For more prolific broccoli, try some disciplined pruning

Three years ago I happened to have a sharp knife in my hand at just the right moment, and ever since then we have enjoyed better-quality broccoli in our home.

Broccoli is rated ahead of cauliflower in most home food gardens because it is a cut-and-come-again vegetable. One harvest can follow another all season long, as the seed catalogs are quick to tell us.

But the size, and even the qualitY, of the side heads that develop after the first head is cut can go downhill rapidly unless the gardener does some disciplined pruning. That's right, broccoli responds to pinching, cutting, or shearing out the excessive growth as dramatically as your favorite apple tree - or maybe even more so.

I discovered the value of pruning broccoli quite by accident. I was inspecting the garden one evening, and happened to have a sharp knife in my hand when I came across the broccoli. Over the weeks that followed the first harvest, the plants had become a mass of closely growing stalks. Obviously, any new side shoots would not readily make it up through that tangled mess.

So I bent down and cut away much of the growth, opening up the plants the way one would a fruit tree. The effect was moderately dramatic. Tiny and somewhat stringy florets gave way to side heads that were larger and more tender - about the size of a half-dollar coin.

Last season this pruning was done even more relentlessly, and a lot earlier as well. The side heads enlarged to about a third or more of the original head.

From time to time I have mentioned this pruning experience to other gardeners , and the idea has always been greeted with moderate surprise. So it was with some satisfaction that I recently read of a gardener who takes the same approach.

An article in Country Journal magazine by Nancy Bubel, whose gardening and garden writing I hold in high regard, quotes the experience of Dr. Peter Cunningham of Guilford, Conn.

Dr. Cunningham, whose broccoli pruning is more disciplined than my own, takes the main head, waits a few days, and then harvests the first crop of mature side heads. After that, the earnest pruning begins. First, he removes all of the small side shoots that have already developed into mature, but tiny (an inch or less across), broccoli heads. These, he says, will never grow any larger.

''Throw them away or add them to the soup,'' Dr. Cunningham suggests. The remaining leaf-covered buds, usually on one- or two-inch stems at this stage, hold promise of developing into much larger side heads.

''Three or four of the one-to two-inch shoots are left, and the rest pinched out. And I mean all the rest,'' says Dr. Cunningham. ''If you leave eight, they will make small heads, but if you leave three or four, they will develop into sizable heads.''

Other tiny stems keep forming, and the thinning process is continued throughout the season.

Often a new side head will develop near the base of the old stem and send up a vigorous shoot as a result of the pruning that later produces many more side heads of its own. Dr. Cunningham also removes some of the leaves as the plants become bushy. This gives developing shoots more space in which to grow.

Broccoli likes warm soil, but resents hot soil. Thus I apply a thick mulch to the broccoli bed in summer, which seems to help the plants produce moderately well even during the dog days of August. And while good initial soil preparation will frequently carry the plants clear through to frost, a side dressing of fertilizer, placed in a ring around the base of each plant midway through the season, is also beneficial. Or you can periodically apply liquid fertilizer, a compost, or manure ''tea.''

While on the subject of pruning plants, don't forget you can get secondary heads from cabbage too. After cutting out the main head, the cabbage will start forming several buds or miniature heads at the base of each remaining leaf. I remove all but two of these buds to get a second harvest of modest cabbage heads.

Cabbage also responds well to a mulch and a midsummer feeding.