THE COASTAL GARDENER:

If the drought in Orange County continues, or worsens, there won’t be enough water for your garden.

That may sound frightening, but surprisingly, it’s not necessarily true. There is water available to our gardens; maybe we just haven’t noticed it. As I write this column, it’s Thursday morning. Forecasters are predicting between a half-inch and one inch of rain in the next 48 hours.

That’s a lot of water — precious water. Water that your garden may need later.

Some of that rainwater is falling onto your garden and watering your plants — hooray. But a lot of it is lost.

Urban roofs and how they deal with rainwater are a fascinating study. Consider that your rooftop has been carefully designed at a gentle slope, for the sole purpose of shedding any water that might fall on it.

Along the edge of that well-crafted, sloped roof is another engineering marvel, a rain gutter. Your roof and rain gutter are perfectly coordinated to collect every drop of precious rainwater.

The collection and diversion of rooftop rainwater is a nearly perfect system. But what happens to all that rainwater, so masterfully collected?

At a couple of roof corners all that precious water is fed into a couple of collection tubes, called downspouts.

So far, so good. In fact, it’s an amazing sight to watch, driving down a street while it is raining and seeing all that water, being perfectly collected, home after home.

But here’s the punch line. What do we do with all that collected rainwater? We move it swiftly and efficiently off our property and into a storm channel. Oops! Our gardens need it, not the storm channels.

Now look at this picture. That’s a 620-gallon water storage tank. It’s sitting next to a rain-gutter downspout.

Let’s assume that a typical home roof in the Newport-Mesa area is 1,500 square feet. That’s probably pretty close. How much rainfall would it take to fill a tank this size? The answer — about two-thirds of an inch would fill the tank with pure, clean and free water; water to garden with.

The tank in this picture isn’t very big and fits flat against a wall, where it is hardly even noticed.

It’s a closed tank with no mosquitoes or algae. Just pure, clean water. A hose bib provides the water to the gardener. Connections are even available to hook it up to an irrigation system.

Of course, larger tanks can store even more water. In a year of average Orange County rainfall a roof this size will yield almost 12,000 gallons of free irrigation water.

In addition to supplying water, rainwater harvesting is also effective in reducing storm-water pollution.

When rain falls, it’s clean, but it picks up pollutants along the way.

This pollution is carried into storm drains and then to channels and ultimately into the ocean.

Collecting storm water from rooftops and directing it to storage tanks so it can later be used for irrigation decreases the volume of runoff and protects our coastal waters.

Before the creation of public water utilities, rainwater harvesting provided water for many American homes. It is still popular in places with limited water resources, such as the Middle East, India, Africa, the Caribbean, Australia and Israel.

In the United States, rainwater collection rebates are becoming increasingly popular, including cities like Albuquerque and Santa Fe, N.M., Austin, Texas, Denver and some communities around San Francisco.

Santa Monica offers rebates of up to $500 for diverting downspout water into these collection tanks.

A couple of large garden centers, including Roger’s Gardens, are now demonstrating these rainwater tanks and even offering installation and setup options.

There is water available for your garden; perhaps you just haven’t noticed it.

ASK RON

Question: After six years in the same spot, my tomatoes were ruined by nematodes. Do nematodes attack other plants? How long before the soil is cleared of them? What else can I plant there?

Christine

Newport Beach

Answer: Nematodes are microscopic organisms that live in the soil of our gardens, usually without the gardener’s knowledge. Although many nematodes are beneficial, keeping other pests under control, one specific species, the root-knot nematode, is the curse of tomato growers. In a vegetable garden, root-knot nematodes are pests of plants in the solanum family (Solanaceae). I suggest moving your tomatoes, eggplants, peppers, potatoes or tomatillos to a new location or into containers. Unfortunately, studies suggest that root-knot nematodes persist in soil for many years, although most garden experts suggest about a two-year waiting period prior to replanting in the same location.

ASK RON your toughest gardening questions, and the expert nursery staff at Roger’s Gardens will come up with an answer. Please include your name, phone number and city, and limit queries to 30 words or fewer. E-mail stumpthegardener@rogersgardens.com, or write to Plant Talk at Roger’s Gardens, 2301 San Joaquin Hills Road, Corona del Mar, CA 92625.

RON VANDERHOFF is the nursery manager at Roger’s Gardens, Corona del Mar