Modern materials pop up for the backyard greenhouse

Jim Ruddy often made it a point to watch Crockett's Victory Garden, filmed in Boston and aired widely on PBS stations nationwide. "It tells me what I want to know," he says. It also gave him an idea for a new garden product.

Two years ago, when Crockett host Bob Thompson was interviewing his counterpart in Britain at the BBC's demonstration garden, the merits of a homemade Quonset hut (a simple wooden frame covered with clear plastic) as a season extender were being discussed.

While the homemade cloche cum greenhouse was flimsy and not too durable (the plastic readily degraded in the sunlight), the speed-up, early-spring growth of vegetables growing under the cloche and the extended growth period at the tail end of the season made it worth while.

Both gardeners wondered why there was nothing commercially available to fill the void between the cold frame and the "hard" greenhouse. Why not, indeed, Mr. Ruddy asked aloud. Then, quite suddenly, he realized a solution lay practically in his own backyard.

Moss Tent Works, recognized worldwide for its innovative designs in backpacking and mountain tents, has its headquarters in neighboring Camden. It was Bill Moss who 25 years ago invented the popup tent that goes up like an umbrella. That just happens to be the most sold tent of its class in the world today and it prompted Mr. Moss to go into business for himself.

"What about replacing the opaque tent covering with a clear one?" Ruddy asked. The result, he suggested, would surely be "a unique form of greenhouse."

Moss agreed and the search was on for a suitable covering.

What they found was a reinforced (you can poke a hole in it but it won't rip) , ultraviolet-inhibited (it does not readily degrade in sunlight) clear plastic that fits over the tent frames to form a 5X5-foot, 3-foot- high, easily portable cloche or cold frame. Despite its total 5-pound weight, the tension- stressed poles and fabric combine to form a tremendously strong structure that readily takes my 160 pounds when I lean on it.

With a mountain-tent ancestry, it is highly wind-resistant as long as it is properly staked to the ground.

The fabric was initially designed to be inflated over outdoor pools, tennis courts, and the like so as to extend these summer-only sports arenas into year-round facilities in the North. The fabric is expected to last at least 5 years, even under long sun-exposed situations, but probably 10, and even more if the cloche is used only in spring and fall.

In tests last year, the cloche was shown to readily add 6 weeks on to either end of the season in New England. That's a total of almost 3 months in which quantities of additional fresh food can be grown in its 25 square feet of protected bed.

As expected, the Moss cloche does not come cheaply. The current retail price is $50 (F.O.B. Camden, Maine).

Other less-expensive heat-trapping materials are currently turning up at garden centers. One such product is conventional plastic sheeting, reinforced with a light wire mesh, and it comes in rolls. The wire enables the plastic to be readily shaped into long tunnels which go over a row of emerging seedlings.

Another more durable "tunnel" option is simply to take a sheet of corrugated fiber glass, bend it into shape, and hold it in place with three U-shaped pieces of wire made from old wire hangers. And, of course, there is always conventional plastic sheeting that can be thrown over a variety of supports to form plastic tents.

That these miniature greenhouses are effective is readily seen in the experience of Bob Mertz of Ballwin, Mo. Mr. Mertz harvests tomatoes three weeks ahead of most folks in his area by making a temporary, although large, cloche out of plastic sheeting and hay bales.

He sets out tomato seedlings in his fields around the first week of April, a month ahead of most others in his region. After planting, a wall of hay bales (one bale high) is erected around the tomato bed. Other bales are placed periodically inside the ring of bales to support the poles or anything else that is used to hold up the polyethelene plastic that goes over the entire bed.

Temperatures in this oversized cold frame will rise to a moist 90-plus degrees F. when outside temperatures are in the mid'60's. At night the temperature often drops into the mid-20's without adversely affecting the tomatoes under the plastic.

On unusually hot springs days - in the 80's, for example - Mr. Mertz folds back part of the plastic to release some of the trapped heat. There is a fold-back flap on the Moss cloche, as well.

But, warns Jim Ruddy, "you need to be alert to zip up the flap ag ain once the sun goes down."