Top Ten Trees and Shrubs with Great Fall Color

 

Sugar maple (Acer saccharum) in full fall color
Sugar maple (Acer saccharum) in full fall color

As promised in my Sept. 9 post of “The Science Behind Fall Color”, I would address trees and shrubs with outstanding fall color. It was hard limiting it to only ten trees and ten shrubs, so I cheated a bit and grouped the maples, oaks, etc. into one group so that my list was not entirely all maples.

'Robin Hill' apple serviceberry (Amelanchier x grandiflora 'Robin Hill')
‘Robin Hill’ apple serviceberry (Amelanchier x grandiflora ‘Robin Hill’)

I have seen the below plants with reliable fall color in northern, southern and eastern landscapes. These plants “light” up the landscape in autumn. For outstanding, long lasting autumn color, plant the below trees and shrubs with herbaceous plants which bloom in fall such as asters, mums, sedums, monkshood, toad lilies, and Japanese anemones. Do not forget ornamental grasses with their showy seed heads extending the season of color and texture.

Sweet birch, cherry birch (Betula lenta)
Sweet birch, cherry birch (Betula lenta)

We used to recommend ash for fall color, but not any more due to emerald ash borer. Japanese barberry and burningbush are tops for fall color, but both species are highly invasive and not recommended. There are more plants with great fall color than the ones below. I would love to hear your favorites!

 

Top 10 Trees for Fall Color

1) Black gum, sour gum, tupelo (Nyssa sylvatica), orange-red, scarlet to purple, outstanding

Black gum, sour gum, tupelo (Nyssa sylvatica)
Black gum, sour gum, tupelo (Nyssa sylvatica)

2) Maples, especially:

Sugar maple (Acer saccharum), bright yellow to orange-red

Red maple (A. rubrum), yellow, orange-red to bright red

Freeman maple (A. × freemanii), yellow, orange-red, red to reddish-purple

Paperbark maple (A. griseum), dark red to bronze

Japanese maple (A. palmatum), orange, red to purplish-red

Korean maple (A. pseudosieboldianum), deep orange to reddish-purple

Three-flower maple (A. triflorum), orange

Full moon maple (A. japonicum), yellow-orange to scarlet-red

Moosewood, striped-bark maple (A. pensylvanicum), bright yellow

3) Ginkgo (Ginkgo biloba), bright golden-yellow

4) Thornless honeylocust (Gleditsia triacanthos f. inermis), bright golden-yellow

5) Quaking aspen, trembling aspen (Populus tremuloides), bright yellow

6) Oaks, especially:

White oak (Quercus alba), dark red to wine

Red oak (Q. rubra), red to russet

Scarlet oak (Q. coccinea), red to scarlet

Black oak (Q. velutina), dark red

Scarlet oak (Quercus coccinea)
Scarlet oak (Quercus coccinea)

7) Apple serviceberry (Amelanchier × grandiflora), yellowish-orange to red

8) Buckeyes, especially:

Yellow buckeye (Aesculus flava), golden-yellow to orange

‘Autumn Splendor’ buckeye (A. × arnoldiana ‘Autumn Splendor’), deep, burgundy-red

‘Homestead’ buckeye (A. × marylandica ‘Homestead’), orange-red

9) Birch, especially:

Yellow birch (Betula alleghaniensis), bright yellow

Sweet birch, cherry birch (B. lenta), bright yellow

Paper birch, canoe birch (B. papyrifera), yellow

10) Yellowwood (Cladrastis kentukea), bright yellow to gold

 

Top 10 Shrubs for Fall Color

1) Large and dwarf fothergilla (Fothergilla major and F. gardenia), yellow-orange to red, outstanding

Large fothergilla (Fothergilla major)
Large fothergilla (Fothergilla major)

2) Common and vernal witchhazels (Hamamelis virginiana and H. vernalis), bright yellow to golden-yellow

Vernal witchhazel (Hamamelis vernalis)
Vernal witchhazel (Hamamelis vernalis)

3) Virginia sweetspire (Itea virginica), dark reddish-purple

4) Black and red chokeberries (Aronia melanocarpa and A. arbutifolia), red-orange, wine-red to purple

Black chokeberry (Aronia melanocarpa)
Black chokeberry (Aronia melanocarpa)

5) Sumacs, especially:

