Our local utility company has been busy butchering trees around the power lines.
Every plant person I know complains about this, but I honestly don’t think there is much hope for a change. Power companies don’t want limbs falling on the electric cables during storms, and they’re not likely to start spending money to hire real arborists to do the pruning.
What I really wish is that people would start thinking a little more before putting in a tall tree directly under electric lines. I’m sure these went in as cute little babies, I know it can be hard to visualize what a small tree will grow into, but we do really need to do a better job of it. If you are looking to plant, take the time to look up the tree in question and see how fast it is going to grow. Google will usually tell you, and if you are planing conifers, the American Conifer Society has an amazing website which will tell you the growth rates in inches per year of just about any conifer you can imagine. Check it out, and do the math, and see just how fast that little spruce is going to be causing problems before you start digging holes.
One of the great things about doing a multi-author science blog is that there will be topics about which colleagues will disagree. One of those topics revolves around the best way to prepare woody rooted plants (trees and shrubs) before planting them. This is an area in arboricultural science that is evolving. A search through our blog archives will find many of these posts and for convenience’s sake I’ve linked one from each of us here.
Rather than belabor the points that Jeff, Bert and I have already made in our posts, I think I can sum up our major difference here: I like to bare-root trees and shrubs completely before planting (so I can correctively prune all flawed roots) while Bert and Jeff prefer a less invasive approach. What we do agree upon, however, is the deplorable condition of the roots of many trees and shrubs that end up in the nursery. Because I do practice bare-rooting trees, I thought I’d use today’s post as a rogue’s gallery of trees that should never have made it to the retail nursery. (All of these trees were ones that I bare-rooted and root-pruned myself before planting – and all are thriving.)
In many parts of the U.S., particularly the northern U.S., we are blessed each year by nature’s display of bright color dotted through the landscape. Fall color of leaves at the end of the growing season provides a remarkable encore in the landscape. There are many trees and shrubs with great autumn leaf color and I will address some of them in my next week’s post, but this week, I will talk about what actually happens inside the plant during autumn.
As the days get shorter and temperatures start to cool, particularly at night, the season changes from green leaves into a kaleidoscope of yellow, orange, red, purple and bronze shades. There are a variety of factors that interact and play a role in determining how colorful the display will be. Plant leaves contain several pigments that determine the color that will appear and variations arise when different concentrations of pigments are combined in the leaf.
Chlorophyll is the green pigment in leaves. This critically important pigment captures the energy from the sun and uses it to change water and carbon dioxide (CO2) into oxygen and sugars (carbohydrates), i.e. the plant’s energy source for growth and development. In autumn, chlorophyll breaks down faster than it is produced, revealing the other plant pigments and their colors.
Carotenoids are responsible for the yellow, orange and a few red pigment colors. This pigment is always present in the leaves during the growing season, but the colors become more evident as the chlorophyll breaks down in the leaf. In addition to providing us with a beautiful display, carotenoids protect leaves from harmful byproducts of photosynthesis. Since carotenoids are always present in leaves, yellow, gold and orange colors are least affected by the weather.
Anthocyanins are responsible for most of the red, pink, and purple colors we long for in autumn. Sugars in leaves accumulate as active growth slows down in autumn permitting the production of anthocyanins. These colorful pigments act as an internal sunscreen to protect the photosynthetic system allowing plants to recover nutrients from the leaves more easily as the temperatures decline. What about plants that have no anthocyanin pigments? These plants are usually more resistant to damage from bright light so they have no need to produce these protective anthocyanin pigments.
Tannins are not considered an actual plant pigment, but are responsible for some of the tan and brown colors we see in oaks and beeches in the fall.
Why Do Leaves Change Color?
During the summer, most of a plant’s nutrients are located within the leaves. The shortening of day length and cooler temperatures, particularly at night, signal the plant to begin preparing for winter by transporting carbohydrates (sugars) and mineral nutrients from the leaves to stems and roots for storage in the plant to be reused the following spring. A layer of cells at the base of the leaf stalk (petiole), called the abscission zone, gradually closes off the flow of sugars and minerals into and out of the leaf. In a process called senescence, chlorophyll breaks down causing the leaves to change color and eventually fall off the plant.
Variability of Fall Color and the Role of Weather
Many factors play a role in determining when fall color occurs and the intensity of the color. We cannot predict each summer how the autumn leaf color will be in the landscape. For example, peak (best) fall color can shift by as much as two weeks ahead or behind the normal time peak color occurs year to year based on the weather.
