FORTRESS OF WATER AGAINST THE FROST. Product protects frost-sensitive plants at temperatures below 20

Early in this decade, a new garden product called Wall O'Water hit the market. It was a type of hot cap, but with a marked difference in the way it performed: Its hollow walls held water. During the day, the sun would heat the water - and the water, in turn, kept the plant warm all night long.

Wall O'Water was a new, if not rev-olutionary, concept. Earlier, a few in-novative gardeners had been standing water-filled plastic jugs alongside seedlings.

It was also an American invention in an era when many innovative horticultural products were coming in from Europe. Reviewers latched onto it.

Most comments were positive. Wall O'Water did advance the growing season by some weeks. But was it worth the cost? Would it last for more than a couple of years? And would it end up as just another five-day wonder?

Now, after seven years on the market, the product has proved itself. Wall O'Water advances the planting date by two months. It protects even frost-sensitive plants at temperatures below 20 degrees F. It lasts, with moderate care, for six or more seasons.

And it's cost-effective enough even for commercial growers to use. For home gardeners, the lengthened tomato harvest more than covers the retail cost (under $2.50) in extra vine-ripened fruit the very first season.

The Wall O'Water ``tepee,'' to give its full description, is 18 inches high by 18 inches in diameter. Its clear plastic walls are made up of a series of hollow tubes the gardener fills with water when he sets the tepee in place. This makes it rigid so it can stand on its own.

To take full advantage of these water tepees, it is best to begin using them a full two months ahead of the normal planting time.

Here is the suggested procedure:

1. Set out the Wall O'Water a week before you intend planting. Have the walls sloping in, tepee fashion at the top.

This allows the soil to begin warming up to more pleasant growing temperatures. 2. Set out the seedlings, preferably on a windless day to avoid any risk of chilling while planting. 3. If the nighttime temperatures consistently drop below 20 degrees F., keep the tepees closed at the top. When night temperatures move above that mark, leave the top open.

The water will have reserves of heat that radiate out to the plant all night long. Even when temperatures dip so low that the water in the tepee freezes, it will protect the plant.

This is because of the tremendous amount of heat given off by water at the moment it freezes - 80 calories per gram - which, for the three gallons of water in the tepee, translates into many thousands of calories of heat energy!

4. Leave the Wall O'Water in your garden for another four weeks after the last frost date. This is recommended because the weather remains cooler than is optimal for rapid growth for several weeks after the last frost, and plants continue to benefit from the tepee's warmth. The plants will never be stressed in this period, because water absorbs so much heat that it prevents the tepee from overheating.

Another point to remember: Plants grow ``logarithmically'' - by a certain percentage each week. So the larger a plant is when the period of optimal growth begins, the more increase in size.

For example, if you set out a four-inch tomato plant at the normal time, it will double in size within, say, 10 to 12 days. But the 18-inch Wall O'Water plant, set out two months earlier, will double to three feet tall in the same period.

Janice Paseka of Ames, Neb., talks of ripe tomatoes in June, rather than the usual August, even though the tepees were surrounded by 18 inches of snow for three days after seedlings were in.

In Burlingame, Kan., Karen Picket had the temperatures drop low enough to freeze the water, but her tomatoes didn't seem stressed. ``They flowered two months early [for her region],'' she says.

And in northern Vermont, Carolyn Ormsbee set out tomato transplants in late March and harvested the ripe fruit on July 4.