IT'S one of North America's favorite vegetables, particularly when covered with a rich cheese sauce. We're talking about broccoli, the vegetable that no fresh produce merchant worth his salt would be without these days.
But Stokes Seeds of St. Catharines, Ontario, and Buffalo, N.Y., can recall a time when broccoli's lack of appeal almost put the company out of business. Sometime in the early 1930s, Francis Stokes, son of the founder, saw broccoli's place in the future. Unfortunately, his substantial investment in broccoli seed came a decade too soon. Conservative American tastes demanded white, not green, ''cauliflowers'' at that time.
Stokes survived, if only just, and today the company sells quantities of broccoli seed annually, as does every other seed supplier. The credit for broccoli's present popularity, however, belongs to the frozen-food industry, which began importing the vegetable after World War II. People who found it in their TV dinners tried it, liked it, and began ordering the vegetable all on its own.
Soon local gardeners and farmers woke up to the fact that they could grow perfectly good broccoli on this side of the Atlantic. There was no need to import it. At the time the folks at Stokes said in effect: ''We tried to tell you that years ago.''
Broccoli is relatively easy to grow given the right conditions (humus-rich soil, cool roots, adequate moisture, and six hours of sunlight a day). As a result, many home gardeners cross it off the supermarket list during much of the growing season. Broccoli even grows readily in containers. Two plants in a five-gallon bucket do nicely.
Right now broccoli plants have yielded the spring harvest over much of the South, and the plants are up and growing in the North. The need now is to prepare for the fall harvest as well as to see how we can boost the production of side shoots from the spring-harvested crops.
While garden centers supply started plants during spring, few, if any, provide this service for a fall crop. The gardener, then, must grow his own from seed; and in the far North that begins during the final week of June. In warm summer soils, broccoli seed germinates readily, and they can be direct seeded at this time. Or they can be started in flats and transplanted just as is commonplace for the spring crop.
Meanwhile, how can the early crop be induced to continue yielding a good supply of side shoots throughout the season?
Here are a few steps that work in my garden:
* Mulch thickly to keep the root system relatively cool. Alfalfa hay is ideal. As a legume it is rich in nitrogen, which it feeds steadily to the broccoli as it decomposes. But any organic material will do (leaves, straw, sawdust, and shredded newspaper).
* Water deeply once a week, and more frequently if it is very hot or if your soil is sandy.
* Feed regularly. I aim (though I don't always get around to it) to feed the broccoli with a liquid fertilizer every second week. This is often manure or compost ''tea'' or sometimes fish emulsion mixed with seaweed solution. Another option is to sprinkle fertilizer or compost around each plant as a side dressing and then water it in. Remember, if you are using shredded newspaper or sawdust as a mulch, it may temporarily tie up some of the nitrogen. Under those circumstances be prepared to increase the nitrogen in your fertilizer mix.
* Prune the plants. Removing some of the shoots allows those remaining to grow more sturdily, yielding larger side heads. I strive to leave no more than three side shoots growing at any one time.
Treated this way, broccoli becomes a cut-and-come-again vegetable that can last all season; this is one reason it has replaced its cousin, the cauliflower, in most home gardens.