Two Tools That Help You Garden Smarter

by Marcia Kirinus, EMGV

Gardeners often rely on seasonal cues or the calendar to determine when to perform specific gardening tasks. For example, they may plant nasturtiums on St. Patrick’s Day or set out tomato plants on Mother’s Day. However, what happens when you move to a new climatic zone and your gardening timing is thrown off? And what if weather patterns change to the extent that plants that once thrived now struggle?

Unpredictable weather patterns or a relocation to a different climate can make it challenging to assess a plant’s adaptability to the local conditions. Fortunately, there are two valuable tools at your disposal: the USDA Cold Hardy Zone designation and the Horticultural Society’s Heat Zone designation.

Image: Tropaeolum majus (Nasturtium)
Sebastian Dario CC BY-NC 2.0

Unpredictable weather patterns or a relocation to a different climate can make it challenging to assess a plant’s adaptability to the local conditions. Fortunately, there are two valuable tools at your disposal: the USDA Cold Hardy Zone designation and the Horticultural Society’s Heat Zone designation.

These designations provide vital information about a plant’s ability to tolerate both winter cold and summer heat. Most gardeners are familiar with the USDA hardiness zone system, which rates zones on a scale of 1 to 12. The Horticultural Society’s Heat Zone system parallels this scale, with higher numbers representing regions experiencing more hot days.

In Durham, for instance, the heat zone map places it at a temperate 7, while the hardiness zone map designates it as 7b. The USDA Plant Hardiness Zone indicates the lowest winter temperature a plant can withstand before facing potential death. In Durham’s case, being in Hardiness Zone 7b means plants in this zone can survive temperatures as low as 5-10°F. If you travel south, the numbers increase, and if you go north, they decrease.

However, the USDA hardiness map does not provide insight into a plant’s ability to endure the heat of summers. For that, the Horticultural Society’s Heat Zone number comes into play. It indicates a plant’s tolerance to heat and specifies the temperature threshold at which it starts to suffer and becomes unable to efficiently process water for normal functions. In Durham, our Heat Zone is 7, suggesting that plants should not be exposed to a cumulative total of 61 to 90 heat days above 86°F. Given that we experience many days higher than 86°F, it is crucial to choose perennials wisely based on the heat zone index number.

To accurately describe Durham’s local climate, we can use the format 7b/7, representing the cold hardiness zone followed by the heat zone designation. This combination of numbers provides a clear picture of the climatic conditions suitable for your plants. You can typically find these numbers listed on informational tags at reputable plant nurseries.

Consider incorporating native plants into your landscape. Native plants are well adapted to the local environment, offering numerous benefits such as enhanced climatic resilience, support for local wildlife, and reduced maintenance requirements. Here are some native shrubs that can thrive in Durham’s summers and winters, typically associated with USDA Zone 7b and Heat Zone 7:

Clethra alnifolia flower spike
Tom PotterfieldCC BY-NC-SA 2.0

This is a short list, and it is advisable to consult your local Cooperative Extension Master Gardeners office for a comprehensive selection of suitable native plants, shrubs, and trees. You can also use the “Find a Plant” tool on the North Carolina Extension Gardener Plant Toolbox to narrow down native plants by your site conditions and needs.

While the heat zone and hardiness zone systems are essential factors to consider when choosing plants for your garden or landscape, other variables like soil type, sunlight exposure, humidity, and drought tolerance should also be considered to ensure successful plant cultivation.


Native Plant Profile:

Spotted Wintergreen (Chimaphila maculata)

By Wendy Diaz EMGV

Foreground: Spotted Wintergreen in bloom near base of Beech Tree. All photos taken on June 4, 2021 by Wendy Diaz unless otherwise stated.

In North Carolina, one of the advantages of my removal of invasive ground cover mechanically rather than chemically and changing my gardening habits – such as no longer mulching with three inches of pine straw – is that remarkable tiny native plants start to appear beneath my beech tree (Fagus sylvatica) like the Spotted Wintergreen (Chimaphila maculata) without any more intervention from me. (I wrote about another diminutive native plant beneath my beech tree, the Cranefly Orchid (Tipularia discolor), in the blog post of July 30, 2020 [1]). 


