July is the ideal month to assess our tree needs. We may not even want to go out in the garden because of the scorching heat, which however could be easily alleviated by planting trees that cool the air.
Not only do trees provide shade, they literally cool the air through “transpiration cooling”. The transpiration has been called botanical transpiration. Just as the function of sweat is to cool us down on a hot day, perspiration cools the leaves and the air around them. Transpiration occurs when water moves from the soil to the roots to the leaves. The final stage of water’s transpiring ascent is when it emerges from the leaves but hovers above them as a gaseous vapor. When the heat passes through the steam, there is a cooling effect. We experience this when we step out of a swimming pool on a hot day and feel cold because as the water on our skin turns to steam, the hot air passing through the steam cools the air.
This also explains why plants can wilt in cool weather if a strong wind blows. If the cooling thin layer of water vapor on the surface of the leaves is dispersed, the air around the leaves dries out. As quickly as water is absorbed through perspiration, it is immediately lost to the dry surrounding air on a windy day. Eventually the leaves lose their turgidity and wilt because the rate of transpiration or water uptake in the plant cannot keep up with the rate of leaf water loss.
Of course, the function of transpiration is not only to keep the leaves cool, but also to bring hydrogen from water molecules to the leaves where photosynthesis, the process by which plants make their own food, takes place. Photosynthesis uses solar energy to split the elements hydrogen and oxygen that make up water. Hydrogen atoms from water combine with carbon and oxygen atoms from carbon dioxide to form carbohydrates or sugar, the homemade food that sustains plants. When we apply fertilizers, we are not actually feeding plants, but rather providing minerals that help plants engage in their photosynthesis and food-making process. Incidentally, the waste product of photosynthesis is the oxygen we breathe (from the oxygen in water and half the oxygen in carbon dioxide), which is released into the atmosphere for the benefit of all living beings.
“Now is the Time for Trees” (Timber Press, 2022) is a beginner’s guide to selecting, planting and caring for trees. It is written by Dan Lambe of the Arbor Day Foundation and Lorene Edwards Forkner, a gardening columnist. Although the information provided is basic, it is definitive and bears the seal of authenticity from the world’s largest non-profit organization dedicated to planting trees. Plus, there’s definitely something new here for you, regardless of your experience with trees.
I’ve learned that staking a tree when planting it is undesirable unless it’s “container grown evergreens or windy sites” because trees establish more quickly when ‘they are not tutored. Also, you don’t want to stake firmly but leave slack in the straps wrapped between the trunk and the stakes, located on opposite sides of the tree, so that “the tree can sway in the wind without the clod does not move”.
I was very interested in knowing what trees the authors recommend planting in our part of the country. Several oaks are recommended, including the cork oak (Quercus suber), a mature specimen of which grows at the northeast corner of Van Nuys-Sherman Oaks Park on Hazeltine Avenue. The cork oak is distinguished by its spongy and hydrophobic bark.
The corks you may be popping tonight come from this tree, grown mainly in Portugal and Spain, but also in France, Italy, Morocco, Algeria and Tunisia. Harvesting cork, or the outer bark of these trees, is a special skill done entirely by hand. Improper harvesting procedures can kill a tree.
Normally a cork oak tree is allowed to grow for 25 years before the virgin layer of cork is harvested. When harvesting is done correctly, the tree is undamaged and will regenerate another layer of cork in about 10 years. There are approximately 10 cork harvests during the productive life of a 150-year-old tree, while a tree’s lifespan extends up to 250 years. Cork oak acorns are eaten for their nutty flavor in their native lands after being boiled like chestnuts.
Cork oak forests are threatened with extinction, but not because of overexploitation or poor management. The problem is that plastic and silicone stoppers are less expensive than real cork and these stoppers are gaining acceptance, leading to a drop in demand for cork and the subsequent neglect of cork oak forests. Yet almost all premium wines continue to be corked.
The cork oak itself is a very drought tolerant shade tree and can be grown in desert conditions from Antelope Valley to Palm Springs, the only caveat being that the foliage may turn yellow in poor soil. very alkaline. To overcome alkalinity, regularly apply gypsum to the soil around the tree, at least once a year. After two or three years in the ground, a well-mulched cork oak will not need irrigation unless the winter rains stop falling.
In an email I recently received, Tamera Rooke posed this question: “How do you control the height of an avocado tree so that it doesn’t get too tall to pick the fruit?”
You can maintain an avocado tree or any other tree at a manageable height by keeping it pruned so that it does not exceed the height at which you are comfortable harvesting it.
If the tree has ever grown so tall that you would have to climb over it to pick the fruit, Greg Alder (gregalder.com), the avocado grower’s guru, suggests two options: you can either cut the tree down to a stump, or selectively cut two large branches each year until the tree reaches the desired harvest height. In the first case, the remaining stump should be about five feet tall and will need to be painted (with 50% white latex interior paint and a 50% water solution) to prevent sunburn. In the second case, you will also have to paint newly exposed bark, which will be more difficult higher up the tree but, by cutting only two branches each year, you will still reap rewards as you reduce your tree. By the way, there is an avocado fruit picking pole with a basket device that reaches up to fourteen feet. Thus, with the help of a high and sturdy ladder, you can pick the fruit from a fairly large tree without having to climb it. .
A decision was made to provide space for a “California native of the week” feature in this column. Many California natives, after being in the ground for a few years, need little or no water in the summer. They also attract wildlife and do not require fertilizer or pesticide applications to thrive. Some Californian natives are also disappearing from their habitat and we can therefore actively participate in their preservation by bringing them into our gardens.
California native of the week: “Now is the Time for Trees” distinguishes the desert willow (Chilopsis linearis) as a small tree native to California, reaching 25 feet in height, suitable for planting throughout the Southwest. The desert willow is so named because of the slender stature of its leaves which resembles that of the leaves of a real willow. Like the true willow, it also grows naturally along watercourses although, in the case of the desert willow, it is arroyos that dry up in the summer. The desert willow is extremely drought tolerant and can withstand scorching direct sun as well as cold down to zero degrees. In its habitat, it grows near two other native trees: palo verde (Parkinsonia species) and mesquite (Prosopis species). The desert willow has the distinction of being a monotypic genus, that is to say the only species belonging to its genus. (The pomegranate is another monotypic genus.) The desert willow’s trumpet-shaped flowers, which are lightly fragrant, attract hummingbirds, and the tree serves as a nesting site for a variety of songbirds.
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