduce the length of stems and branches. This kind of “thinning” is effective as long as the branch that a leader is trimmed to is large enough to resume the hormonal role for that portion of the tree. Arborists have a 1/3 rule that suggests the branch you trim back to should be at least 1/3 the diameter of the stem it is attached to. That is, you don’t trim back to a twig otherwise it is a heading cut and re-sprouting will be abundant and require more pruning. The problem with the 1/3 rule is that trees don’t respond consistently to pruning on this basis. Some will re-sprout at the cut, others will not. In any event if you embark on size reduction pruning you will likely be maintaining that strategy over the life of the tree and this may not be sustainable. Removal and replanting with a smaller tree species that can be cultivated with little or no pruning would be a more sustainable approach.

Deadwood, I see Deadwood

Deadwood is not the end of a tree or necessarily a reason to call the tree trimmer. Deadwood can serve as habitat for cavity nesting birds and is an important part of tree ecology. But dead branches that hang down and have broken from the canopy pose hazards and should be carefully removed. A dead leader can be pruned back and retained especially if woodpeckers have excavated nests that other cavity nesting birds, such as owls, will subsequently use. Of course if the deadwood is part of the active pathology of an ongoing tree disease, its removal may be warranted to prevent disease progress. This diagnosis would involve consultation with your state extension specialist or agent and or qualified arborist with training in tree diseases.

I have a tree with uncorrected branch faults

Perhaps after careful study you realize your tree has co-dominant stems or too many branches coming from one place on the main stem. Both are important branch faults that precede branch failures. Co-dominant stems look like a “slingshot for giants”, two big stems both the same size. Often they are accompanied by included bark that separates the stems. The problem is often that when the stems are large, pruning one will leave a big wound that can let in decay and decrease the life of the tree. If a codominant is 6 inches or larger, it may be best to not remove it. Or remove the entire tree thus fixing the issue. If you want to keep the tree you should choose one stem to thin and prune rather aggressively (not topped) and leave the other stem unpruned. Over time the thinned out stem will grow slower and the unpruned one faster and the codominance will decline. This strategy will require year over year pruning to achieve your goal of shifting the codominants.

If the problem is multiple branches coming from one point the correction is far more difficult. Removing branches from a cluster again provides decay entry and potentially can weaken the attachments of the other branches. Large trees (especially if a branch has already failed) with multiple attachments at the same point are likely to have a failure. The only correction for some cases is tree removal. If there is no or low risk to people or property the tree can remain as such and or pole braces installed to prevent failure. Again, failure likelihood depends greatly on the species. Elms, Oaks, Acacia, Carob, etc, all are at risk of rapid failure.

What if my tree is storm or wind damaged and needs repair?

The occurrence of severe weather events is on the rise. Intense winds, increased rain, high temperatures are all commonplace now. Trees that withstood the elements in the past can now be damaged in these storms. In every case a professional opinion on the salvage of storm damaged trees is important. Expectations for restoring an acceptable crown that is safe for the site and those that use it is highly variable. A qualified consulting arborist can guide you in working with storm damaged trees or providing an opinion on removal vs restoration. It is best to contract an arborist who has no interest in a trimming/removal operation since they do not stand to gain from an easy and profitable removal job.

I realize my tree was harmed by previous tree trimming and I want to make it better

Much like the other scenarios, growth patterns and targeted branch work can fix or resolve past insults to trees from inferior pruning . However, don’t expect to resolve all the issues in a single pruning. Often it takes years of careful work to restore the canopy of an abused tree.

I just want to have my tree thinned

If your goal is to get more light into the garden and the tree is a variety that responds well to thinning, this is great. If you don’t have a reason then this is not a legitimate pruning need. Trees don’t need to be thinned and thinning is actually detrimental to some oak species, depriving them of the inner canopy of leaves that they rely upon. Thinning slows the overall growth of trees and reduced stored carbohydrates. Extreme thinning on a regular basis can predispose trees to some fungal pathogens.

At the end of the day….