Shining sumac, winged sumac (Rhus copallinum), bright red to scarlet

Prairie Flame® shining sumac (Rhus copallinum 'Morton')
Prairie Flame® shining sumac (Rhus copallinum ‘Morton’)

Staghorn sumac (R. typhina), orange to scarlet

Smooth sumac (R. glabra), orange to scarlet-purple

Fragrant sumac (R. aromatica), orange, red to purple

6) Oakleaf hydrangea (Hydrangea quercifolia), reddish-orange to wine

7) ‘Tor’ birchleaf spirea (Spiraea betulifolia ‘Tor’), orange to reddish-purple

8) Viburnums, especially:

Withe-rod viburnum (Viburnum cassinoides), orange-red, crimson to purple

Blackhaw viburnum (V. prunifolium), reddish-purple

Arrowwood viburnum (V. dentatum), depends on cultivar, yellow, red to purple

American cranberrybush viburnum (V. opulus var. americanum, formerly V. trilobum), yellow to reddish-purple

Doublefile viburnum (V. plicatum f. tomentosum), wine-red

‘Wavecrest’ Siebold viburnum (V. sieboldii ‘Wavecrest’), red to burgundy

9) Cranberry cotoneaster (Cotoneaster apiculatus), deep reddish-purple

Cranberry cotoneaster (Cotoneaster apiculatus)
Cranberry cotoneaster (Cotoneaster apiculatus)

10) Virginia rose (Rosa virginiana), deep reddish-purple

 

 

Laura Jull, Ph.D.

a.k.a.: The “Lorax”

The Science Behind Fall Color

Japanese maple (Acer palmatum) in full fall color
Japanese maple (Acer palmatum) in full fall color

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.

Plant Pigments

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.

Eyestopper™ Amur corktree (Phellodendron amurense 'Longenecker'), a male cultivar with bright yellow fall color
Eyestopper™ Amur corktree (Phellodendron amurense ‘Longenecker’), a male cultivar with bright yellow fall color

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.

A cutleaf weeping Japanese maple (Acer palmatum) bright orange fall color
A cutleaf weeping Japanese maple (Acer palmatum) bright orange fall color

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.

Genetics

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!

An unknown cultivar of red maple (Acer rubrum) with uniform fall color
An unknown cultivar of red maple (Acer rubrum) with uniform fall color

Laura Jull, Ph.D.

a.k.a.: The “Lorax”

It’s the Next Best Thing Since Draft Beer: New Plants and Their Proper Evaluation

'Graham Thomas' rose
‘Graham Thomas’ rose

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.

Knockout® rose is very disease resistant with non-stop blooms, but not hardy in zones 3-5 without significant winter protection Photo credit: http://www.gardeningknowhow.com/wp-content/uploads/2011/08/knockout-rose-400x600.jpg
Knockout® rose is very disease resistant with non-stop blooms, but not hardy in zones 3-5 without significant winter protection Photo credit: http://www.gardeningknowhow.com/wp-content/uploads/2011/08/knockout-rose-400×600.jpg

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.

Chicago Botanic Garden Plant Evaluation Notes Source: chicagobotanic.org
Chicago Botanic Garden Plant Evaluation Notes
Source: chicagobotanic.org

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).

Laura Jull, Ph.D. aka The Lorax

Cool Plants for Late Summer

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.

Variegated fiveleaf aralia (Eleutherococcus sieboldianus 'Variegatus')
Variegated fiveleaf aralia (Eleutherococcus sieboldianus ‘Variegatus’)

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.

Variegated fiveleaf aralia (Eleutherococcus sieboldianus 'Variegatus')
Variegated fiveleaf aralia (Eleutherococcus sieboldianus ‘Variegatus’)

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.

NORTH WIND® maple (Acer 'IsINW')
NORTH WIND® maple (Acer ‘IsINW’)

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.

Closeup of leaves of NORTH WIND® maple (Acer 'IsINW')
Closeup of leaves of NORTH WIND® maple (Acer ‘IsINW’)

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.

Laura Jull, Ph.D. aka The Lorax

 

Why doesn’t my plant flower? Part 2

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):

Over fertilization caused leaf burn on Liriope spicata
Over fertilization caused leaf burn on creeping lilyturf (Liriope spicata)

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.

