Trees and shrubs – Page 3 – The Garden Professors™

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

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.

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.

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.

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.

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