Mid size, mid life trees require no pruning. People may have reasons to prune when the tree is too large for the site, has been damaged or needs further training to develop its architecture. Trees don’t require pruning if they grow according to their genetic program. Pruning the maturing tree really seeks to undo damage or correct issues that arise in its culture. Branch faults and tree branching structure may not be apparent to every gardener so seeking a professional opinion is always worth the money if you want to be sure of your tree’s pruning needs.

I got a Facebook message early this week from a friend in Sacramento CA that said after over 200 days with no rain, she got 4.83 inches in a 24-hour period from the latest extreme rainfall that occurred over northern California. Others have reported up to a foot of rain in three days. If you follow the news, you may have heard the term “atmospheric river” used to describe the torrential rains and flooding that have occurred this week in San Francisco and other parts of Northern California. In this post, I want to explain what atmospheric rivers are and how they affect rain climatology in the Western U.S. as well as other parts of the United States and the world.

Tahquamenon Falls–Autumn. Source: Wfgc, Commons Wikimedia.

What is an “atmospheric river”?

The term “atmospheric river” first appeared in the modern scientific literature in the early 1990s. Since it was first used, there has been a lot of discussion about what the term actually means. Commonly, it is seen as a band of very moist air flowing into a coastal area, bringing the potential for a lot of rain to the region that is at the downwind end of the flow. In some respects, it is like being on the receiving end of a firehose streaming high-intensity water right towards you! After a lot of discussion by meteorologists (described in this Bulletin of the American Meteorological Society article) the official definition in the Glossary of Meteorology became:

Atmospheric river – A long, narrow, and transient corridor of strong horizontal water vapor transport that is typically associated with a low-level jet stream ahead of the cold front of an extratropical cyclone. The water vapor in atmospheric rivers is supplied by tropical and/or extratropical moisture sources. Atmospheric rivers frequently lead to heavy precipitation where they are forced upward—for example, by mountains or by ascent in the warm conveyor belt. Horizontal water vapor transport in the mid-latitudes occurs primarily in atmospheric rivers and is focused in the lower troposphere. Atmospheric rivers are the largest “rivers” of fresh water on Earth, transporting on average more than double the flow of the Amazon River.

Source: NASA Earth Observatory

Why do atmospheric rivers produce so much rain?

The strong flow of moisture into a region provides an excellent source of water vapor for the development of heavy rain, especially if it is moving into an area with flow up mountain slopes that can help storms develop vertically. That enhances the rain-producing process. The West Coast of the United States provides a perfect location for the occurrence of atmospheric rivers since there is a broad expanse of ocean to provide the water vapor, dynamic storms that concentrate the flow into bands that can stretch all the way from the Hawaiian Islands to California (which explains an alternate name, “Pineapple Express”), and mountains near the coast to provide lifting for the moist air once it comes onshore. Cliff Mass of the University of Washington often discusses them in his blog on the weather of the Pacific Northwest.

Do atmospheric rivers occur in other places?

The short answer is Yes! While historically they are discussed most often when talking about weather on the West Coast, atmospheric rivers (ARs) can and do occur in other places as well. Anywhere that has a good source of moisture plus dynamic storms with strong airflow can experience ARs. In the Southeastern U.S., we get them when strong flow from the Gulf of Mexico or the Atlantic Ocean feeds into our region, usually ahead of a strong low pressure center that provides the necessary dynamics to create a narrow band of moisture feeding into the region. According to research by University of Georgia researchers, they occur most often in the cold months of November through March but can occur in any month of the year. In the Southeast, we get about 40 events per year that are classified as ARs. I was surprised to read that there are slightly more events on the East Coast than along the Gulf of Mexico, but anywhere along the Southeast coast can be affected. No trend towards more or fewer events was seen in the 1979-2014 period.

NOAA’s Physical Sciences Laboratory’s page describing ARs says that on average, about 30-50% of annual precipitation in the west coast states occurs in just a few AR events, thus contributing to water supply. ARs move with the weather and are present somewhere on the Earth at any given time. This site has some great resources for tracking and forecasting ARs around the world.