Plant Health and Moisture Levels
Plants that are in transplant-shock (newly planted), drought stressed, nutrient deficient or suffer from insects or diseases may have poor fall color or the exact opposite; they may have better fall color due to increased production of red pigments. Good soil moisture levels throughout the growing season followed by a dry fall can improve the intensity of fall color. On the other hand, excessively wet or drought conditions can cause poor color development. Drought conditions can cause leaves to dry, curl and drop before fall color has sufficiently developed, especially on newly planted material. However, moderate drought conditions may actually improve fall color development in some species, though these same plants may suffer during winter and have dieback apparent in spring.
Temperature, Light Levels and Mineral Nutrition
Cool, dry, sunny fall days with cooler night temperatures stimulate anthocyanin production resulting in bright reds and purples. In contrast, very warm autumn weather may reduce the production of these pigments. During unusually warm autumns, plants may accelerate fall color development, shortening the time leaves remain on the tree or shrub. Early, hard frosts may also severely damage leaves, arresting further fall color development before the brightest colors are revealed. Sufficient sunlight is required for leaves to produce the best coloration. Plants growing in dense shade will usually fail to develop the intense red and orange colors we have grown so fond of each autumn. High soil pH and deficiencies of the various essential mineral nutrients, such as nitrogen, phosphorus, magnesium, iron and high levels of the non-essential element sodium can all affect the intensity of color change in leaves. High sodium soil levels, most likely due to excessive road salt (NaCl) application the previous winter, not only stresses plants and prevents proper water uptake, but also can negatively affect autumn leaf color.
Of course genetics play a key role in the intensity of fall color development. This is why some seed produced trees in the forest have great fall color, while seedlings from the same tree may have less intense or even no fall color. This is where plant cultivars come into play. A nursery person will select plants demonstrating superior fall color, improved cold hardiness, increased pest resistance, better growth form, etc. These superior plant choices will often have a cultivar name associated with it. For example, seedling grown red maple (Acer rubrum) will display a range of fall color from green, yellow, yellow-orange, orange, orange-red, red and purple. As gardener, we tend to choose what we like, mostly the orange, red to purple colors. If the plant is not already in commerce, the grower will select the best seedlings for fall color and vegetatively propagate and grow those particular seedlings, offering these new selections to other nurseries or garden centers. It is a win-win for both the nursery and the gardener!
The end of August brought an unseasonable rain- and windstorm to the Puget Sound region. We had some spectacular tree failures which I missed seeing as I was out of town. But one of our Facebook group members, Grace Hensley, was on the ball and took some great photos of a fallen purple-leafed plum. The first thing you see is the complete lack of a stabilizing root system.
Now look at the base of the trunk, which is actually a massive circling root that has girdled the trunk over time.
By now you must be able to see the orange twine extending from the base of the tree to the soil. Yes, those are the remains of the balled-and-burlapped clay root ball that was planted many years ago. Commercial landscapers will assure you that tree roots can grow through the burlap and establish. And this is sometimes true, as in this case.
But what doesn’t happen when the whole B&B mass is plopped into the ground is that circling woody roots aren’t discovered and corrected. Over the decades what started as a small circling root grew bigger and bigger, slowly squeezing the trunk and preventing it from developing girth at that point. It’s kind of like a blood pressure cuff being pressurized but never released.
In time, the constricted point becomes so unstable that the tree breaks. Look are how small the trunk that’s still in the ground is compared to the trunk of the tree itself. Windstorms are often the final push these failing trees need.
Commercial landscapers say it’s too costly to remove the twine and burlap and clay surrounding the roots, not to mention doing any of the corrective root pruning that might be needed. It’s easier to just plant the whole thing and cross your fingers that the tree lives past the warranty date. This is what happens when you consider a tree as just another design element rather than a living organism.
As a homeowner, however, you can insist that your trees are planted correctly (if you have someone else do the work). Or you can do it yourself. The bare-root method (sometimes called root washing) is an emerging science and it requires thoughtfulness, but it’s certainly better than the conventional approach in terms of long term tree health.
With many new nursery catalogs arriving in my mailbox at work for 2016 introductions, I thought I would focus this blog on “new” plants. With all the publicity and marketing that goes on for new plant introductions, you would think that they are the next best thing since draft beer or even bread! I am a bit cynical and question whether these new plants really live up to their performance expectations and ornamental attributes. With so many new hydrangeas, coneflowers, coralbells, spireas, etc. released each year, you may ask why am I so cynical? Why would I not jump on the bandwagon and promote all of these new plants like so many garden centers are doing across America? Let me explain.
