Definition

Hardiness zones are a system to describe what plants can survive the cold season in what area. This is important if you plan to have plants that live more than one season, either biennial plants (=two seasons) or perennial plants (=several seasons).

The most commonly used is the US hardiness zone system, which might be referred to the USDA zone. The Permapeople plant database uses this system and refers to it as a “hardiness zone” only.

The hardiness zone system defines 13 zones the minimal annual temperature. It goes from 0 (coldest minimal annual temperature) to 12 (warmest minimal annual temperature). Sometimes zones have an added “a” or “b”, for example, Berlin in Germany is a “7b”. This is just a more granular definition.

See the following table for conversion of lowest annual temperature to hardiness zone.

Zone   From To
0 a < −53.9 °C (−65 °F)  
  b −53.9 °C (−65 °F) −51.1 °C (−60 °F)
1 a −51.1 °C (−60 °F) −48.3 °C (−55 °F)
  b −48.3 °C (−55 °F) −45.6 °C (−50 °F)
2 a −45.6 °C (−50 °F) −42.8 °C (−45 °F)
  b −42.8 °C (−45 °F) −40 °C (−40 °F)
3 a −40 °C (−40 °F) −37.2 °C (−35 °F)
  b −37.2 °C (−35 °F) −34.4 °C (−30 °F)
4 a −34.4 °C (−30 °F) −31.7 °C (−25 °F)
  b −31.7 °C (−25 °F) −28.9 °C (−20 °F)
5 a −28.9 °C (−20 °F) −26.1 °C (−15 °F)
  b −26.1 °C (−15 °F) −23.3 °C (−10 °F)
6 a −23.3 °C (−10 °F) −20.6 °C (−5 °F)
  b −20.6 °C (−5 °F) −17.8 °C (0 °F)
7 a −17.8 °C (0 °F) −15 °C (5 °F)
  b −15 °C (5 °F) −12.2 °C (10 °F)
8 a −12.2 °C (10 °F) −9.4 °C (15 °F)
  b −9.4 °C (15 °F) −6.7 °C (20 °F)
9 a −6.7 °C (20 °F) −3.9 °C (25 °F)
  b −3.9 °C (25 °F) −1.1 °C (30 °F)
10 a −1.1 °C (30 °F) +1.7 °C (35 °F)
  b +1.7 °C (35 °F) +4.4 °C (40 °F)
11 a +4.4 °C (40 °F) +7.2 °C (45 °F)
  b +7.2 °C (45 °F) +10 °C (50 °F)
12 a +10 °C (50 °F) +12.8 °C (55 °F)
  b > +12.8 °C (55 °F)  

Taken from https://en.wikipedia.org/wiki/Hardiness_zone#Definitions.

Determine your hardiness zone

To start using hardiness zones to choose plants for your garden, you need to find out in what hardiness zone you are living in.

You can do this by either searching on the internet:

Or if you know your lowest annual temperature, you can use the table above.

Hardiness zone of a plant

If you read that a plant has a hardiness zone of 5 this means it will survive low temperatures until this point as described in the table. If you get temperatures below that level, they are most likely to kill the plant.

While mostly the lowest hardiness zone is interesting, some plants also have a maximum hardiness zone, this means that some plants need some kind of coldness over the winter to survive and regrow in spring. For example, Garlic needs some cold period before it grows from a clove.

Use the hardiness zone in the plant database

If you know your hardiness zone, you can search for plants in the database here.

Let’s say you live in an area with hardiness zone 7b and you want to know what plants you can grow in your area without problems, you can search for a maximum hardiness zone of 7 by entering 7 in the max field of the hardiness range filter and search. This means “Give me all plants which have a minimum hardiness zone up to 7”. You will for example get plants like Apple with a hardiness zone of 3, Rosemary with a hardiness zone of 7, and Peach with a hardiness zone of 6.

This means that Apple will grow as cold as hardiness zone 3, which means it is totally fine in your area. Rosemary can only go as low as your hardiness zone, so it might get problems if it gets much colder than expected in a season. Peach has a hardiness zone of 6 and gives you probably enough safeguard.

Growing plants outside of your hardiness zone

Hardiness zones are not pure math but more like a guiding system. If a plant can survive cold temperatures is highly dependent on where it is grown, how it is exposed to sun, wind, rain, and other things.

It is possible to grow plants with higher hardiness zones than the one you are actually in, but this means you need to add protection to your plants. Let’s say you live in an area with hardiness zone 7 and want to grow a plant which has hardiness zone 9 (=it can only survive temperatures as low as zone 9, not as low as 7), you need to make sure it is getting protection. For example, you can wrap them into fabric in cold nights, move them inside the house, grow them in a greenhouse and/or give them some other source of warmth in the cold period.