A gardening zone map is a map that shows where certain plant species can survive based on the high or low temperatures for an area. This information is vital for any outdoor growing of non-native plant species.
The Two Types of Garden Zone Maps:
There are two main types of gardening zone maps. A plant hardiness zone map and a horticulture heat-zone map. The first divides climates into eleven zones and is based on the coldest temperature or winter range for a region. The second divides climates into twelve zones based on summer temperatures. Basically, one map shows how cold a region becomes and the other is how hot a region becomes. Both divide climates into zones that will show what different plants will tolerate within these ranges.
Both of these maps are updated about every thirty years. The most recent update to the widely accepted zone maps was done in 1990, though due to global warming and an increased rate of global temperature changes another update is already being considered. An attempt was made in 2003, but rejected on claims that thirteen years was insufficient data to compile a new map.
Before planting or purchasing any plant for outdoor use, it’s important to determine what zone you are located in. Doing this generally requires reading a gardening zone map. This simple how to guide will explain how to read the two most commonly seen garden zone maps, as well as how to use this information in gardening.
Zone hardiness gardening maps are what you will usually find on the back of seed packets. Zone hardiness maps generally show a specific country or continent, for example North America, the United States, or Europe. The map will show the area covered in colors with a table that explains which “zone” these colors correspond to.
Maps are broken up into eleven zones. Each zone has a ten degree temperature range. Many maps have an A and B division breaking each zone into two five degree ranges for even more accuracy. The coldest and hottest zones have no A/B division, simply anything below -50 F or -46 C is considered zone 1, and anything above 50 F or 10 C is considered zone 11.
Temperatures on gardening zone maps are based off of the winter temperatures for a region. This means a plant with a zone range between 5 and 7 would be able to survive all year long in the zones 5, 6, and 7. A plant may be able to survive the summer in a zone it is not rated for, however, it will likely die over the winter and not return in the summer as even the roots will die.
A heat zone map is identical in appearance in most cases to a zone hardiness map, being that one inspired the other. The first heat zone map was published in 1997, seven years after the zone hardiness map was updated from its original 1960’s version.
In the case of a heat zone map, each color on the map will correspond again to a zone, however, in a heat zone map there are twelve zones. The zones are based off of the number of days an area rises over 68 F or 30 C. The lowest zone 1 indicates the temperature never rises above 68 F or 30 C and a zone of 12 indicates the temperature is above this point nearly all the time, or at least 210 days out of the year.
So for example, a plant with a heat zone of 12 can survive over 212 days of extreme heat without dying, while a plant rated zone 1 cannot likely tolerate temperatures over 68 F or 30 C at all. Some but not all plant tags and seed packets are now coming with a heat zone printed on them as well as a hardiness zone.
To use a gardening zone map, now that you understand how to read one, you need to know the zone for the plant you are planning on planting.
In many cases, as stated, the plant will have at least one of these zones on it’s care tag or the back of the seed package if you are planting from seed. You may see something like, “Hardy to zone 8” or “Zones 5-9”. If both zone maps are featured the plant will likely have four numbers for example “3-8, 8-1”. This would mean the plant was good for zones 3-8 on the hardiness map and 8-1 on the heat zone map. You can also find a plant’s zone online or in gardening books. Your local nursery may also be of help. Knowing both zone ranges a plant can tolerate will give a complete picture of both how cold and how hot the plant can handle, giving you a better idea of whether you should plant it or not.
It should be noted that because the last update for the zone hardiness map was in 1990 and the heat zone map is from 1997, global climate change has in many gardener’s opinion made them slightly inaccurate. One method for beating this is to go ahead and look at the average temperature range for your particular town and then find the corresponding zone for that temperature range. In this way you will be effectively re-zoning your area so to speak for current temperatures. Keep in mind that generally, the zone maps are still fairly close. Global climate change likely won’t allow a plant only rated to zone 5, for instance, to grow in zone 1.
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