Climate Zones: What can I grow in my yard?

What plants will survive in my climate zone?

• This is a critical question that comes up all the time.
• Below are the major factors to consider as well as the tools/tips needed to make the right decision.

 

Most important factor:

• For the vast majority of gardeners, the single most important issue to consider when selecting a plant is its cold tolerance.
• Different plant species have  different low temperature survival thresholds.
• Most of the time, this low threshold temperature is known information for a particular plant species.

 

How low will it go around here?

• The next logical question is, “how low can I expect the temps to go in my area?”
• Well, there’s a map for that… and it’s called the “USDA Plant Hardness Zone Map”
• This is basically a very detailed type of climate zone map (see image below)

 

Here’s how the map works:

1. Identify the color that your home area is given on the map.
2. Match up that map-color to the color coded zone-key on the side of the map
   • (the second map provided below has better resolution for viewing the zone-key)
3. Each particular zone corresponds to a specified “Annual Extreme Minimal Temperature” range.
4. This is basically the lowest temperature range that you can expect in your area (on average).

Specific example:

• Lets say you are living just outside of the San Diego city limits, and somewhere near the coast (like me).
• You can see that this area on the map is given a yellow-orange color.
• On the map’s zone-key, (on the side of the map) that color corresponds to zone 10a.
• Just to the left side of that color coded zone-key box you can see that there is a temperature range in Fahrenheit (F) and on the right side is the same temperature range in Celsius (C).
• Therefore we can see that for zone 10a, the lowest average temperature that I can expect in my area (on average) is 30 to 35 F, or -1.1 to 1.7 C… Another words, right around freezing.

• As reference: Since I live in zone 10a, (and all the plants outlined on this website are  in my yard), then any of the plants you read about on this website should work in zone 10a.  Most of the plants discussed will also do great in many other zones as well.  Just reference the temperature section of the specific plant article.

 

• If your having trouble matching up the map colors to the zone-key on the map, don’t worry, there is another option.
• The USDA has a website where you can just plug in your zip code to get your “Plant Hardiness Zone”

 

How was this map compiled?

• Lots of temperature measurements were taken during the time frame from 1976 to 2005.
• Those temperatures were averaged together and after careful evaluation a detailed map was created.
• Because of the success of this method, it has been adopted by many other countries.
• The table below has a list of US and European cities with their respective zone.

Potential pitfalls of this map:

• Since the provided temperature values are averages, they only provide part of the picture.
• For example, a geographic area that has wide fluxuations in low temperatures may end up in the same zone as a geographic  area with more stable temperatures.
• In addition, the map doesn’t really account for the potential rogue extreme low temperature that might wipe out plants living at the cooler extent of their biological range.

 

Prepare for a potential unexpected cold snap:

• As a backup, it’s always a good idea to have an extreme cold weather plan and supplies ready for those occasional extra cold days that may damage or kill a cherished plant.
• Protective frost cloth/blanket is a great option to consider.
• The Protec brand of frost plant cover,  and the Dalen brand of Harvest Guard Row Cover are pretty similar products.  They are both found on amazon and both have very good reviews.
• Its important to make sure the trunk/bottom of the plant is also enclosed in this material.  I use use duck tape to make a pillow case- like sac out of the frost cloth material and it works great to cover trees.  After the cold has passed, I roll them up and store them for the next season-then just throw them on when needed.

 

Microclimate: 

• Local variation:
  • There is usually at least some climate variation within a particular zone.
  • Some parts of the world have more environmental variations in a small geographic area than others.
  • A lot of this has to do with the terrain, namely the amount of hills, mountains and valleys in a given area.
• Colder areas:
  • Cool air is heavy so it tends to sink to sheltered low areas such as valleys and other geographic depressions. Therefore, low areas are more likely to experience a cooler microclimate.
    • (You may have notice this happening in your home:  it tends to be warmer on the an upper floor than it is on the first/main floor or basement).
  • In addition, an area with persistent shade, such as the low-north side of a hill or house will likely be significantly cooler than an adjacent area just a few steps away. Note, winter shadows are longer-bigger than summer shadows due to the low position of the sun on the horizon,  and this should be taken into consideration when choosing your planting locations.
  • * Take advantage of these cool areas by planting trees that need a chilling period in lower depressions of your yard or in winter shaded areas.
• Warmer areas:
  • An area adjacent to a south facing wall, or on the top/sunny side of a hill will be significantly warmer.
  • Areas adjacent to large dark solid objects such as boulders, cement buildings and black asphalt will also be warmer.  This effect can carry into the night while those objects radiate the heat they absorbed from the sun during the day.
  • * Take advantage of these factors by planting the more cold sensitive plants in these warmer parts of your yard.

 

Other regional growing considerations:

• There are several other big picture issues to consider when determining if a plant will grow in your area.
• Heat tolerance, drought tolerance, soil type preference, sunlight needs, humidity preference, wind tolerance.
• These factors are all important in their own way.  However, in my opinion, the cold tolerance issue that we discussed above is the very most important element.