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

Dogwood rescue – an update

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).

The Lazarus dogwood
The Lazarus dogwood
A "grateful to be alive" floral display
A “grateful to be alive” floral display

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…)

www.soil-net.com
www.soil-net.com

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.

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.

When Size Does Matter: Dwarf Conifers for the Home Landscape

I just returned from another great “Addicted Confer Syndrome” conference. In reality, ACS stands for the American Conifer Society. The meeting I attended was the Central Region chapter of the ACS held in Green Bay, Wisconsin. You might be thinking that only white spruce and tamarack are the only conifers that can be grown this far north, but you would be wrong. There are many outstanding conifers that can grow up here and throughout the U.S. Not all conifers are evergreen as there are deciduous conifers, like larch and baldcypress, but most dwarf conifers are evergreen.

Dwarf conifer garden near Green Bay, WI
Dwarf conifer garden near Green Bay, WI

According to the American Conifer Society (www.conifersociety.org/conifers/conifer-sizes), dwarf conifers are those that grow between 1-6” per year with an approximate size after 10 years between 1-6’. In contrast, large evergreens grow over a foot a year and are 15’ tall or more after 10 years. Size can vary due to climatic, environmental and cultural conditions. These smaller than usual evergreens are a fraction of the size of their species and fit nicely into the landscape often requiring very little pruning or shaping. Dwarf conifers can provide food and shelter for birds and other small mammals as well as year round interest due to their bright colors and interesting form and texture. An otherwise bleak, winter landscape can be accented with dwarf conifers that come in a variety of colors besides green such as blue, blue-green, silvery-blue, yellow, and purplish.

Below are a few of my favorite dwarf conifers that are available at many garden centers and nurseries.

'Silberlocke' Korean fir
‘Silberlocke’ Korean fir
Foliage of 'Silberlocke' Korean fir
Foliage of ‘Silberlocke’ Korean fir

‘Silberlocke’ Korean fir (Abies koreana ‘Silberlocke’, a.k.a. ‘Horstmann’s Silberlocke’) is a unique dwarf conifer that looks spectacular all year round. The soft needles are different than most conifers as they curve upwards, revealing the bright, silvery-white, frosty undersides. The silvery-gray twigs also add to the plant’s interest. ‘Silberlocke’ Korean fir grows slowly up to 5-7’ in height with a 4-5’ spread eventually growing into a small, compact, conical tree. Firs, in general, require a sandy-loam, moist, well-drained soil and are intolerant to heavy, poorly-drained, clay soils. This cultivar prefers morning sun, but some afternoon shade. ‘Silberlocke’ Korean fir is hardy to zone 4b.

'Blue Shag' white pine
‘Blue Shag’ white pine

‘Blue Shag’ eastern white pine (Pinus strobus ‘Blue Shag’) is a dwarf conifer shrub with a compact, rounded form that reaches 3-6’ tall with a 6’ spread. The bluish-green, finely textured needles are very soft and pliable. ‘Blue Shag’ has a slow growth rate and a dense, mounded form making it a great choice for use as a foundation plant instead of the all-too-common yews (Taxus spp.). Like all cultivars of eastern white pine, it grows best in a sandy-loam, slightly acidic to neutral soil. It is sensitive to drought, heavy-clay, poorly drained soil, and road salt. ‘Blue Shag’ eastern white pine is hardy to zone 3a.

'Bergman' Japanese white pine
‘Bergman’ Japanese white pine
Foliage of 'Bergman' Japanese white pine
Foliage of ‘Bergman’ Japanese white pine

‘Bergman’ Japanese white pine (Pinus parviflora ‘Bergman’) is an outstanding, dwarf conifer that forms a dense, compact, wide, rounded to upright shrub. ‘Bergman’ Japanese white pine is a slow grower eventually forming a 4-6’ tall with a 6’+ spread shrub. The blue-green needles are soft, long and twisted. In spring, the immature cones are bright carmine-red contrasting dramatically with the blue-green needles. It is hardy to zone 5a and is adaptable to most, well-drained soils and pH. Unlike many other five-needled pines, Japanese white pine is road salt tolerant.

'Gold Drop' arborvitae
‘Gold Drop’ arborvitae

‘Gold Drop’ eastern arborvitae (Thuja occidentalis ‘Gold Drop’) adds bright color to the landscape. This dwarf conifer shrub grows 4-5’ tall and 3-4’ wide and is shaped like a teardrop; narrow at the top, wider at the base. The soft, aromatic foliage is bright golden yellow when grown in full sun turning a deeper yellow during winter. ‘Gold Drop’ arborvitae is hardy to zone 3b and is adaptable to most soils and pH, but grows best in moist, well-drained, loamy soil. If grown in shade, the golden colored foliage will turn green.