A decade ago, I conducted research trials evaluating 20 new or recently introduced cultivars of “hardy” shrub roses, many of which are not even on the market anymore. I chose three locations in the state of Wisconsin, each having their own unique soil types, pH, soil drainage and fertility, rain/snowfall and cold hardiness zones. I replicated each of the 20 cultivars ten times at each location and arranged them into blocks with each cultivar represented in each of the ten blocks. The roses were randomly selected for each block and planted, mulched, watered with an application of a slow-release fertilizer. Plants were watered for the first year only as needed. To properly analyze plants for various traits, I allowed the roses to establish for a year with evaluation initiated the following spring. The only care the roses received the remaining years were application of a slow-release fertilizer, weeding and pruning of dead wood following winter. I was trying to replicate conditions that are common in most landscape settings. I did not spray any insecticides or fungicides to any of the roses, regardless of how bad they may have looked due to pests.
After the first winter, I evaluated the roses for winter injury, which they all experienced. The roses were all on their own root systems so if they died back significantly, the new growth would come from the same root system and produce flowers that spring. To some extent, they all grew, though voles killed some of the roses. After the roses starting growing, I evaluated them monthly at all three locations for insects and diseases as well as flowering (amount, size, duration of bloom, etc.). I also measured the plant’s height and spread. A few roses had good fall color. The first year of the trial, the roses all bloomed prolifically. So, one would think that all 20 cultivars are ideal. Not so fast, or “but wait, there’s more” as the television salesman would say to viewers in TV land about a new product. The “real” evaluation started in year two.
In year two, amount of dieback and winter survivability was recorded. To my surprise, the roses in the zone 3 location (boy, that’s cold) had better winter survival than the roses in my zone 4 and 5 location! This is due to consistent and significant snowfall in the most northern location compared to sporadic snowfall and lower amounts in the other two locations. I also evaluated the roses during the summer and fall for flowering, pests, and hip production. Contrary to the catalogs, many of the roses had hips, but some of them never colored up before the cold temperatures arrived at the three locations. Flower production was cited as being continuous all summer by their introducers, however, this was not true for some of the cultivars evaluated. Disease resistance was the most alarming quality I evaluated with many of the so-called “disease resistant” roses being the exact opposite. I explain all these variables to demonstrate what is involved in proper plant evaluation. For a complete report of my rose research trial, see: Jull, L.G. 2004. Hardy Shrub Rose Research Trials. Combined Proceedings of the International Plant Propagators Society vol. 54:429-434.
Now, you may ask, “Why are these new plants, including roses, promoted by these large nurseries as being the best plant around when in effect, they are not?” Many new introductions are from nurseries that trial their plants in their location only. So a plant that performs well in the state of California might not perform the same in Michigan and vice versa. There isn’t the scientific rigor applied to these new plant evaluations that would occur by non-biased, university researchers who have no stake in selling or promoting plants to the public. This is where the beauty of applied, scientific, university-based studies can play a huge role.
Also, these new plants should be evaluated over numerous years, at various locations/soil types, climates, with appropriate replications of each new plant in a random arrangement (not all planted together). This type of quality research is done by a few large nurseries but it is seldom done this extensively by others anymore as demand for new plants is never satisfied and the cost of trialing over several years and locations is too costly.
Unfortunately with increased costs and significant budget/program cuts, most university research is now geared toward larger, basic science studies that have high indirect costs built into the grants. These funds, usually 50% or more of the grant total, go directly to the university to cover overhead. The researchers do not see or can use overhead funds. Ornamental plant evaluation research is now considered either non-fundable by granting agencies, not “scientific or scholarly” enough by their own departmental colleagues or provide significant overhead funds back to the university.
Some researchers rely on their various nursery and landscape associations for small amounts of research support, while others try to piece meal together small amounts of research funds. With the increasing costs of land (yes, we do have to pay for research space at university research stations), plants (not all are freely given to the researchers), labor, supplies, etc., it is becoming critically important to seek alternative funding sources as most federal and state granting agencies do not fund ornamental plant evaluation research. Many of the new initiatives for federal grants seek to fund food crop based research, especially in organic and sustainable food production. Applied ornamental horticulture plant evaluation research at universities has plummeted with most new plant evaluations conducted by the large nurseries that introduce these plants.