Of course, atmospheric rivers are not the only source of heavy rain events, but they are one of the primary sources for the West Coast. In other areas, tropical systems like slow-moving hurricanes or stalled fronts can also drop a lot of rain. You can also get very heavy rains from small local systems of thunderstorms if conditions are right, especially if the storms “train” or move one after another over the same area like cars on a freight train. We saw this in the Nashville area a few weeks ago, where the heavy rains resulted in significant flooding over a few counties.

Rain garden in the Allen Centennial Gardens on the campus of the University of Wisconsin-Madison. Source: James Steakley, Commons Wikimedia.

How do atmospheric rivers and other heavy rain events affect gardeners?

If you are a gardener in the Western United States, you are already well aware of the long dry season over the summer followed by bouts of rain that can occur over the winter months. The timing of the switchover from dry to wet conditions depends on how far north you are on the coast, with the summer dry spell coming earliest in southern California and moving northward with the position of the jet stream as the summer progresses. Dealing with the effects of an AR is like any other attempt to protect your garden against heavy rainfall, and can mean proactive action to make sure that water-sensitive plants and trees are not located in low-lying areas where rain collects. This 2013 article from the Garden Professors blog on-site assessment is still good advice for planning ahead for soggy conditions by walking through your property in the rain. Designing for erosion control, such as rain gardens, can also help divert water in high-intensity rainfall.

In spite of the heavy rain that fell in this last atmospheric river event, the rainfall barely made a dent in the long-term drought that is present across a lot of the Western U.S. Drought will continue to be a part of the hydrologic cycle that affects gardeners, farmers, and water managers across that region and across the world.

As the climate warms the value of trees for cooling the environment around buildings, especially in cities, drives tree planting programs. Planting trees is just the first step in growing a tree in a sustainable landscape. Successful plantings require evaluation and guidance of the new tree’s current and future branch architecture. In almost every case, nursery grown trees will require some structural pruning so that a shade tree can develop strong and effective branch attachments that will support the canopy for the coming decades without failure. In this blog I cover maintenance of the newly planted tree including how to structurally prune young trees so that they develop strong and sustainable canopies.

As mentioned in earlier pruning blogs, trees do not require pruning. This is predicated on the assumption that trees are allowed to grow in the way they are genetically programmed to grow without damage. Unfortunately many container nurseries prune trees with a heading cut to the central leader in order to create branches that can further be pruned to make a “lollipop” canopy that mimics the form of a large tree. Consumers have become accustomed to this “in-pot” miniature version of a shade tree and nurseries are accustomed to producing them. Low branches are removed to enhance the tree lollipop shape. Nurseries often stake trees tightly to provide a way to keep them from being blown over in wind events and since all the temporary branches are removed from the low trunk they are top heavy and require rigid staking usually with a stake taped to the trunk. Tightly staked trees grow taller than unstaked trees and their trunks may lack caliper or taper (increase in trunk diameter lower on the stem). This requires that when these trees are planted out that they continue to be staked, otherwise they would fall over. This creates another burden in getting the newly planted landscape tree to survive—helping trees stand on their own.

Nursery pruning creates two kinds of branch faults that if left in the tree canopy will lead to failure later. These result from heading the main leader of the young tree. When buds grow from the pruned tree, they often produce too many branches from the same place or two branches or new leaders that are the same size. We call these faults: too many branches from one point and codominant stems respectively. If the nursery tree retains these branches and they are allowed to mature in the landscape tree, one or more branches may break loose. Almost all structural pruning seeks to correct these faults at some point in the life of a nursery-grown landscape tree. The approaches are different depending on how long the branch fault is left in the tree after planting. Branch faults of newly planted trees are best corrected in the first year–they are easy to correct in the first few years and problematic after that. This is because when poorly attached branches grow well and attain greater size over time, they will pose a problem upon removal as pruning will leave behind a substantial wound which provides an entry point for wood decay. Structural pruning is best done in the nursery or if in the landscape, in the first year after planting.

