Marin Master Gardeners
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Form and function of trees

September 25, 2010
Nanette Londeree

"The best time to plant a tree was 20 years ago. The next best time is now," or so goes the Chinese proverb.

If you've ever basked under the cathedral-like canopy of a magnificent old shade tree on a warm summer day and reveled in the serenity and tranquility it creates, you're enjoying just one of the myriad benefits of a stately, mature tree. And while you may not have the space for such a tree in your garden, it's easy to find a tree that can add stunning dimensions to your landscape.

They're so much more than just a big, pretty plant.

"The best friend of earth and of man is the tree," reflected renowned architect Frank Lloyd Wright. "When we use the tree respectfully and economically, we have one of the greatest resources on the earth."

Trees are vital to the health of our environment - they purify the air by intercepting airborne particles, act as carbon sinks by absorbing and locking away carbon dioxide in their wood, roots and leaves, and they produce valuable oxygen for all of us to breathe. Their roots bind the soil, reducing erosion and slowing storm water runoff, and trees create a habitat for birds and other wildlife.

Some of their economic benefits include: beautiful trees can increase the value of your property, while shade trees can decrease the need for air conditioning in summer and lower winter's heating costs by breaking the force of winds.

Considering that most trees have the potential to outlive the people who plant them, selecting the right tree for the right location can influence a lifetime. The function of the tree, its form and size at maturity, its growth rate and longevity, the planting site conditions and the desired plant characteristics are key things to evaluate before choosing a tree.

Beginning with function, decide what do you want the tree to do - will it serve as an ornamental focal point in your landscape, act as a windbreak or privacy screen, provide fruit for the owner and attract birds and wildlife? Reduce summer heat around the perimeter of the house, or be the subject of climbing fun for children?

Next you'll want to look at form and size.

There are oodles of tree shapes - rounded domes, upright columns, broad conical forms, umbrella-shaped canopies, weeping and more. Choosing a tree that is cloaked in foliage year round (evergreen) or drops its full load of leaves at one time (deciduous) will narrow your range of choices. Size matters - find out the size and shape of the tree at its maturity. What might be a demure looking tree in 10 years may be an outsized behemoth in twenty or more.

How fast a tree grows can impact its stability and longevity. Some types of eucalyptus grow at a rampant rate, though are reputed to be "widow makers" as they can drop enormous branches for no apparent reason. Many common landscape trees have a relatively short lifespan - 20 to 30 years. Slow growing trees may not give you that optimum shade in a few short years, but they're likely to have a long lifetime.

When assessing your planting site, evaluate the exposure to sun and wind over multiple seasons (if possible). What might seem to be the perfect spot for that evergreen in summer may result in unwanted shade during the remainder of year. Winter frosts can damage tender trees if planted in exposed areas.

When choosing your planting location, look up and down. If there are power lines overhead, assess whether they could be impacted by a full-sized tree. Similarly, if there are underground utility lines in the vicinity, trees with aggressive root systems may interfere and cause significant damage. The site should have adequate drainage and space around the perimeter of the tree to avoid soil compaction by pavement, vehicles, active people and pets.

Individual plant characteristics include visual and aesthetic as well as physical ones. In addition to flowering and fruiting, you can extend seasonal interest by using varieties with vivid foliage like the brilliant lemon-lime locust, Robinia pseudoacacia Frisia. Unusual seed pods are characteristic of the castor-bean, catalpa and golden rain tree, Koelreuteria paniculata.

Other features to look for are blazing fall color, uniquely colored or textured bark (many in the myrtle family), and striking tree form. Plant hardiness (their tolerance to hot and cold temperatures), water consumption, resistance to pests and diseases, and even health concerns like allergy and toxicity, are all important considerations.

Depending on your microclimate, you may need a variety of tree that can handle long summer heat and winter frost, or lots of fog. If water is either costly or scarce, don't go for a water hog - some varieties can easily absorb hundreds of gallon of the precious liquid in a single day. Pests or disease can decimate your investment; avoid varieties known to be susceptible to killer pests.

One attribute often overlooked is the season, duration and amount of litter from a tree. The fernlike foliage of the silk oak (Grevillea robusta) looks great much of the year except in spring when the landscape around the base of tree is ankle-deep in leaf litter. Evergreen conifers can produce a long season of leaf drop early summer through fall. Trees with small berries for the birds may result in your patio being pelted with fruit-laden bird droppings.

Fall is a great time to plant a tree. You'll give the tree a chance to settle in while the soil is warm, and be ready for active growth come spring. You may not be able to hang up your hammock in it for a few years, but if you make some careful choices and plant one now, you'll have a tree to cherish for your lifetime and for those that follow.

TREE RESOURCES

SelecTree: An interactive program to help match tree species to desired characteristics, http://selectree.calpoly.edu

Trees Are Good: A website that contains a wealth of information on selecting, planting and caring for trees,www.treesaregood.org

The National Arbor Day Foundation: A website that includes a tree selector tool as well as care information,www.arborday.org/index.cfm

The University of California Marin Master Gardeners are sponsored by UC Cooperative Extension. For questions about gardening, plant pests or diseases, call 499-4204 from 9 a.m. to noon, and 1 to 4 p.m. weekdays, or bring in samples or pictures to 1682 Novato Blvd., Suite 150B, Novato.

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