Rain is nearly always a blessing in dry climates such as our own. Even the many recent downpours, aside from the inevitable tree damage and slope erosion that accompany such storms, have their positive effects in the garden.
Let’s start with the soil. Our sources of water are salty, a result of the arid conditions in our part of the world. When we irrigate our gardens, the salts in the water are left behind and build up in the upper layers of the soil over time. Rain washes these salts down through the soil profile and out of harm’s way as far as plants’ most important roots – those inhabiting the top several inches of soil – are concerned.
Soil salt stresses plants, inhibiting flowering and fruit production. Nowhere is this more evident than in the case of container plants. In the garden, plant roots are distributed far and wide, and salt buildup is dispersed not only over a large root zone but in soil that may not have any roots at all. In a container, however, accumulated salts have nowhere to go except into the root ball, and container plants suffer as a result. Many types of citrus and palm trees, as well as many tropical indoor plants, may show burned leaf tips due to the salt buildup around their roots.
Where container plants are concerned, it is recommended to soak the soil to saturation, and then some, every month or so to drain out the soil salts. A downpour, naturally, will have a similar effect, and you will sometimes see indoor plants purposely set outdoors in wet weather to benefit from the leaching or salt-draining effect of the rain. Aside from diluting and washing away accumulated soil salts, rain also washes dust off leaves. Dust can serve as a nesting ground for spider mites and as a trap for morning dew, encourage the proliferation of foliar fungi such as powdery mildew.
Best of all, rain is economical, since it allows us to forget about watering for many weeks. Given the recent rains, we should not have to worry about watering until the middle of next month, if not beyond, since February is normally the wettest month in Los Angeles. Even if the temperature should suddenly heat up, the ground will hold moisture for some time. If the rain should suddenly stop and the soil surface becomes dry to the touch, the soil below will still remain moist to a considerable depth.
The key to preserving soil moisture is to refrain from cultivation or digging. After it rains, a crust forms over the soil surface that acts as a barrier to evaporation. Overly aggressive gardeners cultivate the soil on a regular basis. This is a practice that makes no sense, winter or summer. Cultivation merely increases loss of water from the soil through evaporation. After a rain, instead of cultivating the soil, cover it with mulch to prolong soil moisture retention.
From a gardener’s perspective, the principal hazards associated with rain involve trees and slopes. Two weeks ago, I saw several mature eucalyptuses fall over. The leaves of unpruned trees take up lots of water when the soil is soaked and become top heavy. At the same time, their roots’ hold on the soil, which has become waterlogged, is compromised. Extra weight on top coupled with weakened support from below can lead to disaster.
Where slopes are concerned, planting ground covers is a necessity for erosion control. Choose from prostrate rosemary, Hall’s honeysuckle, star jasmine, fern asparagus (Asparagus Sprengeri), trailing lantana, perennial vinca, ivy or any one of the many clumping and/or self-sowing ornamental grasses.