Just Planted - and Ready to Pick. Container-grown tomato plants give northern farmers early pickings of premium fruit. BUCKETS OF TOMATOES

BY late May or early June, when frosts have gone from his lands, Iowa farmer Bob Graaf will have mature tomato vines, 4 or 5 feet tall, shipped in from Florida. Within days of their being set out in his fields, he expects to be picking salable, vine-ripened fruit. On the face of it, this sounds too good to be true, a scenario straight out of science fiction. But the concept has already been proved in practice. Last year, Mr. Graaf was one of a handful of growers who tried the new technology, called ``crop migration'' by its developer, William Skaif.

Despite a hailstorm that thrashed the experimental tomato planting last August, Graaf could smile with satisfaction as he showed a visitor the battered vines and fallen fruit a few days later. His trial entry into commercial tomato production, using the portable-bucket approach, has paid off in his view, and the farm's future is the brighter for it.

``If they [the tomatoes] can do this well in a year when we've had so much wind and high temperatures, what will they do in a normal Iowa season?'' he asked, adding: ``I can't wait to find out.''

Graaf is confident this move into a more diversified farming program is a good one. Plantings of chives and statice (for dried flowers) represented other breaks from the corn and soybeans routine of this region last season.

But the tomatoes proved the most promising. He sees them as able to cushion a bad year, such as the one just past, and to turn a good year into an exceptional one.

Under the crop migration system, tomato plants (or other vine crops) are grown to maturity in buckets in Florida during winter, then brought north after frost where they start the new season as fully hardened, ready-to-pick plants. The result is an early harvest of vine-ripened, premium fruit at a time when conventional growers are looking at 6-inch seedlings.

Ray Cook of Monroe, N.C., was another of the handful of test growers to go the bucket route during the '88 season. In the initial year for the still-developing concept, Cook says he was short on basic information and had to ``figure out a lot of things myself.'' Even so he harvested 7,000 pounds of tomatoes from 250 buckets before an outbreak of red spider ended his season in early August with ``three months of picking still to go.'' Without this setback he reckons he would easily have reached 12,000 pounds for the season ``and maybe even doubled [the harvest].''

Standard cultural advice for tomatoes says that flower-bearing, let alone large fruit-bearing plants should never be transplanted because the shock will cut overall production considerably. In the crop migration concept, however, mature plants may be transported a thousand miles or more before being set out in the farmer's field.

The secret: They are never truly transplanted in that they are not removed from the original container. In the field, the 5-gallon bucket is simply set 6 inches deep into the earth so that the plants can send new roots out through holes in the side of the bucket into the surrounding soil.

While liquid fertilizer is continuously applied to the buckets, the roots spreading into the surrounding farm soil support additional growth and high productivity over a prolonged picking season.

All of the people involved with the bucket system this past season were general farmers who paid only as much attention to the new project as their workaday world allowed them. By their own admissions the projects frequently ``got ahead'' of them.

But now Tom McClure of the Economic Development Assistance Program, in cooperation with Western Carolina University, intends to bring scientific scrutiny to bear with a demonstration project slated to run next year.

McClure, who visited the Cook operation on several occasions and talked with other growers, says he's ``excited about the concept.'' Unless something turns up, he expects to prove with detailed recordkeeping the generally favorable observations of the growers.

McClure sees several unique advantages to the system. They are:

Portability. The system substitutes crop shipments for food shipments into the cooler regions of the country in the spring and allows the reverse (cool to warm region shipments) to take place in the fall.

Immediate harvests. Ideally tomato vines are shipped when the first fruits are just turning color. Ray Cook harvested 200 pounds of tomatoes the first week his buckets in place. ``The first few nights I had to spray them with water'' to protect them from late frosts, he says.

Maximum growth. Mature plants are set in place in late spring just as the longest days of the season are arriving. Large plants can capitalize on the long hours of sunlight more than can seedlings. The warm days and cool nights of more northern areas at this time of year are ideal for maximum fruit set.

Prolonged harvest. The system turns what is normally a two-month growing season, followed by a month-long harvest, into one long harvest season.

While importing buckets of field-hardened plants is the simplest way to go, some Northern growers are seeding their own buckets in greenhouses.

Barbara and Keith Eisenmann, Michigan farmers and greenhouse operators, adopted this approach for the cherry tomatoes they grew last year. They went one stage better in the fall. When frost threatened, they severed the soil roots and brought the plants back into the greenhouse. There was some wilting, ``but the plants recovered and continued producing for a few more weeks,'' Barbara Eisenmann said in a recent phone interview. For further information on Crop Migration, write to PO Box 2054, Silver, NC 28779.