I feel fortunate that I have a few plants of this smallest of evergreen shrubs peeking above the dead leaves and pine needles in my woodland garden; although it is fairly common in North Carolina, it is rare, if not endangered, in its northern range near my home town in Ontario and also in Maine[2]. It has become so rare in its most northern range of Canada that there is a recovery program in Ontario[3],[4].

Spotted Wintergreen in full bloom just a few inches high above the leaf litter.

Growing Conditions

Spotted Wintergreen (Chimaphila maculata) is a native, evergreen rhizomatous wildflower or dwarf shrub found on the shaded forest floor. It only reaches a height of 3 to 5 inches tall which makes it a good ground cover if you can get it to spread[5]; it is slow growing. The habit form of the Spotted Wintergreen is classified as a sub-shrub and is considered in some literature as a small broadleaf evergreen shrub[6]. The woody plant is easily found in the forests of the Piedmont due to the conspicuous white mid-rib of the dark blueish-green leaves which contrast with the light brown of the surrounding leaf litter. The genus name is a combination of the Greek word for winter (cheima) and to love (philein). Other common names include Striped Wintergreen, Striped Prince’s Pine, Rheumatism Root[7] and Spotted pipsissewa[8].  Pipsissewa comes from the Cree Indian word pipsisikweu which means ‘break into small pieces’ because mistakenly, they believed that a substance in its leaves would ‘break-up’ kidney stones. Native Americans also used to make tea from the leaves to treat rheumatism and stomach maladies. The foliage is avoided by deer. It has a wide range in Eastern North America from Quebec to Florida and as far south as Central America. It prefers medium to dry forest floors with medium shade and acidic soils. More precisely, it requires dappled sunlight (shade through the canopy all day) or deep shade (less than 2 hours to no direct sunlight) and sandy soils on the dry side with good drainage[9]. It has a stoloniferous root system and spreads by underground stems or rhizomes. The plant does not do well if its roots are disturbed. It reproduces both vegetatively and by seed reproduction following light wildfires.

Conspicuous white mid-rib of the dark blueish-green leaves of the Spotted Wintergreen plant.


The evergreen leaves are a deep blue-green color with a white stripe along the central vein of the leaf with a waxy or leathery appearance. As the larger leaves widen, the white stripe spreads laterally to give a mottled appearance giving it its most distinguishing characteristic. Dentate leaf margins have shallow widely-spaced teeth. The narrow ovate-shaped leaves have an opposite and whorled arrangement and are about 1 to 3 inches in length and less than an inch in width. The leaves are attached to a semi-woody stout reddish-brown stem.

Pronounced white midrib of whorl of bluish-green leathery leaves of the Spotted Wintergreen along with stem with dual buds.


In late May to early June in the Piedmont of North Carolina, small fragrant, pretty white flowers appear from spherical white buds. The flowers are bell shape and open downward or hang (nodding) from the top of long reddish-brown stalk that grows up from the leaf whorl. Each stalk is topped by 2 to 5 curving stems from which clusters of 2 to 5 flowers emerge; looking much like an old fashioned lamp post.

Top Photo: Spotted Wintergreen buds and stalk look like tiny lamposts. Middle Photo: Partially opened blossom, fully opened blossom and missing blossom on one reddish stalk emerging from the whorl of waxy bluish-green leaves. Bottom Photo: Closeup of blossoms and green pistil (early seed pod). Photo taken by Wendy Diaz at Raven Rock State Park on June 7 2015.

Each flower of ½ to ¾ inch diameter has 5 waxy white petals that have small scattered brown spots, 5 light green sepals and ten yellowish or tan colored stamens and a green pistil. After pollination the flower turns upward so that the resultant small (less than an inch in length and 1/3 inch wide) seed capsule that forms is erect and eventually matures to a dark brown color. The dried capsule splits and releases tiny seeds

I was out of town this year when a few buds blossomed on the plants beneath the beech tree. I suspect there wasn’t a bigger show this year than there was in 2021 because we had a very wet winter and spring and they prefer drier soils or I am disturbing their sensitive roots when I often walk over to their colony to admire them.

Same Spotted Wintergreen plants in other photographs above but are
smaller and without blooms this year. Photo taken on May 17, 2023.