There is another source for evaluation of these new plants. Various arboreta and botanical gardens around the U.S. are conducting evaluation trials. I am a fan of these studies as these gardens and evaluators are also not in the business of selling plants and can provide some analysis, though it is usually only at one location. Richard Hawke, Chicago Botanic Gardens Plant Evaluator and Horticulturist, has done an excellent job of evaluating many species of herbaceous perennials and a few woody plants. He publishes Plant Evaluation Notes: (http://www.chicagobotanic.org/research/ornamental_plant_research/plant_evaluation), a series of wonderful publications that help both the amateur and professional gardener to choose appropriate plants for the Upper Midwest. There are other botanical gardens and arboreta that do the same, with evaluations based on their local climatic conditions. I often rely on Mr. Hawke’s recommendations when choosing herbaceous perennials in my Wisconsin garden and have yet to be disappointed.
So the next time a new plant comes across your way, think twice before buying it. There is the philosophy “Buyer Beware”, and I do recommend people to buy plants, but instead of buying 10 of one cultivar, try one or two of the new plant and make a judgment call the following year or two after you planted it. This is especially important for landscapers who design and plant large amounts of plants. You might be surprised to see the “best thing since draft beer” plant being anything but that. As some of us know, there is nothing better than draft beer (or whatever beverage you really like).
I spent last week in Orlando at the ISA annual meeting (that’s the International Society for Arboriculture). It’s a great venue for networking with colleagues and hearing about the latest tree research. And once in a while I’ll have a WTF moment. (That stands for Why Trees Fail in case you’re wondering.)
My WTF experience this year revolved around some new terminology and techniques. I learned there are now “environmental arborists” who practice “retrenchment pruning.” In the last few days I’ve tried mightily to find some standard definitions from reputable sources. I don’t know what an environmental arborist is, since it’s not a certification (like an ISA certified arborist) nor is it a university degree program (like urban forestry or environmental horticulture). It seems to be a self-anointed title.
But the real WTF issue is retrenchment pruning. I looked in vain for published research through my usual data bases and found nothing – other than two articles in Arboricultural Journal (which is not the same as ISA’s journal – Arboriculture and Urban Forestry). Neither of the articles presented experimental evidence to justify this radical approach to pruning trees. Instead, they are more philosophical in nature, with a smattering of ecological theory.
Fortunately, retrenchment pruning methods are easily found on the internet, along with horrific pictures illustrating the results. As described on various websites, retrenchment pruning imitates the natural process of aging. Practitioners remove live branches or partial trunks to reduce the size of the tree and prevent future failure. These aren’t clean cuts, either: they’re “coronet cuts” or “natural fractures.” The rationale described in one of the Arboricultural Journal articles is that these jagged broken branches and trunks “promote specialist habitats and enhance colonisation rates of niche species.” In other words, this technique creates large wounds that are easily colonized by various insects and microbes.
So apparently we’re expected to ignore the well-established field of woody plant physiology (which happens to be my specialty) and related practical bodies of knowledge (e.g., formal and informal pruning techniques of said woody plants) and start hacking away at mature trees. In doing so, we’re removing live tissue and creating large wounds. This has the effect of both reducing photosynthetic potential of the tree as well as opening it up to possible pest or disease invasion. But nowhere are these possibilities discussed as part of the “natural aging process.” Nor was there mention about how to manage the epicormics shoots that result from improper pruning. And they do need to be managed.
I saw some very angry arborists at the ISA meeting who were incensed at the idea that we should deliberately malprune trees. But others seemed quite excited with this new philosophy. To paraphrase one of my plant physiology colleagues, “Give a bad arboricultural practice a catchy name and it magically becomes legitimate.”
I thought today’s post would feature two awesome plants that are relatively new or unheard of in the industry. Both of these plants have performed well in my own garden and survived our two hardest winters recorded since the 1970’s. Many plants suffered severe winterburn or even death due to extremely low temperatures, but not these two plants. They may be harder to find in the nursery/garden center, but are well worth it and have huge ornamental impact all growing season.
