Ornamental plants - suitability | Orange County Master Gardener Volunteers

Soil

Many plants, including natives, require specific soil conditions that may not match those in your garden. This is especially true for homes where the entire property was affected by construction, but remember that the soil adjacent to the foundation of any home can be alkaline due to buried concrete waste. Bulldozing during home construction often inverts soil layers and brings subsoil to the top. In addition, fill dirt has an unknown composition, so test your soil before selecting plants. For plants that will be smallish at maturity, such as perennials and small–medium shrubs, you can correct any deficiencies in soil fertility, pH or composition. For large shrubs and trees, the root system will become too large for amendment to be practical, so select plants based on their ability to survive under your current soil conditions. See Soil for detailed instructions for both situations.

Climate

Plant suitability is dependent on the extent to which our climate is compatible with a plant’s natural growth cycle. This includes far more than simple cold hardiness. Gardeners in the western US use a system devised by Sunset Magazine, available for our area in a more limited form, that considers latitude (daylength cycle), elevation (sunlight intensity, night temperatures, winter cold), the influence of large bodies of water (milder, moister winters) and topography (the Appalachians act as a barrier to shield us from Arctic air) as well as summer high temperatures and winter low temperatures to assess plant suitability. Unfortunately, most plants used in our area have not been characterized with respect to their sunset zone.

In general, plants from a similar climate such as the humid areas of Japan and Korea can be better suited to our area than natives from our own state that evolved in the very different climates of our coastal plain or mountains. Because our area experiences many weather extremes, including summer heat, drought, late spring freezes, damaging ice storms, and temperature lows unaccompanied by snow, a good approach is to visit older gardens to see what has survived. If an unfamiliar plant interests you, consider it an experiment and do a bit of homework before you purchase it. Learn about the native habitat and compare it to ours, also considering the microclimates that exist in your yard for sun/shade, air temperatures, wind, soil drainage, root competition, and ‘rain shadows’ (dry areas) cast by your house, tall hedges/fences or trees. These aspects of a plant’s native environment are especially important:

seasonal precipitation patterns
sunlight intensity and daylength cycle
humidity
seasonal temperature patterns (especially growing-season night temperatures), temperature consistency and extremes
periodic events like drought, ice storms, and frosts

Cold hardiness

The USDA definition of cold hardiness is the ability to tolerate a particular average annual minimum temperature. This definition is of limited value because it does not reflect factors that often are more relevant for plant survival such as the timing or duration of a low temperature or the occurrence/frequency of low temperature extremes. In addition, cold-hardiness describes only one of the three processes required for plants to survive cold weather: acclimation, maximum cold tolerance, and de-acclimation.

Acclimation

Most plants rely on two environmental cues to trigger acclimation, or the seasonal acquisition of cold tolerance: daylength and temperature. Be aware that daylength-sensitive plants from regions much closer to or distant from the equator than NC can misread our local daylength cues and fail to enter dormancy at the appropriate time.

Maximum cold tolerance

Cold damage is usually caused by water freezing inside plant cells, causing them to burst. Plants use either or both of two strategies to prevent cold damage:

deciduous plants lose some vulnerable organs like leaves and stems
evergreen and woody plants increase the concentrations of sugars and various other solutes inside the cells of remaining above-ground plant parts, thus lowering the freezing point of cellular water
Carbohydrate storage plays an important role in the second mechanism for cold tolerance and can also be an acclimation signal. Plants native to climates with long, hot summers need similar conditions to accumulate sufficient carbohydrates for appropriate acclimation and winter survival. Similarly, inadequate water during the growing season can reduce carbohydrate storage and subsequent winter hardiness.

De-acclimation

Daylength-sensitive species from regions much closer to or distant from the equator than NC can exit dormancy inappropriately. In addition, some plants from areas with reliable, persistent snow cover, like Helleborus niger (Christmas rose), do not regulate dormancy hormonally because deep snow melts only after persistent warmth. These species often respond inappropriately to periods of temporary warmth during the cold months.

Dig deeper

Below are several articles that discuss the shortcomings of traditional climate data and explore the major factors determining plant success in greater detail.

Exploring the Complexities of Plant Hardiness
Plant Hardiness Zone Maps: The Rest of the Story
Understanding Cold Hardiness

Provenance

Plant populations vary genetically across their natural range, with local conditions selecting for particular traits. Consequently, the climate in which a source plant originated (not the climate for the nursery where it is currently grown) can play an important role in determining plant performance. For instance, a dogwood that originated in New Jersey may be more cold tolerant but less heat tolerant than one grown in Florida. Similarly, hybrids selected as superior in one climate may not be suitable for a different one. Local sources of information and local growers are often the best approach.

Genetics

Species

Species are collections of individuals that are not genetically identical and thus can vary in habit, form, flower, fall color, etc.

Cultivars

Cultivars are individuals that have been selected from a species population because they possess a desirable trait(s), then propagated vegetatively to avoid genetic variation. Cultivars are often more expensive. However, sometimes the selected trait(s) are only expressed under certain conditions or are not different enough to warrant the extra cost.

Hybrids

Hybrids are crosses between species (or sometimes plants from different genuses). Hybrids can lose the distinctiveness of the parents, but often exhibit one or more of the following advantages: better visual appeal, greater vigor, longer flowering, improved disease or pest resistance or increased tolerance of various environmental conditions. Like cultivars, hybrids can be more expensive. Whether the added expense is warranted depends on your goals and the conditions present in your yard.

Beware of hybridizer myopia. Many breeding programs focus on flower aesthetics without regard for foliage beauty or plant vigor and do not test for performance under typical homeowner conditions or in multiple geographic areas. This is a particular problem for daylilies, heucheras & heucherellas, baptisias, and echinaceas.

As an example, many of the new purple coneflower hybrids with novel colors (colors other than pink or white) perform poorly relative to the parent species, Echinacea purpurea. In the case of purple coneflower, vibrant new colors were introduced by breeding Echinacea purpurea (the target for improvement) with the yellow-flowered E. paradoxa and sometimes additional species. E. purpurea is adapted to damp prairies and thus has a fibrous root system that permits easy transplanting and tolerates clay and winter wetness. However, essentially all other Echineacea species inhabit dry, rocky, fire-prone environments and have evolved deep taproots that are difficult to transplant and that rot in clay or wet winters. Unfortunately, most hybrid roots resemble those of the non-purpurea parent(s).

Wide-leaved hybrids have a greater purpurea component and are the best choice for perennializing, which can certainly be accomplished here. However, for successful perennialization, it is critical for the hybrid to re-establish its taproot before winter. Follow these instructions to maximize the chance of success:

plant in well-drained soil (a raised bed if you have clay soil)
plant before mid-summer, preferably in early spring
cut off all flowers the first season so that stored energy goes into the root system

↑