What Is The Difference Between Watering A Garden And Rain

How long has it been since we had rain like this? A long, long time. So long, in fact, I had forgotten what a significantly rainy winter could do for a garden.

The beneficial effects of this wet winter are in full view. Plants look lusher, greener and cleaner than they have for half a decade at least.

When a gardener says, referring to rain, "We really need it," you have to be a gardener to truly understand. You could deliver 14 inches of water – the amount of rain we have received since October – through sprinklers or hoses or drip emitters and your plants would not look as good or be as healthy as when the same amount of water, in the form of rain, is heaven-sent. This is a good argument for positioning rain barrels to collect runoff from your roof.

Rainwater collection is not just about saving water but about storing up a reserve of high-quality water for irrigation as well.

There are several reasons rainwater is more suitable for plants than tap water, but the most important is chemistry. In tap water, chlorine is a necessary disinfectant and fluoride is added to prevent cavities (as long as you drink one glass a day).

Nearly all plants, however, are susceptible to chlorine toxicity, usually expressed in burnt leaf margins. Indoor plants such as Dracaenas and spider plants (Chlorophytum comosum), pines, yuccas, and fruit trees, in particular, are subject to fluoride toxicity as well, with symptoms ranging from burnt, discolored, or spotted leaves to stressed fruit that may become diseased.

Calcium and magnesium make tap water hard and damaging to pipes, which is sometimes remedied by the addition of sodium as a water softener. But none of these mineral elements – which are much less concentrated in rainwater – do plants much good when delivered through a sprinkler system. The white sediment you see on the leaves of your plants is calcium and magnesium sediment from irrigation water, and sodium, like chlorine, is toxic to plant tissue.

Furthermore, sodium that reaches the ground is damaging to soil structure, too. In a productive garden, soil particles clump together in beneficial aggregates. Sodium, however, disperses these aggregates and creates cracks on the soil surface.

Plants look greener after it rains since air is 78 percent nitrogen and nitrogen, above all other elements, is what makes plants green. Some of this element, in its nitrate and ammonium forms, comes down in the rain and is immediately taken in by plants through roots and leaves.

Rainwater also contains more oxygen that tap water. You might think your plants are dangerously waterlogged as a result of excessive rain. Yet, whereas waterlogging may bring about anaerobic soil conditions and lead to root rot if you overwater your plants with tap water, the fact that rainwater is highly oxygenated may provide a margin of safety when soil is saturated after a downpour.

Carbon dioxide is also brought down to Earth to the benefit of plants when it rains. Carbon dioxide, when it combines with other minerals in the atmosphere, imparts to rainwater an acidic pH. When this acidic rainwater reaches the soil, it helps to release micronutrients such as zinc, manganese, copper and iron that are essential to plant growth but are mostly locked up in our local soil, which typically registers a neutral to alkaline pH. (Excessive pollutants in the air can produce so-called acid rain, which is harmful to plants, but is more of an issue in the Northeast than locally.)

Another benefit of rain is that it leaches salts down beyond the root zone. These salts, which are carried in irrigation water, accumulate throughout the soil profile and inhibit plant growth. When these salts are flushed through the soil after several years of accumulation, as is happening this winter, the effect is pronounced and the growth of plants is explosive.

Rain also distinguishes itself by the simple fact that it falls uniformly in the garden. This means that all of the soil is leached so that even the furthest reaches of a plant's root zone will be bathed and cleansed of salt.

And, of course, rainwater will also wash off the mineral deposits, dust and pollutants that cover the leaves of all our plants, each and every one a survivor of the profoundly anti-horticultural urban environment that we call home.

The glowing visage of foliage after a rain is not just a beautiful sight to see but also a boon to photosynthesis. The process by which plants turn water and carbon dioxide into carbohydrate, which they then consume as life-sustaining energy food, photosynthesis is much more efficient when the light that reaches a plant's leaves is not filtered by a layer of grit and grime.

In my own garden, I have been observing a butterfly bush (Buddleia sp.) for several years that had yet to develop into more than a few sticklike stems upon which a smattering of foliage could be seen. "Must be a patch of bad soil," I muttered to myself.

Suddenly, this winter, said butterfly bush has put on more than 3 feet of lush new growth. I was more than ready to turn it into compost but now can hardly wait to see it bloom.

And my Peruvian lilies (Alstroemeria hybrid), which proliferate to form the classic no-muss-no-fuss ground cover, are already blooming again, much earlier than usual.

For the better part of a decade, I had been playing with a gerbera daisy (Gerbera jamesonii) that had been planted on the edge of a walkway where it could be shown off to best advantage. Sad to say, it flowered sporadically and declined in vigor from one year to the next. But after this winter's rain, it has shown new life, as if only yesterday it was brought home from the nursery.

TIP OF THE WEEK
Gerbera daisies come from South Africa, a habitat shared with gazanias and geraniums. What these plants all have in common is a preference for drip as opposed to overhead irrigation. I realize that lots of gazanias and geraniums are sprinkler irrigated and do just fine, but you will increase the length of their flowering display, as well as their lifespan by preventing fungus disease, if you keep their leaves dry — unless the foliar wetting agent is winter rain.

The gerbera daisy is a clumping plant and can be divided at the root for purposes of propagation. Gerbera daisies may be grown indoors as long as they are exposed to plenty of light and their crowns, where roots meet leaves, do not touch the soil surface. They grow best when temperatures stay between 40 and 70 degrees and flourish at room temperature.

When in active growth, fertilize weekly with any water soluble flowering plant fertilizer that is diluted to one-fourth of the recommended concentration. For a simple yet arresting table arrangement, cut the heads off gerbera daisies and float them in a crystal or glass bowl that is half-filled with water

For more information about area plants and gardens, go to Joshua Siskin's website at www.thesmartergardener.com. Send questions and photos to Joshua@perfectplants.com.