The first is variegated fiveleaf aralia, Eleutherococcus sieboldianus ‘Variegatus’ (formerly Acanthopanax sieboldianus ‘Variegatus’). I know, a really horrible, long scientific name for such a great plant. Now before I go on about this particular cultivar, I need to let everyone know that this is the cultivar you want, NOT the species that has all green leaves. The straight species is weedy, grows too large for most landscapes and is not colorful. The variegated cultivar is a real showstopper! It is low maintenance, grows slowly and rarely needs pruning. This medium-sized shrub grows about 5-6’ tall and wide in the northern U.S. growing larger in the south. The natural form is upright, rounded with long, arching branches. Suckering at the base of the shrub is slow, hence the plant does not become a nuisance in the garden. Variegated fiveleaf aralia is adaptable to most soils and pH, tolerates sandy and poor, dry, clay-based soils, will stay variegated in shade and will not scorch in full sun. It is quite drought tolerant with no pest problems. Deer and rabbits seem to leave it alone.
The foliage is quite clean with 5-7, bright, cream to yellow variegated leaflets with an emerald green center. There is no fall color to talk about, however, the brightly colored leaves mix well with other yellow, white or purple flowering/foliaged plants. The stems do have 1-2 curved prickles on them at a node, but they are short. Unlike its straight species, ‘Variegatus’ rarely flowers or fruits so the plant does not become invasive.
This plant is native to Japan and was highly promoted as an outstanding urban tolerant plant by my former graduate advisor, the late Dr. J.C. Raulston of North Carolina State University. He tested and evaluated thousands of landscape plants for adaptability to the southeastern U.S. and a few of those plants are actually hardy up in zone 4b.
Another great plant that I am more and more impressed with each day is a relatively new release from Iseli Nursery, Boring, Oregon. It is NORTH WIND® maple (Acer ‘IsINW’). This smaller, 15-20’ tall, ornamental maple is part of Iseli’s Jack Frost® series of hardy, ornamental maples. Through testing across the country, this maple has proven hardy to zone 4a without any dieback, unlike Japanese maple (Acer palmatum). One of the parents of this great hybrid is the Korean maple (Acer pseudosieboldianum), which is also zone 4 hardy.
The beauty of NORTH WIND® maple is that it combines the cold hardiness of Korean maple with the outstanding leaf qualities and deeper leaf lobes of the Japanese maple, one of the suspected parents, but with one exception. NORTH WIND® maple new leaves are bright orange! The color of these leaves lasts longer in the growing season in cooler climates. I have a young tree in my yard and the leaves are still orange, even this late into August. The older leaves gradually fade to green in midsummer. The orange-red fall color is superb and makes a real eyestopper in the autumn landscape. For best color, plant the tree in full sun to partial shade.
NORTH WIND® maple is pH adaptable and grows best in a moist, well-drained soil. I do not know yet how heat or drought tolerant this species is, but if anyone is growing this plant in southern climates, please let me know how it is doing.
As promised last week, here is Part 2 of “Why doesn’t my plant flower?”. If any of you know of more reasons that are not listed here or in the previous week’s blog, please let me know. Happy gardening!
There may be several reasons why a landscape plant does not flower (see last week for more reasons):
1) Over fertilization may inhibit flowering.
Do not fertilize newly planted trees and shrubs the first 2-3 years after planting
Plants need to put their energy into establishing a healthy root system to support future shoot growth
Once the plant is established (2-3 years), fertilizers may be added if a soil test suggests fertilization is needed
Over fertilization with quick-release, high nitrogen (N) fertilizers can lead to excessive leaf growth at the expense of flower bud development
Avoid using lawn fertilizers around the base of your plants, as they are often high in nitrogen
More is not better; follow all fertilizer label directions and do not add more than is required
Your landscape plants might not even need fertilization, especially if compost is added or the soil conditions are favorable for plant growth
2) Poorly-drained, heavy clay soils can result in leggy, unhealthy looking plants that may not flower or will die.
Poor drainage will stunt growth, limit flowering, and may make plants more susceptible to root and crown rot
Before planting, assess soil for drainage
Dig a hole in the soil about 12 in. (30.5 cm) deep and 8-10 in. (20.3-25.4 cm) wide
Fill the hole with water to the top and let it drain for one hour
After an hour, measure the water depth with a ruler and record the new water level from the original level
If the water level drops less than 1 in. (2.5 cm) from the original water filled level, soil drainage is slow
The following day, if water is still in the hole, your soil needs much improvement
Desired range for water drainage is 2 to 6 in. (5.1-15.2 cm) per hour
Drainage can be improved by adding organic matter such as compost, composted leaf mulch, or other organic material; works well prior to planting
If possible, plant in raised planting beds amended with compost, topsoil and other organic amendments to improve drainage and root establishment
3) Under watering can cause a lack of flower buds or poor flower development.
