Yardwork's a breeze with 'Green Machine'

If two heads are better than one, then three ought to be a whole lot better still. That, at least, appears to be the reasoning at the Long Beach, Calif., headquarters of Hawaiian Manufacturing (HMC), the company that introduced the string trimmer to the world a decade ago.

HMC's philosophy is to remove time-consuming tedium from gardening whenever it finds a way. In pursuit of this goal it markets several gardening aids under the title ''The Green Machine.'' Recently it decided to bring vacuum-cleanerlike changeability to this home-gardening line by making it possible to use three separate aids with one power source - a lightweight, 1-horsepower gasoline engine.

A few twists of a wing nut and a few seconds of your time make it possible to change the machine from a weeder-cultivator to a string trimmer-brush cutter to a power blower.

Each gardening option includes a machine head and shaft that matches the shaft of the engine.

The Green Machine people had all gardeners in mind when they developed this switch-a-head concept, but particularly women. Where her mother and grandmother traditionally had an hour or so to spare for the garden each day, the woman of today may be busy clinching a real estate deal, anchoring the midday news, or dispensing a bank loan. In short, she hasn't the time anymore and thus must make the most of such gardening opportunities that do come her way.

The new machine is called the ''Expand-It,'' for obvious reasons. Its exceptionally long handle (47.8 inches) provides the operator with considerable reach without the need to bend and stoop. Stand-up gardening, as everyone knows, is the simplest kind. An adjustable handle lets people of varying heights use the machine.

The weeder-cultivator isn't the standard rotary-operated machine. Its twin blades work reciprocally, like two fingers scuffing back and forth in the soil. But it does this at more than 2,000 cycles a minute, churning up the soil to a depth of 5 inches, should you wish deep cultivation, or skimming across the top lightly to cut off the weeds just below the soil surface. Its compact size makes it easy to use among flowers and vegetables.

When the nylon string trimmer was introduced by HMC, it was dubbed a novelty by early critics. But the concept was readily embraced once its capabilities were realized. The Green Machine's Tap-for-Cord head automatically dispenses additional string by tapping the machine on the ground while in operation. The trimmer also accepts circular metal saw blades that slice through heavy weeds, brush, and even small trees up to 3 inches in diameter.

Apparently, garden cleanup, including raking leaves and lawn clippings, is a universally disliked chore. Here the ground-level blower attachment speeds up the process with a powerful jet of air that directs the garden debris into neat piles for easy removal.

Those on a budget may buy only the engine and one attachment, if they wish, adding other attachments when they can afford it or as the need arises. Meanwhile, still other attachments are being developed, among them a snow blower , scheduled to come onto the market in the fall.