Horticultural zone helps gardeners select the plants that will thrive in their regional climate. In this article, we will see everything there is to now about Horticultural Zones.
I highly recommend you find your horticultural zone before you get your hands dirty. This will ensure that you get the best out of your garden.
The US Department of Agriculture (USDA) has developed an interactive map to make it easier.
Notably, this is the first thing gardeners look for before planning their garden. The zone number helps you decide which plants suit your regional climate.
More importantly, the zones are colour coded based on average low temperatures in that region. That being said, it’s the zone that’s used to rate plants based on their hardiness.
Generally, nurseries and garden centers label plants that are hardy to certain zones. As a result, gardeners can easily select the plants that grow easily in their region. However, make sure you verify the data online.
There are 11 horticultural zones in North America. An interesting fact is that each zone varies with a temperature of 10°F.
For instance, a plant that’s hardy to zone 9 will survive in a minimum temperature of 20°F. Whereas, a plant that’s hardy to zone 10 will survive in a minimum temperature of 30°F.
Keep in mind that horticultural zones affect fruits, vegetables and flowers too. Hence, understanding your horticultural zone is a crucial step that will influence your garden’s success. This will ensure that your garden is healthy all-year round.
What’s my Horticultural zone?
Most warmer zones are in the deep southern half and the southern coastal areas of the US. Zones 9, 10 and 11 are considered the warm zones.
However, Puerto Rico and Hawaii have even higher zones. Zone 13 for Puerto Rico and Zone 12 for Hawaii.
Zones 6, 7 and 8 are the middle zones. They make up the southern middle portion of the mainland and central coastal areas.
Whereas, the far northern portion on the central interior are some of the coldest zones. Zone 4 and 5 as well as a small part of zone 3 fall under this category.
Notably, these zones often have much less temperature consistency during winters. Especially since its more continental. And this is one of the flaws of the map.
Even lower zones down to 1 are found around Alaska.
Check out the USDA interactive map to find your horticultural zone.
What are Horticultural Zone maps?
Horticultural Zone maps help gardeners to compare their regional climate with other regions. Naturally, a great way to compare with regions where plants thrive.
Ultimately, these maps show where various plants grow well in the long run. Whether you’re planning on getting tropical, exotic plants, perennials, shrubs or trees, make sure you check the zone maps.
After all, you want them to survive and grow throughout the climatic conditions in your region. For example, they should tolerate both the high and low temperatures as well as rain, humidity, snow and frost.
Popular zone maps include:
- The US Department of Agriculture (USDA) hardiness map
- The Natural Resources Canada (NRC) map
Notably, every country uses different ways to create a zone map.
Other hardiness rating schemes have been developed as well, such as the UK Royal Horticultural Society and US Sunset Western Garden Book systems.
Research shows that USDA plant hardiness zones will shift northward with climate change.
Keep in mind that zone maps are not absolute. Sometimes, you may find the zone information contradicting your growing experience.
For instance, you may be able to grow plants that wont survive in your zone. Or you may find that plants that thrive in your zone are not growing quite well. Especially, if you plant in a micro-climate like a greenhouse.
Some aspects that affect plant growth irrespective of the zone include:
And other factors.
An interesting fact is that the maps have evolved significantly over the years. For instance, the differences in the USDA maps of 2012 and 1990 are huge. Let’s understand them better.
What’s new in the latest USDA map?
The latest UDSA Zone map was released in 2012. And its more advanced and accurate than the previous versions. The USDA requested expert horticulturists to review zones in their regions.
Eventually, they developed trial versions of the map that were revised with their inputs. Finally, they came up with the revised map in 2012.
Compared with the 1990 map, there are plenty of changes. The zone boundaries have moved in many regions. In general, the new map is 5°F half-zone warmer than the old one.
Notably, the latest map uses data from weather station over a 30-year period from 1976-2005. On the other hand, the 1990 map used data from the period 1974-1986. Indeed, a much shorter time period. And this is what makes it more accurate.
Moreover, the old map was very difficult for new gardeners to understand. In contrast, the latest one is available online as an interactive GIS-based map.
However, some changes in zones are thanks to advanced ways of mapping them between weather stations. For instance, new algorithms considered aspects like elevation, terrain, valleys, ridges and proximity to water bodies.
Furthermore, the map perfectly depicts the climates of eastern half of America. This is because the area is relatively flat. And the mapping method is merely drawing lines parallel to the gulf coast while moving north.
An interesting fact is that these lines tilt towards north-east when they’re near the East Coast. Moreover, they also show special climates formed by the great lakes and mountain ranges.