Water immediately after planting for all landscape plants with a hose
Water trees, shrubs and perennials several times a week for the first month or two, depending on weather
Watering may be needed daily especially if windy, dry and hot
Check actual root ball/soil directly where the plants’ roots are located; insert your finger or soil probe as far as it can go
If you feel moisture, the plant may not need additional watering
Plants previously grown in soilless media in containers and bareroot plants require more frequent watering
Do not rely solely on rainfall as the amount may be inadequate to penetrate the root ball
If you see wilting of foliage, the plant is too dry
Plant establishment will take several years or more for trees, slightly less for shrubs and one to two years for herbaceous perennials
After establishment, 1 in. (2.5 cm) of water a week is recommended; check rainfall with rain gauge in yard
Avoid overhead irrigation as this keeps the foliage too wet and may increase foliar diseases
Water at the base of the plant using soaker hoses, drip irrigation or a hose set at a very slow flow rate to allow for percolation into the soil and avoid water runoff
4) Road salt injury can damage or destroy flower and vegetative buds
Fast-moving traffic and high winds from wet, salted roads accumulate on dormant plants in winter
Sodium chloride (NaCl) is most common deicing salt used and causes the most damage
Most damage occurs within 60-100 ft. from road
Desiccates and may kill buds, twigs; can burn foliage of evergreens; reduces water uptake by roots
Also accumulates on soil surface from melted, salted water from roads
Plant salt tolerant plants near roads, use physical burlap barriers to protect plants, leach soil in spring
Use of anti-transpirants and dormant oil sprays are ineffective in prevention of salt spray injury
5) Insects or diseases may be present which can reduce or eliminate flowering
Plant is stressed and may not flower; more energy within a plant used for defense
Various bacterial pathogens can kill emerging buds and flower clusters, i.e. bacterial blight
Fungal pathogens can also do the same or reduce the size or quality of the flowers, i.e. powdery mildew, Botrytis
Certain insects, such as borers and scale, weaken a plant so it no longer flowers or flowering is reduced
Treatment for these pests needed or the plant may die
Improve plant health by watering and judicial pruning of infected/infested branches
Do not fertilize at this time as the plant is trying to recover from pests
6) Black walnut (Juglans nigra) and butternut (J. cinerea) trees can kill sensitive plants nearby.
Both of these trees and to a lesser extent hickories (Carya spp.) excrete juglone, a phytotoxic chemical produced in the leaves, twigs, bark, nut hulls, and particularly in the roots
Not all plants are sensitive to juglone, but those that are sensitive die either quickly (tomatoes, peppers) or over several years
Ornamentals sensitive to juglone include lilacs, crabapples, rhododendrons and azaleas, hydrangeas, etc.
Black walnut tree roots extend far beyond the drip line of a tree
Even if the tree is removed, roots remain in soil and continue to excrete juglone for several years
7) Deer or rabbits may be browsing on the flower buds.
During winter, deer and rabbits like to feed on the twigs, bark, and flower buds of many plants
Once the flower buds are destroyed, have to wait another year for flower bud set unless the plant flowers later in summer
Plants resistant to deer and rabbits are available, but if hungry enough, they will eat most any plant
Rabbits and voles chew on bark of shrubs and trees causing girdling of twigs or major stems/trunk
Girdled stems may die or barely flower or leaf out
Damaged branches should be pruned back to live wood
I often get horticulture questions from county extension agents, Green Industry professional, gardeners and my next door neighbors. One of the most common questions I get is why their plant did not flower that year. There are many reasons why a plant, either woody or herbaceous perennial, will not flower and both new and established plants can be affected. Some of the below reasons are obvious, some not so obvious.
There may be several reasons why a landscape plant does not flower (more to come next week):
1) Plants requiring full sun are not receiving enough sunlight.
Sun loving plants require at least six hours of direct sunlight per day to produce flower buds
Flowering is significantly reduced in areas with too much shade
Foliar diseases may be increased, such as powdery mildew, as foliage stays wet longer after rain
Growth is tall and “leggy” with most of the foliage and flowers occurring at the top of the plant
2) Transplant shock may result in little to no flowering 2-3 years after planting.
