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.


Making your own tree and shrub branch weights

by Eric Wiebe, EMGV

Are out of control tree and shrub branches getting you down? For fruit and nut trees in particular, branch spacing and angle are important for both fruit and branch health. Proper spacing allows for good air circulation and optimal light gain for the leaves. Branches that grow at too steep an angle are also at risk of snapping off under a heavy load of fruit. While branch angle modification is probably most common on edible fruit trees, these techniques can also be used on ornamental trees and shrubs to improve their aesthetics and plant health.

If you have a branch that you want to preserve but it is growing at too steep (vertical) an angle, the two most common methods for getting it more horizontal is either spacers or weights hung off of the branch. Over the course of a season, as the branch continues to grow, it conforms to this new angle of growth and the spacer or weight can be removed. In many ways, weights are the most versatile method of pulling branches down. This blog post will show you how to make your own weights to use at home.

With very young branches, sometimes something as little as a clothespin provides enough weight to pull them down. However, as the branches get thicker in girth or more vertical, more weight is needed. For these cases, additional weight can be added directly to the clothespin in the form of hanging weights made from cement. 

Making these weights only take a few supplies (Figure 1):

  • Powdered concrete patch mix
  • Disposable sample cups (2-3 oz)
  • Large paper clips or other stiff wire
  • A container for mixing
  • A stirring stick (like a paint stirrer)
  • Water
  • Clothespins (for hanging the weights)

Fig. 1: Supplies needed for the project. All photos: Eric Wiebe

Set out as many cups as you want weights. Keep in mind that the cement hardens quickly, so you may want to start with a dozen or fewer. Now bend your wire/paperclips into a shape like seen in Figure 2. Note that one end will sink into the cement and anchor it while the other end will fit into the clothespin.

Fig. 2: Wire bent into a hanger shape

Now you’re ready to mix up the cement. Follow instructions on the cement package. You may want to use disposable gloves, as the cement can be an irritant and will stick to your skin. Add water in very small quantities, as it is easy to get the mix too liquid. You will want something the consistency of a cake batter (Figure 3, proper cement consistency).

Pour or scoop the cement into the individual plastic cups. Note that you will probably want differing weights, so fill your cups to different levels. Immediately work the wire hangers about ½” under the surface of the cement (Figure 4).

Fig. 4, placing the hangers

Make sure all the wire hangers are straight and let the cement harden for the time recommended on the package. Once the cement is hardened, you can cut the cups off the cement and your weights are complete! You can use an old egg carton to hold your weights in (Figure 5).

Fig. 5, Weights ready to use

Try out your weights out on some branches. Your wire hanger should sit comfortably between the two pin halves near the spring. By adjusting the weight amount, the location of the weight on the branch, or both, you can get varying degrees of pull down on the branch. Don’t overdo it, as you don’t want to break the branch off.

Happy growing!


Producing Fruit Trees for Home Use – NC Cooperative Extension

Extension Gardener Handbook – Tree Fruit & Nuts – Pruning and Training