Azaleas planted in poorly drained, heavy clay soil will not live long
Azaleas planted in poorly drained, heavy clay soil will not live long
  • 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.

http://chatham.ces.ncsu.edu/2014/07/helping-your-yard-survive-drought/
Wilted hydrangea (Hydrangea spp.) source http://chatham.ces.ncsu.edu/2014/07/helping-your-yard-survive-drought/
  • 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

Winter road salt injury on eastern white pine (Pinus strobus)
Winter road salt injury on eastern white pine (Pinus strobus)
  • 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

Shoot death of TINY DANCER™ common lilac (Syringa vulgaris 'Elsdancer') from bacterial blight
Shoot death of TINY DANCER™ common lilac (Syringa vulgaris ‘Elsdancer’) from bacterial blight
Oystershell scale on 'Redwine' lilac [Syringa (Villosae Group) 'Redwine']
Oystershell scale on ‘Redwine’ lilac [Syringa (Villosae Group) ‘Redwine’]
  • 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.

Black walnut (Juglans nigra) tree roots, twigs, leaves and roots all contain juglone that can inhibit or kill nearby plants susceptible to juglone
Black walnut (Juglans nigra) tree roots, twigs, leaves, nut hulls and bark all contain juglone that can inhibit or kill nearby plants susceptible to juglone
  • 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.

Stems girdled by rabbit feeding on stems of burningbush (Euonymus alatus)
Stems girdled by rabbits feeding on stems of burningbush (Euonymus alatus)
  • 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

Laura Jull, Ph.D. aka The Lorax

Why doesn’t my plant flower? Part 1

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):

Sun loving plants will not flower properly when grown in too much shade
Sun loving plants will not flower properly when grown in too much shade

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.

Transplant shock of 'Royal Red' Norway maple (Acer platanoides 'Royal Red')
Transplant shock of ‘Royal Red’ Norway maple (Acer platanoides ‘Royal Red’)
  • 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.

Winter flower bud death on forsythia. Notice flowers at bottom of plant due to being under the snow line.
Winter flower bud death on forsythia. Notice flowers at bottom of plant due to being under the snow line.
'Sunrise' forsythia blooms reliably each year in zone 4
‘Sunrise’ forsythia blooms reliably each year in zone 4
  • 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.

'Dark Night' early flowering lilac (Syringa x hyacinthiflora 'Dark Night') has a low chilling hour requirement
‘Dark Night’ early flowering lilac (Syringa x hyacinthiflora ‘Dark Night’) has a low chilling hour requirement
  • 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.

Late frost injury to new growth on boxwood (Buxus spp.)
Late frost injury to new growth on boxwood (Buxus spp.)
  • 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.

Beautiful flowers of Beauty of Moscow common lilac (Syringa vulgaris 'Krasavitsa Moskvy')
Beautiful flowers of Beauty of Moscow common lilac (Syringa vulgaris ‘Krasavitsa Moskvy’)
  • 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.

Before renewal pruning of Chinese lilac (Syringa x chinensis). Notice larger diameter branches crowded together.
Before renewal pruning of Chinese lilac (Syringa x chinensis). Notice larger diameter branches crowded together.
After renewal pruning of Chinese lilac (Syringa x chinensis). Notice thinning of plant.
After renewal pruning of Chinese lilac (Syringa x chinensis). Notice thinning of plant.
Forsythia pruned as hedge copy
Shearing flowering shrubs into meatballs removes flower buds and destroys the natural arching habit
  • 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.

Sporadic flowering on CHINA SNOW Peking tree lilac (Syringa pekinensis 'Morton'). Many tree lilacs will flower heavily one year and sporadically the following year.
Sporadic flowering on CHINA SNOW® Peking tree lilac (Syringa pekinensis ‘Morton’). Many tree lilacs will flower heavily one year and sporadically the following 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

Stay tuned for part 2 of this article next week!

Laura Jull, Ph.D. aka The Lorax

The Heat is On!

We are in the dog days of summer! Record high temperatures across the country have been reported. Let’s face it, climate change is real and the planet is getting warmer, despite record cold temperatures in winter in some areas. With climate change, there are more occurrences of extreme temperatures as well as precipitation such as severe drought, flooding, heavier than usual snowfall, etc. So, how does climate change affect gardeners? To understand the concept, we must first understand how high temperatures affect plant growth and development.