After planting, woody plants are using energy to establish a root system to support future leaves and flowers
Make sure plants are receiving enough water to encourage root growth and plant establishment
For trees, it may take longer than 3 years to produce new flowers
3) A plant may not flower because it is not cold hardy to your area.
If you live in U.S.D.A. Cold Hardiness Zone 4 and plant is rated to only zone 5, buds may be killed over winter
For some plants, like forsythia, vegetative (leaf) buds can be a half to one zone more cold hardy than the flowering buds
Make sure to select a plant and cultivar rated to your cold hardiness zone
4) In warmer cold hardiness zones a plant may not receive enough chilling hours in winter to break dormancy.
Chilling hours are the total amount of time during winter below a certain temperature, called vernalization
Required temperature are either below freezing, 0° C (32°F) or below 7°C (45°F) for temperate species
Plants suited for colder climates, like common lilac (Syringa vulgaris), require at least 3 months of cold temperatures in order to break dormancy and bloom in spring
In areas with mild winters, these plants may not flower or set fruit
Low chilling hour requiring plants are available for warmer areas
There is no definitive data on number of chilling hours required for all species
5) Severe late spring frosts can kill flower buds coming out of dormancy or emerging buds.
Developing spring buds are in advanced stages of development with minimal cold hardiness
Flowers can be killed or severely deformed
Especially damaging if the hard frost occurs after weeks of warm temperatures resulting in budbreak
Little to no fruit is produced that year; a serious problem for fruit growers
6) Pruning trees and shrubs at the wrong time of year will remove flower buds.
Flower buds are produced during the preceding summer for spring flowering plants
Prune within two weeks after flowering in spring
Avoid pruning in mid to late summer as next year’s flower buds are developing for next year’s bloom
Can prune large, suckering shrubs in dormant season, but realize flowering will be reduced that year
For summer flowering shrubs, late winter to early spring before growth begins is a great time to prune as flower buds have not developed yet for that summer
7) Flowering can decrease significantly on older, overgrown shrubs like lilacs, forsythias, chokeberries, and spirea.
Larger diameter branches have reduced flowering as the stems age, especially for lilacs
Flowers may only be at the very top of the plant out of sight and smell
Large, suckering shrubs need renewal pruning, also called thinning
Depending on the species, every 1-3 years, remove about a third of the largest diameter branches (greater than 1.5” in diameter) back to the base of the plant to allow light penetration
Regeneration of new suckering branches will occur at the base of the plant that produce new flower buds the second or third year and fill in the plant
Thinning (renewal pruning) also preserves the overall plant shape
Never shear flowering shrubs as you will be removing flower buds and ruin the plant form
Renewal pruning should only be done for shrubs that sucker
Do not attempt this type of pruning on evergreens or slow growing plants
8) A tree or shrub may be alternate bearing with heavy blooming one year and sparse flowering the next year.
Common with some trees, such as Japanese and Peking tree lilacs (Syringa reticulata and S. pekinensis), and flowering crabapples (Malus spp.)
There is little a gardener can do to avoid this from occurring
If the fruit is not ornamental, removal of old flowers before fruit set may redirect a plant’s energy into flower bud production for next year’s bloom instead of fruit production this year
Select plants that reliably flower each year if you do not want to miss the show
Long-time readers of this blog might remember a Friday quiz I gave back in 2010. It involved the slow but inexplicable decline of our dogwood (Cornus kousa). On the following Monday I revealed the reason for the decline and reported that we were moving this nearly dead tree to another location without the offending perched water table.
In 2011 I posted my first update along with photos of the new leaves and flowers. And today I reveal its obvious recovery to a fully functional if somewhat still spindly tree (several of its multiple leaders died as a result of the rotted root system).
There are several take-home lessons from this example:
1) Don’t assume that tree decline is due to a nutrient deficiency or pest/disease problems. The last thing a stressed tree needs is unnecessary additions of fertilizers or pesticides.
2) Explore soil conditions to find possible water movement disruptions. Our perched water table was discovered serendipitously with our pond installation. You can do the same with a good-sized soil auger. (I bought one of these bad boys, but haven’t had a need to use it yet. Some day…)
3) If a tree or shrub is failing, by all means move the poor thing to another location. In doing so, you may discover that the roots are still stuck in a clay ball and have not established into the native soil. Clean off all the burlap, twine and clay before replanting.
4) Be patient. If it took a while for your tree to reach its current sorry state, it will take a while for it to recover.