Keeping your backyard trees healthy with these tips to help them grow

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“We have a large ash tree in our backyard. For the first time ever I recently noticed around 50 healthy, leafy shoots on the ground underneath it. These shoots were about 6-8 inches long with stems about ¼ inch in diameter, all of which were neatly cut at a 45-degree angle! There had been no wind or microburst that might have explained this and the shoot stems were green and flexible. This must have been done by a critter. Do squirrels to this? Why?” — Brian Westin, Whittier

You are correct in your hypothesis that this was done by squirrels. Tree squirrels are particularly fond of ash trees when it comes to cutting off the kind of shoots that you describe. There are several reasons why they do this. One is to gather material for building dreys or nests. Another is to sharpen their teeth. Still, another is to satisfy their need for sodium which is highly concentrated in the terminal or end portions of shoots. This activity is commonly observed at the beginning of the spring and fall seasons. Moreover, the ash appears on the list of trees that can grow in saline soils, such as the soils found throughout the Southwest, so ash trees might absorb salt into their tissues more readily than other trees and not suffer from sodium toxicity.

Although such damage is not a danger to the overall health of a tree, it may be prevented by leaving small salt licks or blocks at the base of a tree where squirrels climb and cut shoots. Such blocks are available at pet food stores.

Mimosa tree on Burbank Blvd. west of the 405 Freeway. (Photo by Joshua Siskin)
Speaking of trees, I receive many questions about declining arboreal health and in the vast majority of cases the explanation has to do with the age of the tree or some abiotic factor such as poor soil drainage, insufficient light, exposure to strong winds, or crowding. Overwatering may also be involved.

Age of the tree: Certain trees have weak immune systems and seldom live more than a few decades, often going into decline within ten years of planting. Purple leaf or flowering plums are at the top of this list.  They are more suitable to cold winter climates than to our own. Purple leaf plum trees are truly gorgeous when first planted owing to their unusually dark leaf color and cloud of pink or white flowers. However, they do not stand up well to Southern California heat. All trees in the Prunus genus (plum, apricot, peach, and nectarine), whether they are ornamental or producers of edible fruit, are likely to need replacement by the age of thirty if not sooner so if you have one of these trees that is older than thirty and is still attractive or productive, consider yourself blessed.

Leguminous trees also tend to be short-lived. At the top of this list is fernleaf acacia (Acacia baileyana), a wonderful small tree with small, bipinnate blue-grey leaves and fragrant yellow spherical flower puffs. Its form is a shapely dome. There is also a Purpurea cultivar with violet leaves. It quickly grows to a mature height of only twenty-five feet. Alas, the tree seldom lives for more than two decades or so.

Growing to a similar height is mimosa or silk tree (Albizia julibrissin), another leguminous species. In addition to unusual shuttlecock pink and white blooms, its outstanding characteristic is a spreading umbrella form that quickly creates shade below its canopy. It should be noted that a litter problem is created by its faded flowers and dried up seed pods.  Mimosa will begin to show decline in its second or third decade of life as individual branches start dying back from the tips. There is a Chocolate mimosa cultivar with bronzish foliage but it is not as robust as the common mimosa.

Impaired soil drainage: A tree or any other plant for that matter does not stand much of a chance where soil drains inadequately. A tree may live for years without trouble where drainage is inadequate and then suddenly after a winter of heavy rain followed by warm weather, long-dormant pathogenic fungi will suddenly come to life due to excess water in the root zone and bring about the tree’s demise.

Insufficient light: Sometimes a tree will look great for a few years and then due to crowding from surrounding shrubs or other trees start to defoliate or flower less abundantly due to reduced light exposure. Where the ground under a tree’s canopy is continually moist through frequent irrigation and the base of the tree’s trunk stays wet and cannot dry out, the tree’s decline is sure to follow. Insufficient sun exposure may also bring on disease since leaves wet for more than a few hours, even if the wetness comes from morning dew, are likely to develop powdery mildew.

Root congestion: I have seen eucalyptus and redwood trees begin to defoliate from root congestion.  Although known for their massive size, the root systems of both eucalyptuses and redwoods are shallow.  This means that if the trees are grown in a constricted area where roots cannot grow out laterally due to bordering hardscape or thick ground cover, for example, they become compacted and less capable of water absorption. In such cases, use of a deep root irrigator is recommended. This tool is a three-foot long T-handled, small diameter metal pipe with a pointed tip that is inserted into the earth at the depth of your choosing. A hose is attached to the top of the irrigator and water flows through it. You can also utilize the irrigator for liquid fertilization through a hose-end attachment.

Tip of the Week: I wrote last week about the dazzling San Pedro garden of  Richard Lynch. Lynch has solved the problem of backyard flies through cultivation of a carnivorous pitcher plant (Nepenthes sp.) that grows from a hanging patio basket. Pitcher plants contain liquidy nectar which attracts insects before they crawl into and drown in it. Lynch says that keeping the pitchers half-filled with water is recommended while application of any type of fertilizer inhibits pitcher formation. Lynch occasionally opens his garden to visitors and you can contact him about such a visit through this email address: He specializes in growing tropical Vireya rhododendrons which may be ordered from Hawaii by calling Sherla at 808-966-9225 or through the website at Incidentally, Lynch advises any plant lover visiting the Hawaiian archipelago to spend considerable time on the Big Island because that’s where the flora is most diverse and spellbinding.

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