Plant temperature tolerance is the ability of any plant to adapt to a given climate at both low and high temperatures. It is obviously important to use species that not only can survive our winter temperatures, but also our hot summers. Similar to the U.S.D.A. Plant Cold Hardiness Zone map (http://planthardiness.ars.usda.gov/PHZMWeb/), the American Horticultural Society published a plant heat zone map (http://www.ahs.org/gardening-resources/gardening-maps/heat-zone-map). The heat zones are based on the average annual days with temperatures above 86°F (30°C). For example, up in my neck of the woods in Wisconsin, we have four heat zones in the state; zones 2-5 with the cooler (lower number) heat zones in the northern part of the state and the warmer heat zones in the southern and western part of the state and in urban areas. In comparison, North Carolina has seven heat zones (2-8). California has even more heat zones due to its varying climates and elevations.

Unique microclimates can be created at a specific site. Urban areas usually are slightly warmer than rural areas due to the heat island effect created by light reflection off of buildings, heat rising from sewers, and trapping of heat between large buildings. For example, trees planted close to pavement have warmer conditions in summer, than trees planted near turf, especially in the root zone, and have greater water needs. Trees and shrubs in this environment should receive, but rarely get supplemental irrigation compared to those same plants planted in the middle of a lawn or park area with cooler root areas and less heat stress to the tree.

Street trees compete with other plants and are subject to heat and drought stress
Street trees compete with other plants and are subject to heat and drought stress

So what does this mean for temperate woody plants?

High temperature stress is important for a number of reasons. The main effect is increased water use. However, water availability is often limited, especially in urban environments, leading to a net loss of moisture within the plant. Leaves loose water quicker through the stomates via transpiration as temperatures rise. The stomates then begin to close and the cooling effect of evapotranspiration is stopped.

Other physiological processes are impacted by high temperatures such as fewer carbohydrates available for plant growth and development, generation of plant pigments (red, purple and blue pigments) and defense used in protecting plants against insects and diseases. When plants with poor heat tolerance are grown in regions that routinely experience high summer day and night temperatures and insufficient moisture, plants will use many of their stored sugars during the evening hours and during the hottest part of the day. For many temperate woody plants, optimum temperatures for photosynthesis are below 86°F (30°C). Above this temperature, net photosynthesis declines with increasing temperatures. If this continues long term, plants can die, especially young plants that do not have many stored carbohydrate reserves and are under drought stress.

'Ames' Kalm's St. Johnswort (Hypericum kalmianum 'Ames') is heat and drought tolerant
‘Ames’ Kalm’s St. Johnswort (Hypericum kalmianum ‘Ames’) is heat and drought tolerant
Black chokeberry (Aronia melanocarpa) is a three season interest, heat and drought tolerant plant
Black chokeberry (Aronia melanocarpa) is a three season interest, heat and drought tolerant plant
Purpleleaved ninebarks (Physocarpus opulifolius) are very drought and heat tolerant
Purpleleaved ninebarks (Physocarpus opulifolius) are very drought and heat tolerant

 

What can we do as gardeners?

Unfortunately, climate change is occurring at a fast rate and each decade is getting warmer and often drier than the preceding decade. Here’s what we can do as gardeners:

  • Select non-invasive, pest resistant, stress tolerant plants for your landscape
  • Non-drought or heat tolerant landscapes plants will require regular watering to sustain them
  • Use of soaker hoses or drip irrigation is better than overhead watering as the moisture is directly applied to the roots with less runoff
  • A plant is not truly drought tolerant until it has been established in the landscape for at least three years or more to allow for root growth, especially for trees
  • Some native and exotic plants can tolerate these changes, but some native species in natural areas may be lost if our climate continues to get warmer
  • Incorporate compost into the soil prior to planting new areas to improve moisture retention and aeration
  • Apply 2-4” of bark or wood chips to the top of the soil to retain soil moisture. Make sure the mulch does not touch the trunk of trees or base of shrubs. Excessive mulching (mulch volcanoes) can actually kill landscape plants by impeding air and moisture penetration and invite fungal diseases.
  • Cities need to avoid planting trees in tree pits (sidewalk cutouts), i.e. restrained planting areas in sidewalks along streets. These trees are under severe moisture, drainage and heat stress and do not live long. Planting areas that are wide and long work much better for tree health and longevity than planting in tree pits.

Laura Jull, Ph.D.