The Magic of Mulch

Anyone who doubts the power of mulch should examine the Japanese maples at the corner of Hazeltine Avenue and Riverside Drive in Sherman Oaks.
Although Japanese maples (Acer palmatum) are considered shade lovers, here they are growing in full sun. Even more astonishing, they are planted on the hottest corner – the northeast corner, which receives the full force of the afternoon sun – of the Hazeltine/Riverside intersection. If you have had any experience with Japanese maples, you will probably have a hearty laugh at the thought of planting them in full sun. Japanese maples, which lose their leaves in the winter, are notorious for developing burned foliage the moment after they leaf out in the spring. Generally, the more sun they get, the crispier their leaves become.
Yet the leaves of these Japanese maples are without blemish, as if they had been sprinkled with magic dust that suddenly gave them the power of heat resistance. Mulch, in my opinion, explains the ability of these Japanese maples to grow in full sun without showing any signs of stress.
Several inches of aged, shredded bark have been layered on top of the soil. Mulch, by definition, is any material used to cover the soil surface to reduce loss of water from evaporation. Side benefits of mulch include weed control, erosion control and, as the mulch decomposes, soil improvement.
In dry climate areas such as our own, the primary reason mulch is used is to decrease frequency of irrigation – and not only because the price of water keeps going up. Even if water in the Valley were cheap, it would not be a good idea to irrigate any more than necessary since excess water during warm weather can lead to growth of soil fungi. Such fungi, which attack plant roots, can kill a tree practically overnight. Mulch extends the interval between waterings, creating a dry-wet-dry cycle in the soil that curbs the growth of soil fungi.
Water economy and soil fungi notwithstanding, the most important benefit of mulch in the case of these Japanese maples is the regulation of soil and root temperature. Quite simply, mulch protects the soil and the roots growing in it from the sun – and thus keeps roots cool.
Cool roots. In our climate, there is nothing a plant appreciates more than cool roots. Many plants whose performance would otherwise be marginal suddenly become glorious specimens when their roots are kept either shaded or mulched. In this category, I would include gardenias, princess flowers (Tibouchina) and cannas. Even without mulch, one of the best micro-climates for these marginal plants is a side yard. Situated between two homes, buildings, or neighboring fences, a side yard gets enough light for flowers to bloom but, because of adjacent structures, prevents soil and roots below from being pounded and stressed by the sun.
Even plants that grow well enough in our climate without special treatment – from roses to tomatoes – will produce more flowers and fruit when mulched. And when it comes to a garden of California native plants, a constant layer of mulch is an absolute necessity.
If you have a Japanese maple with burned leaves, make sure you fertilize regularly in addition to putting down mulch. Use organic or slow-release fertilizer to create mineral balance in the soil. Leaf-tip burn on many plants, whether outdoor or indoor specimens, has been attributed to the accumulation of a single salt or other mineral compound in the soil that is then taken up by the roots and sequestered in the leaves. In such cases, achieving soil mineral balance may have salutary effects.
TIP OF THE WEEK: In response to last week’s column on tomato blossom drop, Verna Ahl e-mailed as follows: “I have found that too much water will cause blossom drop, especially in container plants.”
If you have decided to grow tomatoes in a container, use the biggest container – whiskey barrel bottoms are popular for vegetables – you can find. The larger the container, the less often you will have to water.
I have occasionally been asked about whether or not to mulch container plants. Mulching container plants is a good idea since the soil in containers heats up quickly, and mulch – as noted above – keeps soil and roots cool.

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