Besides, the latest map used data from a larger number of stations. Hence, all these factors contributed to the higher accuracy and detailed nature of the map.
What is a Horticultural Zone?
A Horticultural zone helps growers select all kinds of plant that can survive in the climate extremes of a zone. As a matter of fact, these are mostly for perennials, shrubs and trees.
In other words, horticultural zones define which plants can survive the weather conditions in that region throughout the year.
When selecting perennials, you should look for those that thrive year-round. Especially, if you live in a zone with extreme winters.
Whereas, the zone temperature range guides your plant choice for annuals like flowers and even vegetables. However, annuals like tomatoes are grown in almost all US zones. Naturally, depending on the time of the year.
In fact, some zones have very short growing seasons. Hence, gardeners in such zones usually grow seeds indoors and move them outdoors during growing season.
Its vital to remember that horticultural zone is to be used as a guideline. Most seed packs and plants in nurseries and garden centers are labelled. And these labels have information about best growing time for each zone.
How does Horticultural Zone affect me?
The USDA map is based on average yearly minimum temperatures during winter. And its divided into thirteen zones, each varying by 10°F with its adjacent zone. Moreover, the zones are divided into sub-zones of 5°F.
Essentially, horticultural zones are most useful for gardeners selecting perennials. This is because perennials often live for many growing seasons.
And they should be able to survive winters in your region. Hence, its crucial to know how cold it usually gets during winter in your zone. So, find out whether the plants you select are hardy enough for the winter.
In fact, perennial flowers, shrubs and trees grow their best in a suitable zone. Evidently, the winter damage to plants is much more when they’re grown out of their zone. Unless, you have a controlled environment like a greenhouse or indoor garden.
On the other hand, annuals like flowers and most vegetables need more care. Since annuals only last for one growing season, zones don’t matter. Hence, its vital to know the growing seasons and frost periods.
Generally, regional plants are the best for a practical and easy to maintain landscape. Moreover, they ensure that your landscape is sustainable for years to come. Ultimately, its relatively easy to grow a vibrant garden with plants that are native to your region.
Always avoid selecting plants that are marginally hardy in your zone. Because they are more likely to have poor growth, reduced flowering and winter damage.
Drawbacks of Horticultural Zone maps
As the USDA map is based on average annual lows in a zone, it fails to accurately describe climatic conditions. Besides annual lows, there are many more aspects to consider while selecting plants.
Although the map acts as a guideline for growers, there are certain drawbacks. Let’s understand them better.
Some benefits are not accounted for
The USDA map does not consider the beneficial effects of the environment. For instance, when perennial plants are covered by snow, it’s very useful. Moreover, the soil drainage and regular freeze-thaw cycles are some of the benefits.
Besides, frost dates and frequency of snow cover can vary widely within a zone. So, all these benefits are not featured by the USDA map. Especially, for the eastern half of the country. And the map fails for the rest of the country. Primarily, west of the 100th meridian.
Another issue with the colder zones is the reliability of snow cover. Evidently, it acts as an insulator from cold. And this protects roots of hibernating plants like perennials. If the snow cover is reliable, roots are exposed to a higher temperature than what the zone indicates.
Furthermore, the zones don’t give any details about summer heat, humidity and insolation. Hence, areas that have the same average winter lows, but very different summer temperatures fall under the same zone.
Issues of the Western half
Things are not the same in the western half. Many aspects like rainfall and altitude influence the growing climates in the West. And its these aspects which are not featured by the map.
Winds from the Pacific Ocean affect the weather in the Western half. As the winds pass by the mountain ranges, they become less humid.
Even though eastern cities in the same zones may have a similar climate and plants. This is not the case for the West. In other words, two western cities in the same zone may have different climates and plant selection.
For instance, the weather and plants in higher elevation Tucson, Arizona are different from those in coastal, low elevation Seattle. Although, they’re both in zone 8 of the USDA map.
Other Climate aspects
Since the USDA map focuses only on winter lows, it misses out on other key climate factors. The Horticultural zone should also consider many more climatic factors.
- Soil type
Also, the site-specific conditions such as:
- Soil drainage
- Water retention
- Water table
- Tilt relative to the sun
- Any kind of protection like greenhouses
If you’re in a hot climate, make sure you check out the American Horticultural Society’s Heat Zone Map. Ironically, it’s the opposite of the USDA map. It divides the country into 12 zones. Each zone is based on the average number of days per year when the temperature is over 86°F.
So, the annual average low temperature is a useful indicator. But, it should not be used as the only factor for plant growth and survival.
I hope you enjoyed this article. Feel free to share your thoughts in the comments section below.