Even though dwarf conifers are often more expensive than other deciduous shrubs, they are well worth it. They have a slow growth rate, require little maintenance and provide year-round color and texture in the landscape.

Laura Jull

There is a Cure for the Summertime Blues

Like some television commercials say about their product, “But wait, there’s more”, this statement can also be said about flowering shrubs. Just because spring is over, it does not mean there is no more color in the garden. Yes, there are herbaceous perennials that bloom in summer, but there are some fabulous flowering shrubs that also shine during the dog days of summer besides roses and Japanese spirea. Here are three of my favorite larger shrubs with big landscape impact.

Roses ('Graham Thomas') are not the only shrub that blooms in summer
Roses (‘Graham Thomas’) are not the only shrubs that blooms in summer

Bottlebrush buckeye (Aesculus parviflora) produces flowers in the northern U.S. in early July. The white flowers are especially unique and are borne on large, 8-12” long panicles rising far above the leaves. The flowers resemble a large bottlebrush and are often home to visiting butterflies and bees.

Bottlebrush-like flowers
Bottlebrush-like flowers
Bottlebrush buckeye in full bloom
Bottlebrush buckeye in full bloom

In autumn, the foliage turns bright yellow early in the season. This large, spreading, suckering shrub needs a lot of space to grow reaching 8-12′ tall and 12-15’ wide at maturity. It is native to the southeastern U.S. and performs best in rich, moist, well-drained soil, but is adaptable to most soils and pH. Bottlebrush buckeye grows in shade to partial shade and out of the hot afternoon sun. Unlike its tree relatives, this Aesculus species is not susceptible to powdery mildew, leaf blotch or leaf scorch and is hardy to zone 4b.

 ‘Nordine’ smokebush (Cotinus coggygria ‘Nordine’) is another large, wide-spreading, upright shrub that grows 10-15′ tall and wide. It produces purplish-red new leaves throughout the growing season that later turn dark bluish-green. When crushed, these leaves smell like a combination of radishes and oranges.

'Nordine' smokebush in full bloom
‘Nordine’ smokebush in full bloom

The individual flowers are not particularly attractive, however, that quickly changes. The hairs on the individual stems of the 6-8” long flower panicle elongate and turn a wonderful smoky pink to purple color that lasts for many months. The orange-yellow to purple fall color is equally appealing.

Hairy flower panicles of 'Nordine' smokebush
Hairy flower panicles of ‘Nordine’ smokebush

The species is native to southern Europe over to central China and the Himalayas. This particular cultivar of smokebush is hardier than other purple-leaved cultivars of smokebush and should reliably flower in zone 4b each year. It is able to withstand most soils and pH, full sun, drought, and urban conditions. Deer don’t seem to bother this plant. Maybe the smoky appearance of the flowers scares them away?

‘Limelight’ panicle hydrangea (Hydrangea paniculata ‘Limelight’) is one of the best flowering cultivars of panicle hydrangea. The flower panicles are huge, up to 6-10” long starting lime-green and eventually turning all white. As the flowers fade, they turn pinkish to brown from fall through winter providing multi-seasonal interest. The added benefit of this cultivar is that the flowers are borne on strong stems that are held upright, so flowers will not flop over like some other panicle hydrangea cultivars.

Green flowers of 'Limelight' hydrangea before they turn white
Green flowers of ‘Limelight’ hydrangea before they turn white

‘Limelight’ hydrangea is smaller than the species growing 6-8’ tall and 5-6’ wide at maturity with an upright, spreading form. The species is native to Japan and China and hardy to zone 4a. Panicle hydrangea does well in an organic, fertile soil, but is quite adaptable to soil and pH. It requires full sun for the best flower display and moist, well-drained soil, but it is not as finicky about soil moisture as some of the other species of Hydrangea.

'Limelight' panicle hydrangea in full bloom
‘Limelight’ panicle hydrangea in full bloom

When designing a landscape, incorporate a variety of trees, shrubs and perennials to insure continual bloom throughout most of the growing season. These three, non-invasive shrubs are all available at most nurseries and garden centers. If you have not tried one of these beauties yet, plant one or all three in your yard. You will be happy you did.

–Laura Jull, a.k.a. The Lorax