There are several goals of early pruning (1-3 years post planting):
-Retain temporary branches on the stem to assist trunk growth (but keep them pruned)
-Remove competing leaders (remove a co-dominant stem)
– Thin clusters of branches (fix the all branches from one point fault)
-Leave the first permanent branch unpruned
-Subordinate all other branches to “temporary” status by heading them back
– Leave unpruned branches along the stem that will take a permanent place in the crown of the tree.
-Leave enough space between permanent branches to support their sustained growth over the life of a tree
-Permanent branches should be spaced vertically and helically around the main or central leader

Most trees will do all of this without any pruning if they are unpruned from the seedling stage. They will shade out their temporary branches and permanent large branches will form strong attachments and uniform spacings. Heading cuts on young trees destroy their form and this should be avoided. In the next blog I will cover pruning young to mature trees.

Do you wish you had a crystal ball that could tell you what the climate will be next year when you plan your garden? So do many other gardeners (and climatologists). But while there is no magic answer, we do know that in many parts of the United States and other countries, year-to-year climate variability is strongly dominated by what is going on in the eastern tropical Pacific Ocean. This is through a phenomenon called “El Niño Southern Oscillation” or ENSO for short.

What is ENSO and how does it affect climate?

ENSO has three phases—a cold phase with unusually cold water in the equatorial Eastern Pacific Ocean (EPO) called “La Niña”, a warm phase with unusually warm water in the EPO, and the neutral phase that occurs between the two extreme phases. The ocean see-saws back and forth between the two opposite phases on a semi-regular pattern that usually lasts between two and five years from one El Niño to the next. Sometimes you can have two La Niña years (or even three) back-to-back (the end of 2021 is expected to be a second La Niña in a row), but you almost never have two consecutive years of El Niño.

In many parts of the world, the phase of the ENSO is highly correlated with the climate. Scientists can use that relationship to predict what the climate might be like in the coming months. That is helpful for gardeners who need to know what to expect both next season and next year for planning purposes. Not all parts of the world have a climate that is well correlated with ENSO, however, and so folks in those areas will have to depend on other methods to look ahead to next growing season. Winter has the best correlation between ENSO phase and climate, while summer is much less predictable. And every El Niño and La Niña is distinct, leading to variations from the statistical pattern we expect.

How does the temperature of the tropical Pacific Ocean affect climate in other parts of the world?

You might think that unusually warm or cold water in the equatorial Pacific Ocean would not have much impact in other parts of the world because of the distances involved, but it does. Since the atmosphere flows like a river, putting unusually warm water (El Niño) into the EPO acts like putting a rock into a stream. The flow of water (or air) shifts around the rock, changing the pattern of atmospheric winds that blow weather systems around. When we are in a warm El Niño phase, the storm track shifts south and covers the southern US, leaving the northern US warmer and drier than usual. When I lived in Wisconsin, we noted that lake ice cover in El Niño winters did not last as long as other years, which made ice fishermen like my dad unhappy. La Niña shifts the storm track in the opposite direction. Because of that, La Niña winters are colder and wetter than average in the northern US since the storm track shifts north into the Ohio River Valley and sometimes even farther. This leads to cold, damp winters in the northern US. Similar correlations, called teleconnections, are seen statistically in climate records at many places on earth.

If we know what the phase of ENSO is likely to be, that tells us what climate conditions are expected in areas where there is a teleconnection between the EPO and that region. While every El Niño and La Niña is unique, statistically they do provide guidance on what to expect in that region, and most years they are correct, although once in a while a wildcard like a Sudden Stratospheric Warming will occur and give us an occasional busted forecast, as it did in February 2021.

What do we expect this year?

Right now, we are in neutral conditions following last winter’s La Niña, but we are headed back towards another La Niña in the next couple of months (almost an 80% chance in the November through January period). That phase should last for most of the winter but is expected to return to neutral by spring.  After that, it is too far out to make a believable prediction. The Global ENSO Temperature and Precipitation Linear Regressions website provides global correlations between the ENSO phase and what kind of temperature and precipitation anomalies to expect. In it, each three-month period shows the relationship between the temperature anomaly of the EPO and other parts of the world (regression) and how strong that relationship is (correlation).

