Factors to Consider

"San Diego County is the most botanically diverse county in the contiguous United States. This county alone is more diverse than many states, and it has even been identified as an international 'hotspot' of biodiversity. Our county has a combination of climatic, geographic, geologic, and floristic features that are unique in the USA. It represents the SW region of the California Floristic Province and the Sonoran Region of the Desert Province, spanning a range of habitats from the Pacific coast to mesas, foothills, mountains, and desert. Over 1500 native plant species and almost 500 non-native species have been identified and documented to date." (from the San Diego Natural History Museum plant atlas project)

Although San Diego's climate can support plants from regions all over the world, it is ironic our urban areas are planting fewer and fewer diverse species. Urban life also disrupts topography, watersheds and soil fertility through our concentrated human presence in this region. As a result, we cannot continue to rely solely on natural species within our cities, especially with our dwindling tree populations.

San Diego's dry summers and mild, wetter winters are considered Mediterranean, and can easily and sustainably replicate the four other natural Mediterranean regions of South Africa, Chile, Australia and Southern Europe. It is provident for us to broaden the palette and number of trees within our urban forest, incorporating plants from these - and other compatible climates. But we must progress wisely.

1. How large is the space?

Many urban trees fail because they are situated improperly, not allowing room for their mature size. Radical pruning frequently results and urban trees often get unfairly maligned as “getting too big”. Trees don’t get too big, but the space may have been too small for the particular species.Evaluate your area and measure it. Locate the trunk of the tree sufficiently away from obstacles based on the normal widths for that species. Beware of potential height problems ahead of time and observe potential overhead obstructions like adjacent trees, utility lines and emergency vehicle access.

2. Is this tree for a sunny, partial shade or a full shade location?

Most trees adapt to varying degrees of sun, although most prefer at least partial sunlight. In San Diego, coastal zones tend to permit greater latitude for plants which may be sensitive to too much sun. However, hot, inland areas may need to place some “full sun” plants in partial shade, especially if situated facing intense southwestern summer sunlight. It is important to note that trees receive up to 90% of their energy from the sun, making them truly "solar-powered".

To choose the best species for a site, locate your western climate zone What is a climate zone? For many years, these were based primarily on the timing of the first frost of the season. Across the USA, 10 hardiness zones typically assessed the whole nation. Unfortunately, this placed San Diego in the same climate zone as Florida - hardly a similar climate. The Western United States has now established 24 different climate zones, and at least 12 of these 24 zones exist within San Diego County ( Sunset climate zones: San Diego region). If using the traditional USDA hardiness zones, San Diego is 9-11.

Microclimate also matters. A microclimate is a local atmospheric zone where the climate differs from the surrounding area. The term may refer to areas as small as a few square feet (for example a garden bed) or as large as many square miles. San Diego has four major microclimates: coastal, inland valleys, mountains and desert (San Diego Microclimate details). Microclimates can even be influenced by slopes and building which may change wind and sun exposure.

3. What type of soil and drainage exist?

It is important to understand your soil type. When growing trees it is impractical to alter the whole root zone where tree roots will subsequently grow. A good basic link to understand your soil type is from The Global Garden Project. Expect to plant your tree in the native soil.

Most trees need good drainage. Test your soil with a drainage test. Dig a hole 18 inches in diameter, 12 inches in depth.  Fill the hole with water and let it drain.  After it has drained, fill it again and time how long the hole takes to empty.  1/2 to 4 hours is good, 5-12 hours in moderate.  If over 12 hours, there is a problem.  If drainage time is greater than 12 hours and the site will still be used, a raised bed of appropriate size may be the only way the tree can be grown successfully.

Some small surface regions can be changed with amendments and mulches. In addition, mulches aid in water conservation and weed control. The City of San Diego at the Miramar landfill is source of mulch and compost, some of it free to city residents, but it is inexpensive for all.

4. Are only native trees desired? Have the ramifications for this been considered?

Native tree species are the best choices for providing habitat for native fauna and ecological balance. However, there are too few native or naturally occurring tree species in San Diego’s region to replace the unnatural heat islands of urban buildings, roads and development. Native trees may also have abundant pollen production and may aggravate allergies in some confined spaces. Choose carefully, as many native trees are vigorous and can become quite large with aggressive roots well suited to seeking hard-to-find water. Some may actually prefer moist locations, while others cannot stand typical summer garden watering practices at all.

5. Will this tree be planted in a lawn?

  • Most trees tend to develop shallow roots when planted in lawns.

  • To reduce competition from turf, mulch at least an 18" radius out from trunk, preferably twice that radius if feasible.

  • Keep the mulch at least 4" away from the root crown.

  • Practice infrequent, but deep watering for all plants (to include lawn areas) to promote their drought tolerance

6. Growth rate desired: What are the trade-offs?

  • slow - trees that grow slower tend to live longer, add more to property value, often have stronger branch attachments and better behaved roots

  • moderate - good choices for most purposes, moderate growth trees balance longevity, durability and overall dollar value with average PACE of growth

  • fast - fast growth trees tend to have aggressive roots, weaker branch attachments and may be shorter lived, but can be useful to provide quick screening