Selecting trees to benefit the landscape

ON GARDENING

California scrub oak (Quercus berberidifolia). This medium-to-small oak tree grows 15 to 20 feet tall and 15 feet wide, with leathery green leaves. The California native supports biodiversity by attracting birds and several different butterflies. It thrives in various conditions and benefits local wildlife, making it a desirable addition to the garden. KRZYSZTOF ZIARNEK KENRAIZ VIA WIKIMEDIA COMMONS

Leather oak (Quercus durata). This small, drought-adapted oak typically grows in serpentine soils and can tolerate a variety of garden soils, including those with clay. It grows slowly into a mounding form, up to 12 feet high and wide, with leathery green leaves that are somewhat hairy on the underside. It serves as a keystone plant in its native California ecosystem, supporting a range of wildlife species. JOHN RUSK VIA WIKIMEDIA COMMONS

Torrey’s scrub oak (Quercus acutidens). This small evergreen tree, growing 10 to 20 feet tall, is a hybrid of two oak species: Quercus cornelius-mulleri and Q. engelmannii. Its California native habitat includes pinyon, juniper and oak woodlands, margins of chaparral, arroyos, slopes and bajadas. It has small, attractive leaves and develops green and brown flowers in March, April and May. JOE DECRUYENAERE VIA WIKIMEDIA COMMONS

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Our recent column, “Thoughts about trees,” described the large trees in my garden, provided basic guidelines for tree care and promised a column about the criteria for selecting the right trees for the garden. Let’s get started.

First, we present several reasons for incorporating trees into the landscape and then recommend selection criteria.

Today’s image gallery includes close-ups of smaller trees, selected from Wikimedia Commons (commons.wikimedia.org), and captions edited from California Native Plant Society’s database of California’s native plants (calscape.org) and other sources. These are examples of small trees for the garden, rather than recommendations.

Why include trees in the landscape?

The reasons for including trees in a garden depend on the gardener’s priorities and the site’s character. Deciding on these reasons establishes the planning framework. This section describes the significant objectives for selecting trees.

Trees can enhance a residential garden in several ways, particularly by adding to the landscape’s beauty. They provide one or more vertical counterpoints to low-growing annuals and perennials in garden beds and areas of lawn grasses. They can also balance the scale of the residence and other buildings.

Trees also bring functional benefits to the landscape. When well-placed, deciduous trees can shade the home from the summer’s heat and create comfortable outdoor living spaces.

Trees can also serve as windbreaks when the site experiences prevailing winds and as privacy screens when the homeowner wishes to block views of neighboring properties or provide a feeling of enclosure. More generally, trees can create a green space that reduces stress and improves one’s mood.

On sloped properties, deeply rooted trees can stabilize hillsides, anchor soil and reduce runoff.

From the environmental perspective, trees mitigate the effects of climate change by absorbing carbon dioxide and contribute to cleaner air by filtering airborne pollutants and trapping particulates.

The ecological benefits of trees include providing food and nesting sites for birds, pollinators and beneficial insects, as well as fostering biodiversity in the garden and its surrounding area. These benefits depend, to an extent, on the tree’s compatibility with the local ecology.

Let’s not overlook the financial benefit that trees add to the property. Well-placed, well-chosen, healthy mature trees can add 5 to 15% to the site’s market value and can also extend the lifespan of site improvements that the trees shade. Long-established trees can define the property and serve as legacy elements passed from one generation to the next.

Many gardeners find trees attractive in their foliage, blossoms and overall form, and appreciate the seasonal interest that trees contribute to the ever-changing character of the garden.

How to select trees for the garden

Numerous trees are well-suited to California’s Central Coast. These include varieties native to the Mediterranean climate, existing in California’s coastal natives, the Mediterranean basin, South Africa, Australia’s southwestern coast and Chile’s central western coast.

Selecting trees from this large range of options would yield specimens that will thrive without irrigation (once established) during the region’s summer drought, grow easily through the region’s seasonal climates and (if near the coast) tolerate coastal breezes and occasional salt-laden air.

The Monterey Bay area’s favorable environment can also accommodate trees from beyond the Mediterranean climate areas, but such trees might require extra care, such as summer irrigation.Another important criterion is the appropriateness of a tree’s mature size and scale, relative to the landscape of interest. The height of the residence effectively establishes the scale for the landscape. Assuming a level property, a 20- to 30-foot-tall tree would complement a single-story residence, while a larger tree could work well with a two-story or higher residence.

A sloped property may require visualization to determine a good height for a new tree.

Ultimately, deciding the appropriate tree height for a given landscape is a subjective matter and could be aided by second opinions.

The homeowner’s preference for the tree’s form is another subjective consideration. A fundamental issue is the choice between deciduous and evergreen trees. Some gardeners enjoy the seasonal changes of deciduous trees, while others appreciate the year-round continuity of evergreen trees.

The California Native Plant Society’s database of California native trees is a useful resource for deciding between deciduous and evergreen trees.

Our search of the Calscape database revealed 76 evergreen trees, of which only 13 are 30 feet tall or smaller. A follow-up search identified 88 deciduous trees, of which 48 are 30 feet tall or smaller.

Aesthetics present another subjective consideration for the gardener. Many gardeners find several trees pleasing in appearance for different reasons. Several factors deserve attention, including the attractiveness of the branching structure, the canopy shape, the foliage shape and color, the bark color and the seasonal blossoms’ color and form.

Additionally, relatively minor selection criteria involve site characteristics. For example, when the intended site is near foundations, pavement or pipes, it is best to avoid selecting trees with aggressive root systems.

Another example: select a tree compatible with the intended site’s soil conditions, such as clay soil, serpentine soil, low nutrient or rocky soil or poor drainage soil.

Then, when selecting a tree for a shady site, consider varieties that would thrive with less than full exposure to sunlight. Calscape lists 27 deciduous trees and seven evergreen trees that would grow well in semi-shade or deep shade.

There is a lot to consider! Choosing your favorite tree among scores of options is more challenging than the parents’ choice of the favorite child among two or more.

This week in the garden

This week’s project, if you wish, is to add one or more trees to your garden. Scan the reasons (above) for adding trees to the landscape and decide on your priorities.

Then, if you prefer California native trees, browse Calscape.org and use its selection filters to produce lists of trees that meet your criteria. When you focus on a tree of interest, click on the link “View Availability at Nursery” for a list of nurseries that offer that tree.

We haven’t found similar searchable databases for plants native to other Mediterranean regions. One approach to focus your selection is to use the Wikipedia list, “Trees of Mediterranean climate.” Browse tinyurl.com/27dx89k5.

Local garden centers’ inventories usually include such trees (and other plants), so if an available tree is of interest, conduct an online search of its botanical name to learn more about it.

A landscape contractor should be willing to help search the availability lists of nearby wholesale nurseries to locate trees of interest.

Enjoy your garden!

Tom Karwin is a past president of Friends of the UC Santa Cruz Arboretum and the Monterey Bay Iris Society, a past president and lifetime member of the Monterey Bay Area Cactus and Succulent Society, a Lifetime UC Master Gardener (certified 1999-2009), past board member of the Santa Cruz Hostel Society and active with the Pacific Horticultural Society and other garden-related societies. To view photos from his garden, visit facebook.com/ongardeningcom-56651 1763375123. To review the archive of recent On Gardening columns, visit santacruzsentinel.com and search “Karwin.” Go to ongardening.com to review columns from 2012-2020 (and soon) from 2025. Send comments or questions by email to gardening@karwin.com.