In the map below for December-February (DJF) temperature, it shows that if the EPO is unusually warm (+) in an El Niño, then the northern part of the US will also be unusually warm (+) while the southern states are cooler than normal (-). The storm track over the southern US in an El Niño year brings rain and clouds to that region, keeping conditions wet and cool due to lack of sunshine. A La Niña year is just the opposite. The strong correlation in both southern and northern states shows that it happens most of the time, but in areas with little correlation, you can’t use ENSO reliably to predict seasonal conditions. If you have a hard time interpreting these maps, the website has a tab that explains it in more detail.

The bottom line

For this coming winter, I expect warmer and drier conditions than usual in the southern tier of US states as the storm track shifts north. That means more overwintering of insect pests and diseases; an early start to the growing season is also likely. The northern US is expected to see colder and wetter conditions than usual, which means a later start to the 2022 growing season but less chance of drought next year, although fungal diseases could be bad if the damp conditions continue into spring and summer. Western Europe could see warmer conditions than usual but the correlation is weak so that is not a strong forecast. Australia is likely to be colder than normal, with a fairly high probability because the correlations are high, at least near the coasts. This should last until spring, when the La Niña ends, and we swing back into neutral conditions when other climate factors become more important. In the Southeast, the summer after a La Niña ends is also a hot and dry summer due to the lack of recharging rain over the winter, so I think we have the potential for drought in the Southeast next summer.

As we head into Fall garden routines and leaves start to turn color, the smell and feel of the Fall weather is in the air. Winter is just around the corner and with those horticultural routines comes the urge to prune stuff . Both fruit producing and shade producing trees often get a hair cut during fall and winter months, herbaceous perennials are often cut back in the fall after bloom and before their winter rest so it seems a good time to blog about pruning before you get the urge!   After years of pruning demonstrations for Master Gardeners and the public I have noted a common thread in how gardeners think about pruning. Pruning is a mysterious process. How we take that tangled mess of a plant (tree) and fix it? What do we prune? And the less frequently asked question: What do we not prune? To add confusion, some plants such as roses seem to have their own pruning “culture”.  In this blog post I will cover the basic principles that apply to pruning all plants and then expand into specifics in upcoming blogs.

Plants don’t want to be pruned!
The first point is that no plant wants to be pruned. Gardeners prune plants because they think it is necessary for horticultural, aesthetic or safety purposes. Gardeners should temper their pruning by understanding plant responses that result from pruning. Generally plants don’t respond much when dead portions are pruned away. In some cases removing large dead portions of a plant will allow more light to enter and some response can occur, e.g., damage to portions of a plant not used to such sunlight intensity. So even a dead plant part may be doing something that you don’t understand. Also dead wood or dead plant parts may be part of some other organism’s home. Owls and other birds nest in cavities, some kinds of bumble bees will reproduce in old flower stalks of desert plants such as Nolina, etc. So dead plant parts are not always useless. If you are pretty sure that nobody else is using dead material go ahead and remove it if bothers your garden aesthetic.

There are two physiological responses to pruning

The two principles of pruning can be used to train plants and in the case of trees, produce a strong architecture that will not easily fail (drop branches). To achieve pruning goals two kinds of pruning cuts are used: the heading cut and the thinning cut. Heading cuts are often made in the middle of stems and do not have a branch that can take over the terminal role of the removed portion. Heading cuts are often used to reduce size or volume of plants. Thinning cuts remove branches at their origin. If thinning cuts are not too large and don’t allow excessive light into a canopy the plant will not respond by invigorating buds. Thinning cuts are used to maintain the natural form of a plant but can be overdone. Over-thinning results in plants that have so much light now entering that buds are invigorated and new shoot form in overwhelming and unnatural locations just as when heading cuts are made. Excessive use of thinning cuts can also produce trees that are “lion-tailed” where all the leaves occur at the end of Pom Pom branches. Remember from a tree or shrub point of view they don’t need or want to be pruned.

