How do Professionals Select Trees?

Of the 1517 trees that grow in California how can you pick the best trees for your project? When I started specifying trees in southern California most landscape architects called The Western Garden Book the bible. Today we have something almost as good as divine guidance, we have a searchable data base.

A searchable data base is an interactive program that lets you sort trees based on criteria you select. There are several of these search engines which I will describe shortly. But first, of the dozens of tree characteristics you can chose from which do you start with? Which will help you whittle down that list to a manageable number of choices? I start with three criteria:

  • Will it grow in this climate?
  • How much water does it need?
  • How big will it get?


How do you know a tree that will grow on your site? By climate zone. Most of the country does this using the USDA Plant Hardiness Zone Map. It predicts which plants will grow where based on minimum winter temperature. It works well in flatter continental areas but not out west. That system puts the Olympic rain forest in the same zone with parts of the Sonoran Desert.

Given our unique compartmentalized terrain, we needed a system that includes “a broad range of factors such as cold, heat, humidity, wind, proximity to the Ocean, snow and length of growing season.” (Sunset. p. 17) Sunset introduced the West’s climate zone maps in 1954.

Always know your climate zone. Experts look it up. At least they should but as the cold snap of 2007 illustrated that doesn’t always happen. After a decade of mild temperatures some plant specifiers were left red faced when insurance adjusters pointed out that dead trees were planted out of zone.

ABOVE: Sunset. “Sunset climate zones: Los Angeles region” 2015. August 12, 2015.

In southern California a few miles can make a huge difference. Here at world quarters out on the far edge of Ventura we have a blend of zones 23 and 24. Perfect weather. Ojai is just ten miles away but separated from us by four climate zones. When using a searchable database start with the trees that grow in your Sunset climate zone. That’s your first criteria.

Above: Ventura, CA – Zone 24 – Quercus agrifolia, below coast live oaks – May 2015

Above: Ojai, CA Zone 20 – Quercus agrifolia, coast live oaks – May 2015


In California knowing how much water your trees need is critical because that will limit the shrubs and groundcovers you can use. All the plants in a given irrigation zone (valve) need to share the same water needs because they share the same water. The University of California has built a searchable data base that is really simple to use. Just pick your city and quickly get a list of site appropriate trees, shrubs and groundcovers grouped by water needs. This is WUCOLS IV with the classifications you need to comply with state law.

If you have a project in Thousand Oaks go to the website and select your city under the South Coastal dropdown menu. That takes you to a page where you can generate a list of the 215 “Low” water use trees that will grow in T.O. You have answered both your first and second critical questions with one data base query.

Above: The University of California has built an excellent searchable data base


When specifying a tree pretend you live in a small apartment and you are getting a puppy. No matter how cute it is you have to ask, “How big will this thing get?” Too big in the photos below. You can search for trees by size using SelecTree hosted by UC San Luis Obispo.

Above: screen grab from SelecTree foretells trouble ahead for these fig trees.

Above: screen grab explains why this sycamore looks like a knight in a Monty Python skit.

Once you know your site’s climate zone and your desired water use you can use SelecTree to find trees that are the right size. If you are honest the list will be shockingly short. It takes some experience and judgment but if you don’t do it those puppies are liable to get huge and you will be whacking away at it like it’s a medieval knight piñata at a Monty Python festival.

In out hypothetical project above we were looking for low water use trees for a project in Thousand Oaks. That’s Sunset climate zone 21. If I search SelecTree for trees that grow in zone 21 and are less than or equal to 35 feet high, it returns a list of 592 trees. If I add “Soil Moisture Dry” I get a list of 216 trees (recall WUCOLS IV listed 215).

That’s still a lot of trees but we’ve taken our list of 1517 trees and found 216 small to medium size trees that will grow on our project site with little water. When it comes to tree size you also need to think about the canopy shape and branching structure. If the space is tight you might want to do a massing study like the one shown below.

The most important lesson I have learned specifying trees in southern California is that experts look it up. The web has excellent free resources to help. You can begin to create a manageable list by answering these three critical questions:

  • Will it grow in this climate?
  • How much water does it need?
  • How big will it get?

In future posts I will discuss some techniques I use to further refine my choices and pick the perfect trees for each project. But next time I will be asking that burning question: How much are my trees worth? Like Madonna said, it’s a material world.

John Burke, Ventura, CA August 2015

Kathleen Brenzel, ed. The New Sunset Western Garden Book (New York: Time Home Entertainment, 2012).
Agricultural Research Service. “USDA Plant Hardiness Zone Map.” 2015. Aug 12, 2015.
University of California. WUCOLS. “Plant Search Database.” 2015. Aug 13, 2015
SelecTree. \“SelecTree: Search Trees by Characteristics.” 1995-2015. Aug 13, 2015.

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