Back to plant responses. There are two responses that most plants have to pruning. When living portions are pruned the remaining portions are then invigorated. This implies that dormant or “latent” buds will grow that would not ordinarily grow so plants will produce flowers or foliage in new places. In this way we can re-direct the growth of plants to achieve pruning goals we may have. This is how we can pleach a tree to grow flat along a wall or produce topiary shapes with shrubs. These kinds of pruning that dramatically alter form of a plant will require successive and significant pruning to maintain the altered form or shape. Not all plants can tolerate this and even those that do can be subject to sunburn or other processes that cause them injury. The second common response to pruning is that the more a plant is pruned, the less it will grow—pruning is a growth reducing practice. Even though buds are invigorated through pruning they can’t make up for the lost leaves and buds taken away without utilizing stored energy. The overall effect of having leaves removed is to slow the growth of the entire plant. Pruning when used as high art results in Bonsai plants that are really stunted individuals with highly stylized forms.

Pruning devigorates plants

Since pruning removes leaves and buds (which make more leaves) it is a devigorating process. You are taking away a plant’s ability to harvest light energy and convert carbon dioxide and water to sugar. All this happens in leaves. The fewer leaves a plant has the less sugar it can accumulate and then the less work it can do in terms of growing. On old or slow growing plants pruning removes energy needed for growth and also the energy needed to make secondary metabolites or chemicals which fight insect and pathogen attacks. This is why old trees pruned hard often died not soon after or become susceptible to pathogens they may have been able to fight before the pruning happened. Whenever you prune something think about how you are taking away photosynthate and what it might mean to the plant.

Fruit Trees and Roses

We have to prune fruit trees to make them fruitful? NO. Fruit trees produce lots of fruit when they are not pruned. The goal of pruning fruit trees is to modify trees so fruit is:
• in an easy to pick location,
• so there is less of it
• and so the fruit that forms is of higher quality.
An unpruned tree will make the most fruit but it may not be the quality or size you desire or where you want it in terms of picking height.

The same goes for roses. There are many pruning schemes for roses, but the most flowers will be found on the least pruned shrubs. Flower size is mostly determined by genetics. Shrubs that are severely pruned will have fewer flowers than their unpruned counterparts.

Pruning and Disease

Pruning to remove diseased parts is often cited as a common garden practice. With some diseases like cankers and blights it is a good idea to prune out infected portions before they make spores or other inoculum to further infect the rest of the plant. In most cases it is important to prune well beyond the diseased portion so all of an infection is removed. Some diseases are “systemic” such as wilt diseases and while pruning will remove a dying portion it will not rid the plant of the infection. It is always best to identify the cause of disease even before pruning it from the plant.  As we will learn in an upcoming blog I rarely recommend sterilizing your pruning equipment with disinfectants.  A stiff brush and water is all that is needed when removing most diseased plant parts.

Pruning is a useful tool for gardeners. To get the most from the practice it should be conducted with knowledge of the effects it will have on the plant that is being pruned. This is quite variable and in some cases pruning is really contraindicated. While some plants like herbaceous perennials will be pruned to the ground either by the gardener or by frost, others maintain above ground architecture and pruning choices make permanent impact to many woody plants. In the next blog I will write about pruning young trees to create strong structure.

References:

Downer, J., Uchida, J.Y., Elliot, M., and D.R. Hodel. 2009. Lethal palm diseases common in the United States. HortTechnology: 19:710-716.

Downer, A.J., A.D. Howell, and J. Karlik. 2015. Effect of pruning on eight landscape rose cultivars grown outdoors Acta Horticulturae 1064:253-255

Chalker-Scott, L. and A. J. Downer. 2018. Garden myth busting for Extension Educators: Reviewing the Literature on Landscape Trees. J. of the NACAA 11(2). https://www.nacaa.com/journal/index.php?jid=885